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HomeMy WebLinkAboutagenda.council.worksession.20180205 CITY COUNCIL WORK SESSION February 05, 2018 5:00 PM, City Council Chambers MEETING AGENDA I. Mud and Debris Flow Study II. RFTA Integrated Transportation Systems Plan (ITSP) P1 MEMORANDUM TO: Mayor and City Council FROM : April Long, P.E., Stormwater Manager THRU: Trish Aragon, P.E., City Engineer Scott Miller, Capital Asset Director DATE OF MEMO: February 1, 2018 MEETING DATE: February 5, 2018 RE: Mud & Debris Flow Assessment Project – Final Report SUMMARY: In early 2016, the City retained a team of consultants led by Wright Water Engineers to perform a Mud and Debris Flow Assessment to evaluate the potential risks to the City of Aspen from mud and debris flows originating from Aspen Mountain and to recommend effective mitigation strategies. The results of this assessment have been detailed in the City of Aspen Mud and Debris Flow Assessment (Attachment A) and will be discussed in this work session. PREVIOUS COUNCIL ACTION: In February of 2016, Council approved a contract with Wright Water Engineers, Inc. (WWE) to complete the Mud & Debris Flow Assessment Project. BACKGROUND: Debris and mud flows are rivers of rock, earth, and other debris saturated with water. The debris flows are usually triggered by rapid melting of the snowpack or an unusually intense rainstorm. Several mud and debris flow prone areas are located on Aspen Mountain. The City completed a mudflow analysis in 2001 as part of the Aspen Mountain Surface Drainage Master Plan (2001 Master Plan). The 2001 Master Plan analyzed the Aspen Mountain drainage basin for mudflow potential and impact and estimated the depth of mudflow throughout downtown. The 2001 Master Plan evaluated system upgrades and on-mountain capital projects to mitigate the impacts from a mud or debris flow. However, those improvements ranged in cost from $8 million to $11 million. Therefore, the preferred alternative was to guide development within the mudflow plain to understand and limit the impacts of development on the mudflow plain. This guidance in provided within Chapter 7 of the Urban Runoff Management Plan (URMP). In 2014, the National Oceanic and Atmospheric Administration (NOAA) updated the rainfall data for Aspen, which is different than the data used in the original 2001 Master Plan and could potentially change mudflow risks and impacts in town. Additionally, in recent history, the frequency of wildfires in the western United States has increased. After a wildfire, the charred ground where vegetation has burned away cannot easily absorb rainwater, increasing the risk of flooding and mudflows for several years. Properties directly affected by fires, and those located downstream of burn areas, are most at risk P2 I. Therefore, in February of 2016, the City retained a team of consultants, led by WWE, to perform a Mud and Debris Flow Assessment to evaluate: 1. The potential risks to the City of Aspen from mud and debris flow originating from Aspen Mountain, 2. Representative estimates of cleanup costs for the City and for private properties in the mudflow hazard areas, 3. The potential impacts of wildfire on mud and debris flows in Aspen, and 4. Potential mitigation strategies, projects, and design guidelines to prevent or lessen the impacts from mud and debris flows from Aspen Mountain. DISCUSSION: In the spring of 2016, WWE, with the assistance of geologist, an international mudflow expert, and the developers of the FLO-2D mudflow model, began evaluating the possibility of and effects from mud and debris flows from Aspen Mountain. Below is a description of the major tasks completed and the results derived from those tasks: · History and Background: The consulting team reviewed available studies, met with locals and professionals familiar with mudflows in the area, and conducted field investigations of Aspen Mountain and other nearby areas of geologic interest to determine geologic and land use conditions in the study area and to find available documentation of past mudflow events. o Aspen Mountain is a well-managed ski area and the sediment supply on the mountain is greatly reduced through the Ski Company’s revegetation efforts, stormwater management, and snow management practices. o In the mountainous drainage basins above the City, sudden and severe thunderstorms can produce heavy rains and flash floods capable of transporting large boulders, trees, and sediment. Events like these have been reported in and around the Aspen area historically, and off Aspen Mountain in 1919, 1964, and 1997. · Mud and Debris Flow Design Storm and Modeling: The consulting team reviewed rainfall and snowmelt data, past hydrologic studies, and ran several models to determine that: o The largest total rainfall depths and greatest rainfall intensities are associated with storms in late-spring and summer months, when snow water equivalent (SWE) is low, if not zero. o Mudflows are more likely to occur at lower recurrence interval events such as the 10- or 25-year event. o Large flood events such as the 100-year event may contain too much water to produce viscous mudflow – while the available volume of sediment might be the same, the sediment concentration would be much lower compared to smaller events. o While the consulting team ran models for events ranging from 2 – 100 year storms, they focused their analysis on the 25-year event with a 45% sediment concentration. o The model predicts that, in the 25-year event, 21.5 acres would experience mudflow, with 5 acres experiencing mudflow deep enough to put the public and buildings in significant danger (see Table 9, Table 13, Figure 18, and Figure 20 in Attachment A). o These estimates are based on a storm event occurring over the entire study area. It is very unlikely that a storm event will trigger a mudflow out of all three source areas at the same time. · Wildfire Risk Assessment: Several wildfire scenarios were evaluated for Aspen Mountain to look at potential increases in mudflow depths and volumes in the event of a wildfire. P3 I. o Aspen Skiing Company actively manages the vegetation on Aspen Mountain, and the risk of a major wildfire is considerably lower than other local forested watersheds. Based on the scenarios evaluated, peak runoff rates and volumes would be expected to increase in all of the major drainages following a wildfire. The relative increases in runoff rates and volumes are greatest for the more frequently occurring events, and the differences between burned and unburned scenarios diminish for larger storm events. · Damage and Financial Impacts: A damage estimate was performed to quantify the potential impacts to public and private property associated with the target mudflow events. o It is estimated that in areas affected by a mudflow or mud flood with maximum depths of 2- to 3-feet, costs for cleanup and restoration could exceed $5,000,000 per block with damages occurring on lower levels of buildings and sub-grade areas such as sunken terraces and below ground parking garages at the greatest risk. o The time and costs associated with the cleanup of public parks, streets, and the storm drain system following a mud and debris event are significant. The total cost associated with cleaning up the City’s streets and parks could total over $445,000 during the 25- year event, with the jetting of the City’s storm drain system costing over $350,000, for an estimated total of almost $800,000. The time required to remove mud and debris on the impacts to local businesses would be far greater that the cleanup costs. · Mitigation Assessment: The consulting evaluated common mitigation techniques to determine viable and recommended options to reduce risk and costs associated with mudflows from Aspen Mountain. o The most viable mitigation alternative to address some of the areas of greatest mudflow depths is floodproofing at the lot scale – temporary and/or permanent flood barriers that could be structurally and architecturally integrated with the buildings and/or hardscaping and barriers that can be raised and retracted could also be considered. o Additional mitigation techniques recommended include: warning system, reducing sediment supply, and slight modifications to City parks. · Update Criteria: The existing City mudflow regulations require that debris flow analyses be conducted using the 100-year peak rainfall event and maximum sediment concentrations of 45- percent. This criterion is conservative given that debris flows typically occur in the 10- to 25- year recurrence interval range, and that the 100-year peak flows events more typically behave as mudfloods as opposed to mudflows. For these reasons, it is recommended that the regulatory design events are evaluated for the following two events and criteria: o The 25-year, 2-hour rainfall event with maximum sediment concentration of 55-percent and an average sediment concentration of 30-percent by volume. o The 100-year, 2-hour rainfall event with an average sediment concentration of 20- percent. o A 0.5-foot tolerance is recommended when evaluating effects of a proposed development on adjacent properties that are within the mapped mudflow zone. This tolerance would allow for model results to show up to a 0.5-foot rise on neighboring properties that are already affected by mudflows. This would not allow a property to cause mudflow/mud flood impacts to a neighboring property that is not already affected by this hazard. · FLO-2D Training: The consulting team conducted two training sessions for City staff and the development design community to instruct use and evaluation of mudflow designs with the updated FLO-2D model. P4 I. CITY MANAGER COMMENTS: Attachment A: City of Aspen Mud and Debris Flow Assessment – Wright Water Engineers, Inc. 2017. Attachment B: City of Aspen Mud and Debris Flow Assessment Appendices – Wright Water Engineers, Inc. 2017. P5 I. DRAFT City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-1 November 2017 Appendix A. Summary of Field Observations Kirkham Field Observations On September 1 and 2, 2016 fieldwork was conducted in the drainage basins on and below Aspen Mountain and on the southwest and southeast sides of Red Butte. The participants included: April Long, P.E., City of Aspen Stormwater Manager Mike Horvath, City of Aspen, Civil Engineer II Victor Gerdin, Aspen Skiing Company, Director of Mountain Planning Peter King, Aspen Skiing Company, Aspen Mountain Manager David Clark, Aspen Skiing Company, Associate General Counsel Joe Giampaolo, Aspen Skiing Company, Aspen Mountain Trails Director Rana Dershowitz, Aspen Skiing Company, VP and General Counsel David Corbin, Aspen Skiing Company, VP of Planning and Development John Mechling, P.E., CTL Thompson Inc., GWS Branch Manager David Glater, P.E., CTL Thompson Inc., Geologist/Engineer Andrew Earles, Ph.D., P.E., WWE, Senior Engineer Dai Thomas, Ph.D., P.E., TetraTech, Senior Engineer Jim O’Brien, Ph.D., P.E., Riata Catherine Berg, P.E., WWE, Engineer Bob Kirkham, P.G., C.P.G., WWE Adjunct Scientist, Geologist City of Aspen staff and WWE staff and their consultants participated in all aspects of the field tour. The representatives from Aspen Ski Company and CTL/Thompson led the field tour on Aspen Mountain, and some also attended a summary meeting on September 2. The field tour was kicked off on September 1 with a brief meeting in the City of Aspen's new engineering conference room offices. Next on the agenda was the examination of conditions on Aspen Mountain, which was led by representatives from Aspen Ski Company. Some participants rode the gondola to the top of Aspen Mountain Ski Area and made aerial observations of the mountain, while others traveled by 4wd vehicles to the top of the ski area, where they met up with the gondola riders. The 4wd vehicles used during the tour of the ski area were graciously provided by Aspen Skiing Company. Most of the group then conducted a foot traverse down the mountain, following the valleys and topographic lows in the western part of the ski area. Drivers of the 4wd vehicles descended the mountain via Summer Road and frequently met up with those on the foot traverse. The foot traverse first crossed the uppermost reaches of Spar Gulch, then continued down the mountain into the lower part of Pioneer Gulch, at which point all participants then rode in vehicles to additional locales visited in the lower part of the ski area. September 1 ended with brief reconnaissance of the areas where runoff from Aspen Mountain entered into the city's storm drain system. September 2 began with additional reconnaissance of the area where discharge from Lower Spar Gulch enters the city's storm drain system at the base of Aspen Mountain on the east side of the P6 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-2 November 2017 Vallejo Gulch landslide. Then the team viewed the southwest and southeast sides of Red Butte. A lunchtime meeting was held in the city's new engineering conference room to summarize observations and initial interpretations and to determine whether additional fieldwork was warranted for the afternoon. After the meeting two participants went to the Aspen Historical Society to view historical photographs of the project area and assess the availability of them for our use. Figure A is a Google Earth image of Aspen Mountain looking south. Major geographic features are noted in the figure to help understand the field observations. Upper Spar Gulch extends from near the top of Aspen Mountain to the confluence with Copper Gulch. Lower Spar Gulch is the section of the gulch below the confluence. A chronologic summary of the observations made during the fieldwork follows: Several unvegetated mine dumps were observed during the gondola ride up the mountain (see Figure B). As evidenced by rills eroded into steep slopes on the mine dumps, they are prone to erosion and are potential sources of sediment during future flooding events. Areas with poor vegetative cover in the Tourtelotte Park area also were observed during the gondola ride (Figure C). These poorly vegetated areas may be associated with areas where old mine dumps once existed in Tourtelotte Park (Figure D) and have been partially reclaimed. These areas also may be potential source areas for debris during future intense rainstorms. As observed during the gondola ride, and confirmed during the foot traverse, the terrain on the mountain generally is very steep. The valleys of Copper Gulch and much of Spar Gulch are V- shaped in cross section, with steep walls and narrow valley floors. In contrast, the valley floors in Pioneer Gulch and the uppermost part of Spar Gulch are much wider, and the transition from valley floor to steep valley wall is more gradational, although the hillslopes overall still are steep. These geomorphic differences may create somewhat different runoff conditions during storm events. The type of bedrock into which the valleys have eroded probably controls their geomorphic configurations. Copper Gulch and much of Spar Gulch are eroded into hard indurated rock (mostly lower Paleozoic age sandstone, quartzite, and limestone), whereas Pioneer Gulch and the uppermost part of Spar Gulch are eroded into the Belden Formation, which consists of thin beds of relatively easily eroded shale, limestone, and dolomite (Bryant, 1971, 1979). The vegetative cover in a small, heavily trafficked area at the top of the mountain near and immediately below the top of the gondola is relatively poor (Figure E). Bare soil was locally present in this area. Active revegetation efforts by Aspen Skiing Corporation are underway in this area and should improve the vegetative cover. In the upper and middle parts of the mountain that were viewed during the foot traverse the vegetative cover is generally well established on valley floors (Figures F and G). The valleys had broad floors and drainage channels that typically were subtle. Most valley floors appeared to contain veneers of unconsolidated soil that overlain bedrock. The thickness of the unconsolidated soil on most valley floors is unknown. This material could be available for erosion and potentially be incorporated into future mud floods or mudflows under the right P7 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-3 November 2017 conditions. Unconsolidated soil is not present at all locations on valley floor in the upper and middle parts of the traversed area, because small outcrops of bedrock, primarily the Leadville Limestone and an intrusive igneous rock called the aplite porphyry, were observed in a few small areas. Much of Aspen Mountain is underlain by underground mine workings. The rock and soil over the underground mine working may cave into the mine workings. Where the collapse extends to the ground surface closed depressions called subsidence features can develop. Open subsidence features such as the small one shown in Figure H, may intercept and divert surface water runoff into the subsurface, increasing the moisture content of soils in the vicinity of subsidence features. Soils with high moisture content are prone to mobilization during intense rainstorms and can be sources of sediment during flooding events. Saturated soils also are more prone to landsliding than are dry soils. Mantles of colluvial soils also appear to blanket many valley walls in the upper and middle parts of the traversed area. Although many of the valley walls in the upper and middle parts of the traversed area appeared to be generally fairly well vegetated, some were not. Several hillslopes in Tourtelotte Park are poorly vegetated and are potential sources of sediment during future flood events (Figure I). These hillslopes in Tourtelotte Park with poor vegetation may coincide with old mine dumps, such as those seen in Figure D. These soils also can be sources of sediment in future flood events. Rock outcrops were locally prominent on some valley walls, and aprons of talus were observed below some rock outcrops. Blocks of rock within the talus also potentially could be incorporated into future sediment-laden floods. The topography in many of the valleys in the upper and middle part of the mountain locally was hummocky, suggesting either the presence of old landslides, human disturbance, or perhaps periglacial solifluction. The foot traverse in the upper and middle parts of the mountain also crossed the Tourtelotte landslide, described by Hepworth-Pawlak (1998) as having open surface cracks during the Spring of 1997. No open surface cracks were observed during our reconnaissance transect across this landslide or in other areas with hummocky ground where older landslides potentially exist. The landslides were a focus of the reports by Chen & Associates and Hepworth-Pawlak in the 1980s and 1990s, in part due to their potential to be sources of sediment entrained in flood events during heavy rainstorms. Although open cracks and other features suggestive of active or very recent movement were not observed on the Tourtelotte Park landslide, some slopes on the landslide are poorly vegetated and can be sources of sediment during floods (Figure J). Minor channel incision near Bonnie's restaurant exposed gravelly sediment that could be mobilized during flood events (Figure K). Roads within the upper and middle parts of the mountain are potential sources of sediment during flood events. The road adjacent to Bonnie's restaurant (Figure L) is an example of a road that potentially could be eroded during a flooding event and contribute sediment to flood waters. Minor incision was noted in the borrow ditch adjacent to some of the roads, like that seen in Figure M. Summer Ditch intercepts surface water runoff from much of the upper part of Aspen Mountain. Where observed during the field traverse (Figure N), the ditch appeared potentially to be undersized for large storm events. A culvert that carries the flow in Summer Ditch (Figure O), P8 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-4 November 2017 as well as a half culvert employed where the ditch crossed the Zaugg Dump landslide (Figure P), also may be inadequate. The foot traverse in the lower part of the mountain focused on Pioneer Gulch. Two landslides that were evaluated by Hepworth-Pawlak (1998) and earlier geotechnical studies were traversed in Pioneer Gulch: Roch Run landslide and Strawpile landslide. No evidence of active or very recent movement was noted where these landslides were traversed, and the landslides were fairly well vegetated with only relatively small areas of weak vegetative cover. All participants in the field investigations were transported by vehicle during the latter part of the day. One of the first stops was at the confluence of the narrow valleys of Spar Gulch and Copper Gulch. While driving to the confluence unvegetated mine dumps were noted on the steep slope on the west side of Spar Gulch. The channel in Copper Gulch immediately upstream of the confluence was incised 1 to 2 feet deep into unconsolidated, gravelly, surficial soils (Figure Q). Our next stop was on the road below the Compromise Mine portal, which is located in Vallejo Gulch. Considerable unvegetated soil and small mine dumps exist in the vicinity of the mine's portal (Figure R). Note the absence of a well-defined channel. Discharge from Vallejo Gulch has had insufficient time to erode a channel into the landslide deposits. Sediment carried by floodwater issuing from the upper part of Vallejo Gulch could be deposited on the Vallejo Gulch landslide or on either side of it. If floodwater from Vallejo Gulch moved to the east side of the landslide it could discharge into the flow from Lower Spar Gulch. If it moved to the west side of the landslide it could reach the base of Aspen Mountain between the Pioneer Gulch fan and the Vallejo Gulch landslide. Exposed soil also exists in road cuts and road fills in the lower part of the mountain, and a small slump with unvegetated head scarp was observed in a road cut near the Commodore Mine (Figure S). A cluster of plastic sub-horizontal drains installed into the lower end of the Strawpile landslide were briefly examined (Figure T). One of the drains discharged a small flow of water. The other drains were dry, but appeared to be functional. The drains may have been installed by Schmueser and Associates (1984). The caravan of vehicles also stopped near the bottom of the Bell Mountain ski lift. This location is on the eastern margin of the Vallejo Gulch landslide. Here the discharge from Lower Spar Gulch is carried in what appeared to be a small, shallow, constructed channel that contained a series of small rock drop structures (Figure U). During a large flood event the discharge from lower Spar could overtop the constructed channel, allowing flood water to be diverted onto the Vallejo Gulch landslide and potentially reaching the base of the mountain between the west side of the landslide and the Pioneer Gulch fan. Such a flow probably also would include any discharge from Vallejo Gulch. During the remainder of the afternoon of September 1 the inlets into the City of Aspen's storm water system at the base of Aspen Mountain were examined. The only significant flow of water noted at the base of Aspen Mountain on September 1 was the discharge of from the Lower Durant Tunnel (Figure V). No surface water was observed in either P9 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-5 November 2017 Pioneer Gulch or Lower Spar Gulch at the base of Aspen Mountain. Apparently, some of the precipitation that falls on Aspen Mountain infiltrates into the underground working beneath the mountain, and a significant part of the water in the underground mines issues from the portal of the Lower Durant Tunnel. The mine water is believed to eventually flow into the Glory Hole Pond. On September 2, the field investigation began at the base of Aspen Mountain on the east side of the Vallejo Gulch landslide. This was in the vicinity of South Alps Road and South Ute Road. The purpose of the work was to locate the channel of Lower Spar Gulch at the base of the mountain. One of the surprising observations was the absence of a distinct channel for Lower Spar Gulch, as well as the relatively minor amount of sediment that has been deposited by Lower Spar Gulch at the base of the mountain. Although it had been raining for several hours, there was only a minor flow of surface water coming off of Aspen Mountain and into the City's storm sewer system in this area. Next the team moved to Red Butte to assess conditions on the southwest and southeast sides of the butte. Figure W is a photograph of the southern part of Red Butte taken the previous day while the team was on Aspen Mountain. Hillslope processes on the southwest side of Red Butte appear to be dominated by colluvial deposition, not sediment-laden floods, and as such the use of FLO-2D would not be appropriate to use here. Our observations are in agreement with the mapping of Bryant (1971, 1972), which shows the surficial soils on the southwest side of Red Butte as colluvium. Instead of using FLO-2D for the southwest side of Red Butte, unit rates of runoff and representative bulking factors will be determined. This information can be used by the development community for identifying hazards and planning appropriate mitigation. The hillslope on the southeast side of Red Butte is mantled with talus (rockfall debris) that has broken loose from the rock outcrops high on the hillslope and rolled, bounced, or slid down the hillslope (Figure X). Mitigation efforts include concrete retaining walls to prevent the talus from damaging structures and roads (Figures X and Y). A local homeowner reported to us that the mitigation work was undertaken by the local property owners. Since rockfall is the geologic hazard on the southeast side of Red Butte, not mud floods or mudflows, a FLO-2D analysis will not be performed for the southeast side of Red Butte. P10 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-6 November 2017 Figure A. Annotated Google Earth image of Aspen Mountain. View is to south. P11 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-7 November 2017 Figure B. Photograph of an unvegetated mine dump in Spar Gulch viewed during the gondola ride. Note the rills eroded into the steep slopes on the dump. Figure C. Some hillslopes in the Tourtelotte Park area are poorly vegetated and are potential sources of sediment during future flooding events. Photograph taken during gondola ride. P12 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-8 November 2017 Figure D. Historical photograph of the Tourtelotte Park area in the 1890s (courtesy of Aspen Historical Society) Note the dense concentration of mine dumps, mine buildings, and access roads. Figure E. Poorly vegetated area near top of Aspen Mountain. P13 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-9 November 2017 Figure F. View of a well-vegetated, broad, valley floor in the upper part of Spar Gulch. Note the absence of an incised channel. Figure G. Another example of a well-vegetated valley floor in upper Spar Gulch that lacked an incised channel. P14 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-10 November 2017 Figure H. Small, recent subsidence feature over underground mine workings in the upper part of Aspen Mountain. Figure I. Photograph of a poorly vegetated area in Tourtelotte Park that may coincide with an old, partly reclaimed mine dump. P15 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-11 November 2017 Figure J. Small areas on the Tourtelotte Park landslide are poorly vegetated. Figure K. Minor channel incision near Bonnie's restaurant exposed gravelly sediment that could be mobilized during flood events. P16 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-12 November 2017 Figure L. Photograph of a road near Bonnie's restaurant on Aspen Mountain. Roads can be sources of sediment during flooding events. Figure M. Note the shallow, eroded borrow ditch with unvegetated banks along a road above Bonnie's restaurant on Aspen Mountain. P17 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 November 2017 Figure N. Photograph of Summer Ditch. Figure O. Culvert that carries the flow in Summer Ditch. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) Wright Water Engineers, Inc. Figure N. Photograph of Summer Ditch. O. Culvert that carries the flow in Summer Ditch. Page A-13 P18 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 November 2017 Figure P. Half culvert that carries flow in Summer Ditch across the Zaugg Dump landslide. Figure Q. The channel in Copper Gulch immediately upstream of the confluence with Spar Gulch is incised 1 to City of Aspen Mud & Debris Flow Assessment (APPENDIX A) Wright Water Engineers, Inc. Figure P. Half culvert that carries flow in Summer Ditch across the Zaugg Dump landslide. Figure Q. The channel in Copper Gulch immediately upstream of the confluence with Spar Gulch is incised 1 to 2 feet deep into gravelly surficial soil. Page A-14 Figure P. Half culvert that carries flow in Summer Ditch across the Zaugg Dump landslide. Figure Q. The channel in Copper Gulch immediately upstream of the confluence with Spar P19 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 November 2017 Figure R. Overview of the Compromise Mine area showing the unvegetated soil and mine Figure S. Small slump with unvegetated headscarp along the road near the Comprise Mine. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) Wright Water Engineers, Inc. Figure R. Overview of the Compromise Mine area showing the unvegetated soil and mine dumps near the mine portal. Figure S. Small slump with unvegetated headscarp along the road near the Comprise Mine. Page A-15 Figure R. Overview of the Compromise Mine area showing the unvegetated soil and mine Figure S. Small slump with unvegetated headscarp along the road near the Comprise Mine. P20 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-16 November 2017 Figure T. Drains installed in the toe of Strawpile landslide. These subhorizontal drains probably were installed by Schmueser & Associates (1984). Figure U. Runoff from Lower Spar Gulch is carried by this shallow constructed channel located on the west side of the Vallejo Gulch landslide near the bottom of the Bell Mountain ski lift. P21 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-17 November 2017 Figure V. Water discharges from underground mine workings at the portal of the Lower Durant Tunnel, located near the base of Aspen Mountain east of the Vallejo Gulch landslide. Figure W. Overview of the south end of Red Butte. Photograph taken from Aspen Mountain. P22 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-18 November 2017 Figure X. Note the tall, red-colored concrete wall uphill of and behind the structure, which is located on the southeast side of Red Butte. The wall is designed to prevent talus (rockfall debris) from impacting the structure. The short concrete wall in the foreground also is a rockfall barrier. Figure Y. A series of concrete retaining walls are used on the southeast side of Red Butte to prevent rockfall debris from falling onto Pitkin Mesa Drive and/or impacting homes located on the east side of the road. P23 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-19 November 2017 O’Brien Field Observations Overview This memo reports observations from the Aspen Mountain field trip to investigate potential flood and mudflow hazards for the City of Aspen. The memo covers four topics: · Hydrology · Fluvial Geomorphology and Historical Flooding/Mudflows · Sediment Supply · Flood Modeling Approach · Mitigation Concepts Some photos are presented to support observations. A more extensive set of photos are available. Hydrology One of the more interesting observations was the lack of drainage channels in the upper portions of the Aspen Mountain contributing to the Spar, Pioneer and Vallejo watersheds. Considering the mining history and open areas associated with the ski area, this is an indication that the rainfall is not very intense on the upper Mountain. It would be valuable to review any rain gage record that might represent the high altitude rainfall on the mountain. If the rain gage data is available in hourly intervals or less, then some conclusions might be drawn about the orthographic effects on rainfall intensity. To establish the various project frequency-duration rainfall events, in addition to the application of the NOAA-14 Atlas, a review of other regional Rocky Mountain rainfall studies is suggested. Tetra Tech in Breckenridge can be contacted to assess their use of rainfall frequency in high altitude environments. It is suspected that the rainfall intensity is higher in Glenwood Springs, but a review of their local rain gages and studies may lend some insight on potential upper range of rainfall intensity. It is suggested that the raingage data used to justify the rainfall frequency and duration should be distributed for review. Two precipation parameters that should be compared with other local or regional studies are the percent rainfall runoff versus percent loss for the 100-year storm of a given duration and the 100- year rainfall unit peak discharge (cfs per acre or cfs per sq mi) for high elevations in the Central Rockies. After the rainfall runoff hydrology has been analyzed, it is recommended that a letter of confirmation be submitted to the City of Aspen describing the analysis and results and indicating the storm duration and frequency to be used in the study. The letter should have a placeholder for the City to sign in concurrence. P24 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-20 November 2017 Fluvial Geomorphology and Historical Flooding/Mudflows The Aspen Historical Society and the Aspen Library were visited to ascertain the condition of the watershed morphology pre-mining (prior to 1880). Most of the photos reviewed were taken after mining was already established. Aspen silver mining began in 1879 and production peaked in 1892. Reviewing the pre-1880 photos lead to the following conclusions: · The alluvial fans at the base of Aspen Mountain were limited in spatial extent. · Lower Aspen Mountain was vegetated with primarily scrub and aspen trees (few coniferous trees). · The Spar Gulch alluvial fan and drainage was impacted by the Little Nell landslide (Figures 1 through 4). · There were no pictures or accounts of historical flooding or mudflows in any of the references reviewed including a number of library history books. There were only two Aspen Times newspaper accounts over the past 130 years as present in the Wright Water Engineers May 13, 2016 memo. · The alluvial fans along the base of Aspen Mountain have been obscured or obliterated by encroaching development (Figure 4). Figure 1. 1982 Photo Showing Little Nell Landslide with Spar Gulch (near the ‘Y’ in “Property”) P25 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-21 November 2017 Figure 2. Spar Gulch Alluvial Fan (shown in red oval circa 1890). Reference: Frank L. Wentworth Aspen on the Roaring Fork (1976). The Little Nell Landslide is the dominant feature in the center of the photo. Mountain communities with frequent flooding and mudflow problems generally have periodic photographic and noteworthy newspaper reports. These are important new stories. It is conspicuous that the City of Aspen has only two accounts. As indicated in the Wright Water Engineers May 23, 2016 Memo, the Chen & Associates 1984 and 1985 reports discussed only two historical sediment loaded flood events (Sept 1919 and August 5, 1964). These were reported in the Aspen Times. The 1919 “cloudburst” had large volumes of mud and water but apparently no significant property damage. This event occurred during a period following extensive vegetation removal and exposed mine dump materials that would have excerbated sediment loading and runoff. The rainfall during the 1964 storm was only 1.13 inches in a 1 hour interval as reported by Chen & Associates resulting in mudflow deposits of up to 5 ft deep. P26 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-22 November 2017 Figure 3. Spar Gulch Alluvial Fan (circa 1885 – red oval). Flooding and mudflows from the Aspen basins may be infrequent and did not evolve a extensive alluvial fan as a result of the following conditions: · Small watersheds; · Orthographic effects on intense rainfall in the Aspen area; · Pre-mining vegetation cover. With the advent of silver mining, the vegetative cover was significantly altered, large areas of potenial sediment supply became available (mining dumps, roads and excavation in Tourtelotte Park community), and in response runoff increased. This may have led to the 1919 flooding or mudflow event. During the extensive mining in the Aspen mountain basins, small, frequent rainfalls, should have expanded the alluvial fan depositions, but this does not appear to be the case. Apparently there was not another significant flood event until 1964, an interval of 45 years, during which time the watershed healed from the effects of mining. There has not been another significant flood or mudflow event in the last 52 years. The inference is that any appreciable flooding/mudflows associated with damage or clean-up may be more infrequent than anticipate and may be on the order of 50 year return period. Chen & Associates (1985b) in the WWE memo reported that based on five test pits on the Pioneer Gulch fan that at least three debris flows (and most likely numerous small flood events) contributed to the geomorphic evolution of the alluvial fan over the last 5,000 years. The lack of frequent flooding is the primary factor in the small alluvial fan deposits. P27 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-23 November 2017 Figure 4. Spar Gulch Channel. The Spar Gulch Alluvial Fan (in Red) and the Spar Gulch Channel (blue line). Sediment Supply The potential for a mudflow event in response to intense rainfall on Aspen Mountain is primarily controlled by the sediment sources on the lower subbasins. The upper Mountain basins have few exposed areas of sediment supply (mostly limited to roads – Figure 5 and 6). No significant conveyance channels are present (Figure 7) in the upper basins, only small rills and gullies. During the late 1800’s and early 1900’s, mining activity in the basin significantly increased the available sediment supply (Figure 8). Figures 9 and 10 indicate that the basin has significantly recovered since the silver mining period. The lower portion of the mountain has some sediment sources in the water courses (including large cobble and boulder size material), and potential sediment supply from exposed hillslopes, road cuts and a few remnant mine dumps (Figures 11- 14). P28 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-24 November 2017 Figure 5. Typical Upper Aspen Mountain Watershed – Well Vegetated Ski Slope Figure 6. Road Erosion - Potential Sediment Source in Upper Basins P29 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-25 November 2017 Figure 7. Concentrated Flow in Small Rills and Gullies in the Upper Basins Figure 8. Historical Photo show the Extent of Mining Activity and Denunded Landscape on Aspen Mountain Around the Turn of the Century P30 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-26 November 2017 Figure 9. Photo After the Ski Area Development with the Mining Buildings Removed and the Basin Recovering Figure 10. 1916 Photo of the Tourtellote Park Area – More Complete Recovery of the Basin P31 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-27 November 2017 Figure 11. Sediment Charged Spar Gulch Channel – Potential Flooding Down Road Figure 12. Ground View of Spar Gulch Channel with Sediment Loading P32 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-28 November 2017 Figure 13. Lower Spar Gulch Channel Bed as a Source of Sediment Figure 14. Exposed Hillslopes and Roadcut in Lower Aspen Mountain Basin P33 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-29 November 2017 Sediment loading during flooding can be derived from five potential sources (not including volcanic origin): · Landslide · Hillslope sloughing or failure · Overland sediment yield · Channel bank failure and erosion · Channel bed scour Depending on geology, slope and, sediment size, and source of water, some sediment sources will be dominant over the others. For Spar Gulch, the predominant supply is channel bed material that has accumulated over a number of frequent rainfall events (Figures 11 and 12). During a flood event, channel scour to bedrock can result in increased channel bank failure and possible hillslope sloughing. There is sufficient large sediment accumulated in the lower basin channel of Spar Gulch to generate a mud and debris flow event depending on the water volume and peak discharge. Fine sediment, silts and clays, are available in sufficient quantities to support the generation of a mudflow event with high sediment concentrations. During the field trip, a light rainfall event produced a trickle of runoff that was turbid with fine sediment. Flood Modeling Approach From a hydrology persective, assuming there is sufficient sediment supply, the return period for a mudflow should be based on the frequency assigned to the various design storm. For example the 25-year mudflow should be considered has a having a probability of 25-year rainfall event for the selected design duration. Although there is a combined probability associated with the sediment supply being available, the worse case scenario is that the sediment sources would generate a mudflow for a given rainfall event. The estimated sediment supply volume from the five sources previously listed, should then be compared with the water volumes associated with the various return period rainfalls. This can be done in conjunction with running the FLO-2D model for a very short simulation time (e.g. 0.01 hrs) comparing the water and sediment volumes listed in the SUMMARY.OUT. In order to produce mudflows, the average sediment concentration by volume should be in the range from 25 to 35% with peak concentrations from 45 to 55% by volume. Using a conservative estimate of the sediment supply, this average concentration by volume will indicate what return period storm would associated with a mudflow event. The following simulations should be performed: · Water flooding · Water flooding bulked by a uniform 20 percent concentration by volume (XCONC factor in CONT.DAT). This increases the volume without modifying the flow behavior. · Mudflows with temporally variable concentrations by volume associated with the estimated sediment supply. The ratio of sediment to water governs the ability of the mixture to flow and determines whether the flow will pile up at the fan apex or will flow over significant distance over the alluvial fan. Sediment concentration varies temporally and spatially throughout the flood event with surging P34 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-30 November 2017 and flow cessation. The distribution of sediment in the flood hydrograph will control frontal wave celerity and magnitude, flow bulking, surging, recessional limb flow dilution, and deposit reworking. When assigning sediment concentrations to a flood hydrograph the following guidelines are suggested: · The average sediment concentration should reflect the estimated water and sediment volumes for the design storm. · The sediment volume should not exceed the estimated maximum potential sediment yield observed for the basin. · The steep rising limb of the hydrograph should be bulked with the highest sediment concentrations to simulate the frontal wave. · The peak discharge should be assigned a sediment concentration slightly less than the frontal wave to account for water dilution. · The rising and following limbs of the hydrographs should not have less than 20% sediment concentration by volume. · To generate a mudflow the average sediment concentration for the entire hydrograph should be in the range of 25% to 35% by volume with the frontal wave peak concentration on the order of 45% to 55% concentration by volume. For the mudflow simulations, the various Aspen Source sample viscosity and yield stress as a function of sediment concentration by volume shown in Table 1 can be tested. By selecting a given concentration by volume (e.g. 30% and 45%) and computing the viscosity and yield stress for each source sample, an appropriate sample can be applied to the simulations to predict the area of inundation. One sample may result in very fluid flows while another sample may cause the mudflow to stop abruptly. P35 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-31 November 2017 The watershed FLO-2D model could consist of 50 ft grid elements to predict the inflow hydrographs to the Aspen urban area. The urban area model could be more detailed with a 20 ft grid system. Since the area being modeled is not that large, one model of 20 ft to 25 ft could be considered. The infiltration and roughness parameters in the watershed should be spatially variable differentiating the ski area runs from the forest patches. To achieve unit peak runoff values and appropriate time of concentrations for the flood wave, it may be necessary to model some rills and gullies in the watershed areas. One-dimensional channels might be considered for the lower basins, however the channels appear to be obscured when reaching the urban interface. For the urban areas, streets, buildings and walls should be modeled. For a worse case flood hazard condition, the storm drains could be considered to be plugged by the frontal wave. Mitigation Concepts Based on the field investigation a few preliminary mitigation concepts can be suggested for later discussion. These suggestions not meant to be all inclusive but rather represent some mitigation ideas that can be folded into an overall mitigation scheme. Central to a flood hazard mitigation plan for Aspen Mountain is the recognition that the watershed/urban interface has been severely altered by development at the base of the steep slope of the mountain front. This restricts any potenial storage opportunities on lower mild slopes. Conveyance to avoid flood hazards remains an option but is limited because of property values, easements, and lack of any significant P36 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX A) 161-012.000 Wright Water Engineers, Inc. Page A-32 November 2017 existing channels through the urban area. The best approach would probably consist of reducing runoff, eliminating or reducing sediment supply, raking off large boulders and debris in small detention areas on the ski area, storm drain system expansion and some flood conveyance. In the upper watershed, the mitigation focus should be to spatially distribute concentrated flood waters to overland flow and capture and divert overland flow into forested areas. This upland mitigation would include creating small diversion rills and gullies on the ski runs in a downslope to direct captured flow to the patches of forest between the runs. It is recommended to seed any exposed slope areas with native vegetation and improve road drainage so that roads are not scoured by concentrated flow. In the mid and lower portion of the Aspen Mountain watersheds, all exposed slopes, road cuts and mine dumps should be stabilized and treated to reduce or eliminate sediments from entering water courses, no matter how small. The concentration of sediment laded overland flow in rills and gullies can accumulate and contribute to more extreme flooding downstream. There may be opportunities to have small detention storage on the ski area to rake off boulders and debris. Detention storage on Spar Gulch near the lift might include storage berms, deflection berms to protect the lift, and sloped rail rakes on outlet faciilties to retain large boulders and logs. The channels should be maintained free of large quantities of sediment and boulders. Soil concrete might be considered to stabilize channel beds. Flood conveyance should be expanded from the ski area to the urban interface after which a combination of improved storm drain capacity and increased conveyance would minimize the urban flooding. The streets may be restructured to safely convey some flood waters through the urban area with consideration for emergency access and evacuation routes. These mitigation concepts can be discussed and conceptually designed in more detail once the flood hydrographs and mudflow volumes have been determined. P37 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-1 November 2017 Appendix B. Chronologic Summary of Geologic, Geologic Hazard, Engineering Geology, and Geotechnical Data Sources The following sections summarize the geology, geologic hazards, and geotechnical data contained in numerous publications and unpublished reports that cover the project area. The Project Team located and examined many of these documents. Some documents were not located, but if they were cited and summarized in other documents, they are included in this section. All publications and unpublished reports that are described in this section are included in the References section of this report. The list of references also includes list of reports we are aware of from references that may be relevant to the project but were neither located nor summarized in other reports. Brunton, 1888 The earliest known report on the slope instability of Aspen Mountain were articles by Brunton (1888) in the Engineering and Mining Journal, who stated "All over Aspen Mountain land-slides small and great are of frequent occurrence." Although his study focused on ore deposits, he described evidence of mine claim posts moving downhill, and also the deformation of the upper sections of numerous vertical mine shafts by shallow downslope movements. For example, careful surveys of claim posts situated over the Veteran Tunnel, which is located on the west side of the Little Nell earthflow, indicated 3.4 feet of downslope movement in two years. It is possible that these slope movements were triggered by or enhanced by then recent mining activities, including the building of roads and the loading of slopes with the mine waste materials removed from the many underground mines. Brunton (1888) was the first to note the large landslide from Vallejo Gulch that about 100 years later Chen & Associates (1985b) named the Little Nell earthflow and McCalpin (1997) called the Vallejo Gulch landslide. Brunton described the feature as "the result of an enormous land slip, the debris from which covers an immense area at the foot of the mountain.” Some of the slope instability features described by Brunton were mistakenly interpreted by him as active fault movements. He also did not mention the fan sediments at the base of the mountain. Spurr, 1898 Spurr (1898) produced a large monograph with many topographic maps, geologic maps, and cross sections. He focused on the bedrock geology and geologic structure as it related to the old mines and mineralization. He also gave short descriptions of the glacial geology. Spurr (1898) had access to many of the underground mines, which allowed him to examine and describe both the bedrock formations and structural folds and faults on Aspen Mountain in detail. Spurr did not discuss the landslides on the mountain nor the sediment eroded from it and deposited at the base of the mountain. However, his work on the bedrock and structure laid the foundation for the modern understanding the landslide, slope instability, and sediment-laden flows that occur on the mountain and that can affect areas at the base of the mountain. One difference between his work, which was done over a century ago, and more recent geologic studies involve the name of an important bedrock formation. What Spurr called the Weber P38 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-2 November 2017 Formation later became known as the Belden Formation. The Belden plays major role in the slope instability on the mountain. Bryant, 1971 Bryant (1971) published the first geologic map to show landslides on Aspen Mountain and also the fan deposits at the base of the mountain, which he called alluvial-fan deposits. His map, published by the USGS on modern topographic base, depicts the bedrock formations and complex structural geology of Aspen Mountain at a scale of 1:24,000. His mapping of Aspen Mountain relied heavily upon the work of Spurr (1898). The part of Bryant's map that covers Aspen Mountain is shown in Figure B1. He identified two landslides on the mountain slopes above town that drain into Aspen: one in the lower part of Pioneer Gulch that was later evaluated by consultants and named the Strawpile landslide, and a second landslide in the upper reaches of Spar Gulch, which was later called the Zaugg Dump landslide. His fan deposits included sediment at the mouths of Pioneer and Spar Gulches and the elongate, steep-sided deposit of debris immediately uphill from the Gondola Plaza that later was called the Little Nell earthflow or Vallejo Gulch landslide. Bryant's geologic map, as well as his USGS Professional Paper 1073 on the Aspen 15-minute quadrangle that is summarized in a later section (Bryant 1979), serve as the basis for all subsequent geological studies of the project area. The following paragraphs summarize the geologic setting of the project area; they also are based chiefly upon Bryant's 1971 geologic map and his 1979 professional paper. Structurally, the rocks on the mountain slopes above the town are folded into a north-south- trending syncline that is broken by many faults. The syncline formed on the east side of the Castle Creek fault zone, probably during the Laramide Orogeny in late Cretaceous to early Tertiary time. A syncline is a trough-like fold that is concave upward, with the core or axis of the syncline containing the youngest rocks. In the west limb of the syncline, located along the west side of the Aspen Mountain Ski Area, the rocks dip east. In the east limb of the syncline, which is located along the eastern side of the ski area, the rocks dip west. The northern part of the syncline axis is in Pioneer Gulch. The oldest sedimentary rocks folded by the syncline include several Lower Paleozoic sedimentary formations; they rest on Precambrian quartz monzonite. The lower Paleozoic sedimentary rocks are overlain by the Mississippian Leadville Limestone, which hosts much of the silver-lead-zinc mineralization on Aspen Mountain. The Pennsylvanian Belden Formation overlies the Leadville and is the youngest sedimentary bedrock formation on the mountain. An igneous sill composed of aplite porphyry intruded into the lower part of the Belden Formation during the late Cretaceous to early Tertiary Laramide Orogeny prior to the synclinal folding of the sedimentary rocks and prior to the introduction of the hydrothermal fluids responsible for the silver-lead zinc mineralization. The aplite porphyry has a fine-grained crystalline groundmass or matrix. There are larger crystals, called phenocrysts, which are visible to the unaided eye and scattered throughout the rock. The phenocrysts consist chiefly of a feldspar mineral called plagioclase that has altered to a P39 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-3 November 2017 secondary mineral called sericite. Quartz and seriticized plagioclase and potassium feldspar constitute the bulk of the fine-grained matrix. The aplite porphyry is intensely fractured, which allows it to weather into blocks of rock readily available for transport during flooding events. Sericite consists of a fine-grained mica that weathers to tiny flakes of mica. The mica flakes can be readily entrained in sediment-laden floodwaters. Although the potential importance of the sericite micas has not been considered in prior investigations, they may play a role in the sediment-laden floods by increasing the density of the flood waters and increasing the ability of the flood waters to transport or float large-diameter, rocky debris during flooding events. The axis of the synclinal fold is in the central part of the ski area; its northern end coincides with Pioneer Gulch. The Belden Formation crops out in the axis of the syncline. It contains many thin beds of shale that weather to clay. These shale beds and clayey soils derived from it probably host the slip planes for some or many of the landslides and soil creep areas on Aspen Mountain. The clay particles are readily entrained in sediment-laden floods, which gives the flood waters their muddy character, as well as increasing the density of the flood waters and increasing its ability to transport large-diameter debris during floods. There are brecciated zones in the Belden that are subparallel to bedding. These may be related to faults that follow bedding in the formation or to dissolution and collapse of underlying beds. The Belden also contains thin limestone beds, some of which also hosted silver-lead-zinc ore deposits. The axis of the syncline plunges or dips to the north, which causes the bedrock contacts in the axis of the syncline to also dip north at approximate the same angle as the slope of the ground surface. This relationship further exacerbates slope instability conditions on Aspen Mountain. For example, the Strawpile landslide initiated in mine dump material where the Belden Shale and clayey surficial material weathered from it occur along the axis of the syncline. Bryant (1971) was also the first to map the geology of the Red Butte area in detail (see Figure B2). The rocks that crop out on Red Butte are younger than those on Aspen Mountain. They include the Triassic and Permian State Bridge Formation and Triassic Chinle Formation, which consist chiefly red siltstone and are responsible for the name of the butte. Other bedrock formations on Red Butte include the Jurassic Entrada Sandstone, Jurassic Morrison Formation, and Cretaceous Dakota Sandstone and Burro Canyon Formation. These rocks on Red Butte lie between strands of the Castle Creek Fault Zone and are very deformed structurally. They are broken by many small faults and fractures and are so strongly tilted that they generally are overturned. Most of the bedrock formations on Red Butte dip 40 to 55o east, but due to the overturning they essentially are upside down. Bryant (1971) mapped the surficial deposits at the southwest base of Red Butte as colluvium with a small area of talus. He mapped the surficial deposits at the southeast base of Red Butte as talus. Bryant, 1972a, 1972b, and 1972c In 1972, Bryant published several 1:24,000-scale derivative maps based on his 1971 geologic map. One of these (Bryant 1972a) was the first geologic hazard map of the area (see Figures A3 and A4 for the parts of his map that cover Aspen Mountain and Red Butte, respectively). He identified the alluvial fan areas at the base of the mountain and also one in the upper reaches of the mountain as potential geologic hazard areas, and he mapped two landslides on Aspen P40 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-4 November 2017 Mountain, one of which was later named Strawpile landslide. Bryant briefly mentioned the 1964 sediment-laden flood in his description of alluvial fans, but recalled that it occurred in 1965. Pioneer Gulch and large parts of Spar, Copper, and Vallejo Gulches are depicted by Bryant (1972a) as potentially unstable slopes. The northern end of the Aspen Mountain ridgeline, known to some as West Aspen Mountain or Shadow Mountain, is mapped as a rockfall area. The southeastern slopes of Red Butte are classified as a rockfall area, and the northeastern side of the butte is considered potentially unstable. Other useful derivative maps by Bryant include his map showing groundwater potential (Bryant, 1972b) and map of types of bedrock and surficial deposits (Bryant, 1972c). F.M. Fox and Associates, 1974 F.M. Fox and Associates (1974) conducted an environmental and engineering geology for the Colorado Geological Survey of an area that included Aspen. Their geology and geologic hazard mapping relied upon that of Bryant (1971, 1972a), but was at the less detailed scale of 1:48,000. The polygons on the geologic map by Fox and Associates are nearly identical to those on Bryant (1971), although Fox and Associates lumped many of Bryant's map units because their base map was at a smaller scale. For example, all of Bryant's many surficial deposit units were placed in only three units by Fox and Associates: colluvial deposits, younger alluvial deposits, and older alluvial deposits. The two landslides mapped by Bryant, as well as his talus and colluvium units, were included in a single unit called colluvium by Fox and Associates. The geologic hazard map by Fox and Associates (1974) is called an environmental and geologic constraints map. The portion of their map that included Aspen Mountain and Red Butte is shown in Figure B4. Their hazard map is similar to Bryant (1972a) in some ways, and different in others. Fox and Associates include the landslide in the upper part of Spar Gulch in their colluvium/landslide hazard unit, but they lump the landslide in lower Pioneer Gulch in their colluvium/talus unit. The fan deposits at the mouths of Pioneer and Spar Gulches and the steep- sided elongate deposit of debris above the Gondola Plaza are in their younger alluvium/alluvial fan unit, which is prone to flood hazards. They also used a stipple pattern for the fan deposits; this pattern indicates a geologic flood plain that is subject to flash flooding. Fox and Associates do not have a map unit called potentially unstable slopes, but instead map much of the mountainous slopes above town as areas of potential avalanches and/or rockfall. House Bill 1041 Geologic Hazard Map of Aspen Quadrangle In 1974, the State of Colorado legislature adopted House Bill 1041 (C.R.S. § 24-65.1-103), titled "Areas and Activities of State Interest." In response to this legislation geologic hazard mapping was performed in all or selected parts of many the counties in Colorado, including Pitkin County. These maps commonly are referred to as House Bill 1041 geologic hazard maps. Most were completed in mid to late 1970s. A copy of the 1040 geologic hazard map for the Aspen 7.5-minute quadrangle was obtained from the Colorado Geological Survey Archives (2016). Unfortunately, no geologic hazard mapping was done within the City of Aspen, including the Red Butte area. Also, the lower half of Aspen P41 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-5 November 2017 Mountain was not mapped. Geologic hazards on Aspen Mountain were mapped only in the area south of the uppermost reaches of Pioneer and Vallejo Gulches. Laing and Lampiris, 1980 Laing and Lampiris (1980) published a geologic guide book to the roads and trails of the Aspen high country. It includes a description of the geology visible from the Ute trail, which climbs up the ridgeline on the west side of Spar Gulch. They pointed out that the Belden Formation underlies Vallejo Gulch and stated that on occasions in the past the formation was "sufficiently lubricated by groundwater to slide as mudflows down the gulch towards Aspen. Notice the fan- shaped deposit at the lower end of the gulch. This unstable history has not deterred the Aspen Ski Corporation from building a ski lift up the full length of the fan. More vulnerable than the lift are the various buildings on the downhill edge of the fan." Schmueser & Associates, 1984 Although the unpublished report by Schmueser & Associates (1984) was not located by WWE, it is cited and briefly described in Chen & Associates (1985a, b). Our information in the following paragraphs on the Schmueser & Associates (1984) report is summarized from the descriptions of it that are in Chen & Associates (1985a, b). Cracks in the ground surface in the Strawpile ski run were first reported on June 6, 1984. This prompted city officials to temporarily evacuate part of the city due to the potential threat of an imminent "mudslide." Survey monuments were installed in the area with ground cracks and monitored for about three weeks. The landslide movement was reported to have subsided by this time, with the rate of movement being about 3 to 4 inches per day with a total movement of several feet during the monitoring period. Six horizontal drains were installed near the suspected toe of the landslide near Tower Ten Road at the end of June 1984. The drains did not produce flowing water when installed, and the slide movement was reported to have stopped. Later in 1984, a half culvert was installed along the upper edge of Dago Road to prevent runoff from infiltrating into the landslide. It is not clear in the 1985 reports by Chen & Associates report whether this work was done by Schmueser & Associates, by Chen & Associates, or by others. Chen & Associates, 1984 Chen & Associates (1984) evaluated the potential for debris flows at the then proposed Top of Mill complex. The report was dated November 14, 1984. The Top of Mill complex was located at the junction of two geomorphic fans: a western fan at the mouth of Pioneer Gulch and an eastern fan located at the mouth of "Copper/Spar Gulch and Vallejo Gulch." Their investigation included a review of newspaper files, city records, and meteorological records, an interview with Bruce Bryant, and the excavation, logging, and sampling and geotechnical testing of materials from four test pits. The Chen & Associates map showing the "Engineering Geology of Site and Areas Upslope" is reproduced in Figure B5. Chen & Associates (1984) discovered newspaper accounts that described two historical sediment-laden flood events that flowed into the City of Aspen. One article reportedly was in an P42 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-6 November 2017 Aspen Times newspaper dated September 5, 1919. The second event was on August 5, 1964 and was described as being in the August 7, 1964 issue of the Aspen Times. Chen & Associates (1984) concluded that the historical flooding events that flowed into the city were mud floods. They stated that mud floods with sediment concentrations of about 40% by volume posed the greatest hazard to the Top of Mill site and that the design volumes of water floods "should be increased to 50% to account for transport of solids in the flows." Two of the four test pits by Chen & Associates (1984) were located on the Pioneer Gulch fan. Test pit TP-3 "encountered man-made fill to a depth of approximately 8 feet" and was of limited use to characterize past flood events. Their test pit TP-4 was 8.5-feet-deep and exposed several layers of sediment that they interpreted to result from multiple debris flows. Laboratory test results indicated the sediment in TP-4 was chiefly clayey gravel (Unified Soil Classification soil type GC), with minor silty or slightly silty gravel (GM soil type) and clayey sand (SC soil type). These soils had liquid limits of 27 to 32% and a plasticity index ranging from 8 to 10% (see Table A-1 in Chen & Associates 1984). Carbon-14 dating of a piece of carbonized wood from a depth of 4 feet yielded an age of 5270 ± 110 years. The sample came from near the base of the 3rd debris-flow layer exposed in the trench, which led Chen & Associates to conclude that at least three debris flows had deposited sediment at the location of the test pit in approximately the past 5,000 years. They noted that erosion may have removed sediment and evidence of younger flows at the location of the test pit and that other debris flows may have deposited sediment in other parts of the fan and left no record at test pit TP-4. They concluded that Pioneer Gulch likely produced more than three debris flows during the past 5,000 years, perhaps many more than three events. Test pits TP-1 and TP-2 were excavated into the elongate, steep-sided deposit at the mouth of Spar Gulch, which in 1984 they called the Little Nell fan. TP-1 was 10.5 feet deep, and TP-2 was 11 feet deep. The soils exposed in these test pits also classified mostly as clayey gravels (GC soil type), but they contained more cobbles and boulders than the sediment observed in the test pit on Pioneer Gulch fan. The coarser material seemed to be tightly bound in a matrix of clay or silt. The sediment exposed in test pits TP-1 and TP-2 contained no evidence of stratification or old weathering profiles, indicating at least the 11 feet of sediment exposed in the test pits was a result of a single event like an earthflow or landslide, not multiple episodic events as on the Pioneer Gulch fan. Chen & Associates (1984) also described several other slope failures that occurred on Aspen Mountain during 1984. A small debris flow formed west of Pioneer Gulch during the spring; the failed slope was regraded during the summer and a blanket drain was installed. A small slump was observed in an aspen grove next to the Corkscrew ski run on the east side of Pioneer Gulch. Another slope failure occurred during the spring "adjacent to the upper reaches of Pioneer Gulch" at the location of a spring. The 1984 Chen & Associates report noted that studies were "currently underway to evaluate the large Aspen Mountain landslide" at the western edge of Pioneer Gulch that was observed to have moved during June 1984. This probably is the landslide later called the Strawpile landslide. P43 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-7 November 2017 Chen & Associates, 1985a Chen & Associates (1985a) is a preliminary geotechnical report dated January 4, 1985 that focused on the landslide on Strawpile Run in the Aspen Mountain Ski Area. We obtained a partial copy of this report from the Colorado State Archives that had been placed there by the Colorado Geological Survey. Chen & Associates (1985a) conducted geologic mapping of the landslide, installed three inclinometers and several piezometers in it, and analyzed numerous soil samples in the laboratory. They were in the process of installing survey monuments to monitor slope movement, but the surveying was not complete at the time of their preliminary report. The report stated that in June 1984 landslide fissures were observed above Dago Road upslope of Strawpile Run on Aspen Mountain. Schmueser & Associates (1984) had identified and monitored the landslide in conjunction with the City of Aspen. "The landslide was viewed as a potential hazard to a fairly large area of the City of Aspen including Top of Mill and Aspen Mountain Lodge." The movement was monitored using electronic distance measuring equipment. This monitoring indicated a slowing in the rate of movement at the end of monitoring during July 1985. "Schmueser & Associates in conjunction with the Colorado Geological Survey and United States Geological Survey concluded that the risk to the City of Aspen at that time was no longer significant. Concerns were expressed as to the long-term threat to Aspen in wet seasons and the upcoming spring." The investigation by Chen & Associates (1985a) was undertaken to address those concerns. The Strawpile landslide was described by Chen & Associates (1985a) as involving chiefly mine dump material and having a relatively shallow failure surface. They qualified that interpretation by saying that their monitoring program had not yet indicated any significant landslide movement during the short period of monitoring and that the lack of data showing a clearly defined failure surface limited their ability to analyze the landslide and propose corrective measures. Chen & Associates (1985a) concluded that the "potential for a debris flow from the landslide reaching the Aspen Mountain Lodge and the Top of Mill sites as anything but a fluid 'mud flood' appears relatively low." The Chen & Associates (1985a) report was preliminary, and they released a comprehensive report on this topic later in 1985. Please refer to our summary of this later report (Chen & Associates 1985b) for more detailed information about the investigations by Chen & Associates into the landslide and debris hazards posed to the Top of the Mill site. An earlier report on the engineering geology and geologic hazards at the Top of Mill site (Chen & Associates, Job No. 4 163 84) is mentioned but not described in detail in Chen & Associates (1985a). This reconnaissance report apparently identified potential geologic hazards related to construction-induced slope instability, stormwater runoff, and abandoned mines. WWE was unable to locate a copy of this report. WWE assumes that the subsequent reports by Chen & Associates (1985a and 1985b) that we describe herein cover these topics. Chen & Associates, 1985b The Chen & Associates (1985b) report dated September 20, 1985 consists of two volumes that focused on their investigations of the Straw Pile landslide and the debris-flow potential it posed P44 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-8 November 2017 to the Aspen Mountain PUD project. This project area included the Top of Mill site, Aspen Lodge site, 700 South Galena Street site, and Summit Place site. This report included results described their earlier reports and also new data and conclusions. Volume I of Chen & Associates (1985b) contains an Executive Summary and the report text; Volume II contains five appendices with numerous figures and tables. The copy of the report that WWE borrowed from Roy H. Spitzer, one of the preparers of the original Chen report, was lacking Figure B-1, which is a plate showing the surficial geology of Aspen Mountain. Efforts to locate this plate elsewhere were unsuccessful. Chen & Associates (1985b) pointed out that the landslide and debris flows could affect not only the Top of the Mill site, but also other development at the base of Aspen Mountain. They recommended remedial actions for the landslide and debris flows, and suggested that the debris- flow mitigation efforts be incorporated into an overall stormwater and debris-flow management plan for the City of Aspen. Three additional test pits (TP-5 to TP-7) were excavated on the Pioneer Gulch fan during the late spring of 1985 near where test pits TP-3 and TP-4 were excavated in 1984. Carbonized wood was present in all three of the new test pits; the material was interpreted to be in the same stratigraphic horizon as the carbonized wood in test pit TP-4 that was dated in 1984. A carbonized wood sample from TP-5 yielded a carbon-14 age of 5310 ± 90 years before present, which was similar to the age of a sample from TP-4 obtained in 1984. This supported the conclusion by Chen & Associates (1984) that at least three debris flows deposited sediment on the Pioneer Gulch fan during the past ~5,100 years, and that observed fan stratigraphy probably does not record all of the flow events. Debris flows exposed in the five test pits excavated into the Pioneer Gulch fan in 1984 and 1985 deposited sediment layers that ranged in thickness from 0.5 to 1.5 feet. The debris-flow sediment exposed in the three new trenches excavated in 1985 was similar to the sediment exposed in the 1984 test pits. Based on laboratory testing, a sample of debris-flow sediment from 3 to 4 feet deep in test pit TP-5 was classified as clayey sand (SM to SC soil type) and had a liquid limit of 31% and plasticity index of 12%. Bulk soil samples for laboratory testing were obtained from five locations. The locations of some of bulk samples are described differently in the text (page A-3) than are shown on the location map (Figure B-1). It appears that bulk samples 1, 4, and 5 were collected from sediment in drainage basins outside the limits of the Strawpile landslide to characterize in-place sediments that potentially could be mobilized during future flood events. Bulk sample 1, collected in a tributary drainage to Pioneer Gulch, was a clayey gravel (GC) with liquid limit of 30% and plasticity index of 10%. Bulk sample 4 was collected in Spar Gulch near the confluence with Copper Gulch. It was non-plastic silty gravel (GM). On Figure B-1 the location of bulk sample 5 is shown in Pioneer Gulch a short distance southeast of Strawpile landslide. It classified as clayey sand (SM-SC) with liquid limit of 26% and plasticity index of 6%. Chen & Associates (1985b) identified four debris-flows source basins on Aspen Mountain, two of which were thought to potentially affect the Top of Mill site: Pioneer Gulch and an unnamed interbasin area east of Pioneer Gulch (see Figure B6). Debris flows issuing from Vallejo Gulch and Spar/Copper Gulch were less predictable, because those basins have shallow channels where P45 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-9 November 2017 they discharge onto the Little Nell earthflow. Because of this, debris flows from Vallejo Gulch and Spar/Copper Gulch potentially could be deposited on the Little Nell earthflow, or they could spill into the areas on either side of the Little Nell earthflow. Chen & Associates (1985b) described several methods to mitigate debris-flow hazards from the Pioneer, interbasin, and Vallejo source basins. They pointed out that the engineering practice of analyzing debris flows and the design of debris-flow mitigation were in developmental stages at the time of their study. For design purposes, they assumed a rainfall event of 1.95 inches in a one-hour time period, with a probability of occurrence of once in 100 years. Refer to Table E-1 in Chen & Associates (1985b) for the design debris-flow parameters for Pioneer Gulch and to Table E-2 and E-3 for the design debris flow parameters for the interbasin area and Vallejo Gulch, respectively. Figure B7 is a map of the Strawpile landslide by Chen & Associates (1985b) that shows the approximate limits of the landslide and locations of surface monuments, inclinometers, and piezometers used to evaluate the landslide. Figure B8 is their south-north cross section through the landslide that also shows the locations of six test holes drilled into the landslide. One additional test pit was excavated into the landslide during the late spring of 1985. Many soil samples were analyzed in the laboratory; index property testing included grain-size analysis, water content, liquid limits, plastic limits, and dry density; the strength tests included triaxial shear tests, direct shear tests, and unconfined compressive strength. Viscosity tests were conducted on samples from both the fan and debris-flow source areas. The Strawpile landslide was about 15 acres in size and ranged from 28 to 62 feet deep at the time of the Chen & Associates investigation in 1985. The landslide involved mine dump material that overlies colluvial surficial deposits. According to a report by Schmueser & Associates (1984) that is cited in the Chen & Associates report, the landslide moved several feet between June 6, 1984 when ground cracks were first noticed and July 1984. Inclinometer and surface survey monuments installed in the fall of 1984 and spring of 1985 indicated about one foot of movement during the spring of 1985 when piezometers recorded increases in ground water levels resulting from infiltration of snowmelt. Three inclinometers were installed in December 1984; they recorded 0.14 to 0.32 inches of average displacement per month between December 1984 and early spring 1985. Movement rates increased starting in about mid-April. The casing for inclinometer number 1 was sheared sometime between April 18 and May 7; casings for inclinometers 2 and 3 were sheared sometime between May 7 and May 28. Twenty-two survey monuments were installed both inside and outside of the landslide. Three of the survey monuments were the tops of the inclinometer casings and were initially surveyed on November 19, 1984 and again on December 17, 1984. The other survey monuments were installed on May 21, 1985. All were surveyed on May 24 and 28, June 4, 8, and 18, and July 2. During this 42-day time period, the upper part of the landslide experienced greater movement (~6 to 14 inches) than the lower part (~2 to 10 inches). The small slump next to the Corkscrew ski run that was reported in 1984 also experienced additional movement during the spring of 1985. P46 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-10 November 2017 Chen & Associates (1985b) concluded that the long-term behavior of the landslide was difficult to predict, but that at the then current rate of movement it did not present an immediate hazard to the Top of Mill site or to nearby areas. To stabilize the landslide, Chen & Associates recommended remedial actions including excavation and re-grading of material above Dago Road and installation of horizontal drains in the landslide below the road. Test pits TP-8 and TP-9 were excavated into the upper reaches of the steep-sided, elongate landform immediately uphill of the Gondola Plaza. Although Chen & Associates (1984) referred to this landform as a fan, Chen & Associates (1985b) called this landform the Little Nell earthflow, which is a more accurate description of its origin than calling it a fan. The material in the upper part of the earthflow was similar to the material in the lower part of the earthflow that was exposed in test pits TP-1 and TP-2 in 1984. The sediment from 6 to 8 feet deep in test pit TP-8 classified as clayey gravel (GC), with a liquid limit of 32% and plasticity index of 10%. The material from a depth of 6 to 7 feet in test pit TP-9 was clayey gravel (GM-GC), with a liquid limit of 27% and plasticity index of 7%. Bryant and Martin, 1988 Bryant and Martin (1988) is a layperson's guide about the geologic story of the Aspen region. They briefly mention the sediment-laden flood in 1964, although they indicate it occurred in 1965, as did Bryant (1972a). They attributed this flooding event to heavy rains on Aspen Mountain and reported that it caused mud to flow down Pioneer Gulch and into town. They described the flow as small, and noted that its effects were not obvious in 1988 when the book was published. Mineral Systems, Inc., 1989 Charles S. Robinson described his observations and recommendations on the 1989 slope failure in Pioneer Gulch in a letter from his company Mineral Systems, Inc. (1989) to Beryl Eylar, Pitkin County Engineer. Dr. Robinson reported that his understanding was that cracks and slumping of fill were noted on May 22, 1989. He visited the site on May 25th. It appeared to him that the landslide occurred in colluvium overlying the Belden Formation and that the removal of vegetation and "dressing" of the slopes by the ski company increased infiltration of precipitation in the colluvium and increased the rate of slope movement. Dr. Robinson also indicated that snowmaking by the ski company increased the amount of water available for infiltration. He described the potential damage to life and property under normal conditions as minimal, but that severe storm conditions could trigger a debris flow that affected areas at the base of Pioneer Gulch. He recommended that a detailed engineering investigation of the slope be conducted. Hepworth-Pawlak, 1996 Hepworth-Pawlak (1996) contains the results of a geotechnical investigation of the 1995 slope movements in the Roch Run and Spring Pitch areas in the Aspen Mountain Ski Area. Figure B9 shows their overview map of the landslide area, and Figure B10 is their more detailed map of the landslide. Slope movements were first noted by the ski company in late May 1995. Measurement of the ground cracks by taping between survey pins was initiated by the ski P47 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-11 November 2017 company on June 6, 1995. On June 16, they began to survey the landslides using an electronic distance meter. Hepworth-Pawlak mapped surface crack patterns on July 12, 1995, and they drilled five test borings into the Roch Run landslide during August 1995. Engineering properties of soil samples from the borings were tested in the laboratory. Two areas of ground cracking and slope movement were reported: an area about 1 acre in size on Roch Run and an area of about 1/3 acre on Spring Run. The ground surface in both areas slopes at an average of about 50%. Based on drill-hole data, the areas with slope movements are underlain by the Belden Formation. In addition to containing beds of shale, there are breccia zones associated with faults that subparallel the bedding in the formation. Slope movements had occurred in both the Roch Run and Spring Run areas prior to 1995. The small debris flow in 1984 in Pioneer Gulch described by Chen & Associates (1984) apparently was in the same area where ground cracks were noted in the Spring Pitch area in 1995. The 1995 Roch Run landslide occurred in the same area where minor slope displacement were observed in 1989 by Chen-Northern (1989), which is summarized in Hepworth-Pawlak (1996), and by Mineral Systems (1989). The rate of movement in 1995 was estimated to be about two inches per day or less, with total displacement estimated at less than one foot. Ground cracks had also been noted in the Spring Pitch area during late May and early June of 1991 (Chen-Northern, 1991; cited in Hepworth-Pawlak, 1996). The primary crack was in Summer Road at the same location where cracks formed in 1995. During late May and early June of 1992, slope movements were noted in the Roch Run area (Chen-Northern, 1992; cited in Hepworth-Pawlak, 1996). This movement had a total displacement of about five feet in the same vicinity as the 1989 and 1995 slope failures, but it involved a smaller area of only about 1/4 acre. During the 1995 investigation, the Roch Run landslide was mapped as having a main landslide body area with four smaller interior landslide units. Material within the slide was chiefly colluvium and artificial fill; the landslide apparently did not include the Belden bedrock. The basal shear surface of the landslide appeared to be in colluvium in the lower part of the landslide; in the upper part of the slide the basal shear plane was at or near the contact between the colluvium and Belden Formation. The maximum rate of movement in the main body of the landslide was 3.5 inches /day, whereas the smaller interior landslides had maximum displacement rates as high as 41.7 inches/day. All landslide displacements abruptly declined in mid to late June. The 1995 slope failure on Spring Pitch was much smaller than the one on Roch Run. The total displacement on the Spring Pitch slide was less than 1 foot in 1995, and all movement ended by late June. The slope movements on Roch Run and Spring Pitch occurred as the snow pack in the area was melting and the ground was water saturated. Water levels were much lower when the test bore holes were drilled in August in the Roch Run landslide. Hepworth-Pawlak concluded that the ground water within the landslide was a result of infiltration of local snowmelt, not a regional ground water system. They also noted that the 1984 and 1995 slope movements occurred during the peak years of wet cycles. Winter precipitation from November, 1994 through May, 1995 was a record high. 1989 also was a year of heavy precipitation, but slope movements also occurred in 1991 and 1992, which were years of average or below average winter precipitation. Hepworth- P48 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-12 November 2017 Pawlak suspected leakage from a sewer line may have contributed to the 1992 Roch run movement. Hepworth-Pawlak (1996) stated that the historic slope movements on Roch Run and Spring Pitch "have not resulted in problems outside of the immediate area of the slope movements." They noted that under certain conditions large slope movements could be mobilized into debris flows that potentially could reach developed areas on the Pioneer Gulch fan. Because of this potential, they recommended that mitigation efforts be undertaken to improve slope stability and reduce the potential for a debris flow. Their mitigation plan was developed and submitted to the client on September 6, 1995, and it was included in their 1996 report as Appendix A. Unfortunately, the copy of the 1996 report received by WWE does not include the appendices or tables, and figures 6 and 7 are missing from the report. Hepworth-Pawlak (1996) mentioned that some but not all of their mitigation recommendations were completed before snow prevented access to the area in the late fall of 1995. They reported that the skiing company planned to bulldoze the snow off of the landslide areas in the spring of 1996 and to prevent surface water runoff in the Summer Road ditch from infiltrating into the landslide as temporary mitigation measures. McCalpin, 1997 The geologic and hydrologic impacts of proposed snowmaking on nearly 60 acres located on upper Aspen Mountain were the subject of McCalpin (1997). Using field observations, interpretation of aerial photography, and library research, McCalpin documented the geology and hydrology of the uppermost part of the ski area, predicted if the new snowmaking might increase erosion and slope instability, and made several recommendations for routing the additional runoff resulting from the new snowmaking. As noted in previous investigations, McCalpin described most of the bedrock on Aspen Mountain as consisting of hard, indurated rock that is not prone to erosion or slope instability. In contrast, McCalpin noted that the Belden Formation contained many beds of shale, had interstratal breccias resulting from faulting and dissolution and collapse, and it weathered to produce clay-rich surficial deposits. These characteristics convinced McCalpin that the Belden and its weathering products were prone to erosion and slope instability. McCalpin described several of the landslides and slope failures on Aspen Mountain. One of these is the landslide that originated in Vallejo Gulch and created the elongate, steep-sided, tongue-shaped deposit of debris immediately uphill of the Gondola Plaza. McCalpin pointed out that although Bryant (1971) mapped the deposit as alluvial fan deposits, there is strong evidence to indicate the deposit is a result of landsliding. The evidence included the geomorphology of the deposit, as well as its composition as exposed in road cuts along Summer Road, where it consisted of angular blocks of aplite porphyry floating in a matrix of brown sand and gray silt and clay, the latter derived from the Belden Formation. McCalpin (1997) apparently was unaware of the test pits excavated into the landslide by Chen & Associates (1984, 1985a, b), which exposed similar sediment that was massive and unstratified. P49 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-13 November 2017 McCalpin called this feature the Vallejo Gulch landslide. He described the aplite porphyry in the landslide source area as being fractured and shattered by two north-northeast-trending faults, the Schiller fault on the west and the Silver fault on the east. The fracturing was presumed to have weakened the rock and was a contributing factor to the landslide. Two other inferred causes of the Vallejo Gulch landslide were the oversteepening of the base of Aspen Mountain by glacial erosion and the northerly and valleyward plunge of the synclinal fold axis. McCalpin noted that the Vallejo Gulch landslide formed sometime after the glacial ice in the Roaring Fork Valley that was associated with the most recent major ice age retreated upvalley beyond the base of Aspen Mountain, which he assumed was around 15,000 to 20,000 years ago. This interpretation is at odds with the mapping of Bryant (1971) that shows the terminal moraine for the most recent glacial ice advance a short distance upvalley from the landslide deposit. This suggests the terminal end of the last major glacier did not extend as far downvalley as the Vallejo Gulch landslide, and it was not responsible for oversteepening the slopes at the base of Aspen Mountain where Vallejo and Spar Gulches are located because the glacier did not extend to that location. Older glaciations, however, did extend beyond the mouths of Vallejo and Spar Gulches and they probably oversteepened the slopes near the base of the mountain, but not the most recent major glacier. Stronger evidence of the maximum age of the Vallejo Gulch landslide is that the landslide deposits overlie glacial outwash from the most recent major glaciation (Bryant 1971), which suggests the landslide formed sometime after about 13,000 to 15,000 years ago. As documented during our study, the Vallejo Gulch landslide apparently blocked or dammed Spar Gulch, and neither Spar Gulch or Vallejo Gulch have eroded clearly defined channels through or adjacent to the landslide. This suggests the Vallejo Gulch landslide may have moved much more recently than 13,000 to 15,000 years ago. McCalpin (1997) was unaware of any evidence of historic movement of the Vallejo Gulch landslide and pointed out that the Homestake Deep Shaft, located in the landslide, apparently was never abandoned or relocated due to post-construction downslope movement of the landslide. McCalpin (1997) mentioned the landslide in the "Strawpile area" of Pioneer Gulch mapped by Bryant (1971), and also the letter report by Charles Robinson (Mineral Systems 1989) that described movement of the landslide in 1985 and 1989. McCalpin apparently was unaware of the monitoring of the landslide movement by Chen & Associates (1985a, b). McCalpin devoted considerable text to the May 13 and 14, 1996 Keno Gulch debris flows, in part because his study addressed the potential effects of increased runoff from the proposed snowmaking. Two large debris flows occurred, one at ~4 PM on May 13 and a second at ~4 PM on May 14. Several smaller debris formed between the first and second large events. Although this debris flow happened a short distance outside of our project area, it is discussed below because it may be helpful to understanding the debris-flow hazards on Aspen Mountain that potentially could flow into the city. McCalpin recounted the prior written studies of Keno Gulch debris flows by Mears (1983, 1992, 1996), Bussone (1989), and Wright and Rold (1996), and a phone conversation on November 20, P50 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-14 November 2017 1997 with Jeff Hynes, Colorado Geological Survey, who had accompanied Mears during their site visit after the initial debris flow. McCalpin also spent some of his time in the field during 1997 examining the landslide in the upper part of Keno Gulch where the 1996 debris flow originated and the Summer Ditch. McCalpin described the differences in opinion between Wright and Rold (1996) and that of Mears (1996) and Jeff Hynes on the cause of the 1996 Keno Gulch debris flows. Mears (1996) and Hynes had concluded the landslide toe moved down the hillslope and into the drainage that contained the discharge from the Summer Ditch. The debris flows were created when the landslide was saturated and liquefied by Summer Ditch water, causing blocks of soil to cave from the toe of the landslide into the drainage that carried the Summer Ditch discharge. In contrast, Wright and Rold (1996) described strong evidence that the debris flows initiated at the toe of the landslide while it was still upslope from the drainage carrying the Summer Ditch discharge. In that interpretation, the debris flows would have traveled down the tributary drainage before entering the drainage with Summer Ditch discharge, and that the water discharged by the Summer Ditch would not have triggered the debris flows. See Figure B11 for a map by Wright and Rold showing the relationship between the landslide and the drainage carrying discharge from the Summer Ditch. McCalpin diplomatically concluded that both interpretations may be correct, but that they apply to different debris flows. Mears and Hynes visited the site around noon on May 14, soon after the initial debris flow at ~4 PM on May 13, and before the second large debris flow event at ~4 PM on May 14, whereas John Rold didn't examine the site until May 17 after the second event. McCalpin's three-phase interpretation of the 1996 Keno Gulch debris flows is as follows: Jim Blanding (cited in Wright and Rold, 1996) reported movement of the landslide during the three days prior to the first debris flow on May 13. Hynes inferred that the landslide, in part still frozen, advanced slowly down the hillslope during this time period and reached the drainage carrying the discharge from Summer Ditch on May 13, temporarily blocking the ditch drainage. This would explain why the surface flow at the mouth of Keno Gulch temporarily ceased during the afternoon of May 13 immediately before the first debris flow event, as described by Mr. Goers in Wright and Rold (1996). The discharge from the Summer Ditch would have ponded upstream of the landslide, eventually saturating and flowing over the top of the landslide and thawing the frozen part of the slide debris. When the landslide dam failed, it would have mobilized into the first debris flow, ending phase 1 of McCalpin's model. After the initial failure of the landslide toe, the remaining part of the slide began to extend and tension cracks formed, allowing small blocks of the slide mass to fall into the drainage with the Summer Ditch discharge. This process created the smaller debris flows reported between the two larger events. These smaller events comprise Phase 2 of McCalpin's model. Phase 3 began shortly before 4 PM on May 14 when the second large debris flow ran down Keno Gulch. Although no one was present to directly observe this failure, evidence described by Wright and Rold (1996) indicates the landslide toe was located well upstream of the tributary drainage carrying the discharge from Summer Ditch when the second large debris flow formed. P51 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-15 November 2017 The evidence observed by Rold on May 17 included a narrow-incised channel in the tributary drainage between the landslide and Keno Gulch, mud splatters on trees, and debris-flow levees (Wright & Rold 1996). This evidence apparently was not observed by Mears or Hynes during their visit prior to the second large debris flow. McCalpin concluded that Mears' and Hynes' interpretations apply to Phase 1 and 2, and that the interpretation of Wright and Rold applies to Phase 3. McCalpin (1997) also examined the second landslide mapped by Bryant (1971) on the slopes of Aspen Mountain above the City of Aspen. This landslide is on the wall of Spar Gulch immediately north of Short Snort Run. The landslide extended from about 60 feet below Summer Road down to the floor of Spar Gulch. It formed in the Belden Formation (or colluvium overlying it) where it is faulted by the Hallet fault of Spurr (1898). McCalpin described the toe of the slide as having the appearance of an earthflow. McCalpin (1997) also reported a nearly identical landslide about 1,200 feet to the south near the F.I.S. Run. Both slides were described as overlying east-west-trending faults with down-to-south movement. McCalpin attributed these slump-earthflows to (1) crushing and weakening of the rock in the fault zones; and (2) damming and ponding of northward-moving ground water against the east-west-oriented faults. McCalpin also commented on the potentially unstable slopes mapped by Bryant (1972a). Much of those areas are on dipslopes underlain by hard Paleozoic rocks older than the Belden Formation that he felt were unlikely to fail. He concluded that dip slopes mapped by Bryant as potentially unstable "pose very little threat of a slope failure" unless undercut by high cuts, "whereas areas underlain by the Belden Formation (such as Pioneer Gulch) pose a continuing threat of sliding." In his section on geologic impacts of proposed snowmaking, McCalpin mentioned the Deer Park landslide, an old, thin earthflow on the southwest side of Bell Mountain with no known historic reactivation, and the Tourtelotte Park landslide, which was partially reactivated during the spring of 1997. Both landslides are in areas underlain by the Belden Formation, and both were located just downslope of the then proposed snowmaking areas. McCalpin recommended that no additional runoff be diverted into those ski trails. McCalpin discussed in detail multiple methods by which to control the runoff off from the ski area, as well as the pros and cons of each method. Since it is currently unknown which, if any of these methods were utilized, no further discussion of the geologic hazards associated with those methods are described herein. Hepworth-Pawlak Geotechnical, Inc., 1998 Hepworth-Pawlak (1998) also evaluated the potential slope stability effects of proposed new snowmaking in the upper part of the Aspen Mountain Ski Area. They concluded that the new snowmaking would likely increase deep percolation of surface water during the spring snowmelt and that some reduction in slope stability might result. They did not think the new snowmaking would cause extensive slope instability, and that if local instability developed it could be mitigated. They noted that the Summer Ditch diversion provided a benefit to the City of Aspen P52 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-16 November 2017 by reducing snowpack melt runoff, but additional runoff in the diversion could increase the debris-flow potential in Keno Gulch. Figure B12 shows the landslide and drainage basin map by Hepworth-Pawlak (1998). They identified thirteen landslides and slope-creep areas, of which ten are in areas on Aspen Mountain that drain toward the City of Aspen. According to Table III in the Hepworth-Pawlak report, these ten landslides and slope-creep areas are named as follows: #3) Tourtelotte Park; #4) Zaugg Dump; #6) Snow Bowl; #7) Spring Pitch; #8) Roch Run; #9) Corkscrew; #10) Strawpile; #11) Old Pioneer Dump; #12) un-named; and #13) un-named. Only the Tourtelotte Park landslide was said to be within the proposed new snowmaking area, which contrasted with McCalpin (1997), who said this landslide was a short distance downslope of the new snowmaking area. Table III in Hepworth-Pawlak (1998) also contains brief descriptions of the landslides and slope- creep areas, including any known recent movement. In Figure B13, the thirteen landslides and slope-creep areas identified by Hepworth-Pawlak (1998) are overlain on the geologic map by Bryant (1971). All of the identified landslides on Aspen Mountain originated in either the Belden Formation or in surficial deposits that overlie the Belden Formation. They pointed out that Roch Run landslide has been active at times since 1984. Long-term monitoring of Strawpile landslide indicated it had moved episodically at an annual average rate of 0.24 to 1.0 inch/year and that the basal slip plane was 40 to 60 feet deep. Some remedial actions had been implemented at Strawpile, Roch Run, Pioneer Dump, Spring Pitch, and Snow Bowl landslides, primarily monitoring, sealing of surface ground cracks, and control of surface drainage. Hepworth-Pawlak stated that shallow landslides in colluvium can mobilize into debris flows given the right conditions. This usually happened on slopes steeper than 50%. Significant parts of Aspen Mountain are steeper than 50%. Both unusually intense thunderstorms and the rapid melting of snowpack have triggered historic debris flows in the Rocky Mountains. The report also described the 1996 Keno Gulch debris flows, based mostly on the descriptions of Mears (1996) and Wright and Rold (1996). Hepworth-Pawlak (1998) described the Tourtelotte Park landslide as about 5 acres in size, on slopes the average 32%. It formed in colluvium that overlies the Belden Formation. Open surface cracks were present during field work on July 24, 1997. They recommended that the cracks be sealed and the area graded. If new cracks developed, they recommended measurement of them, and that the landslide be characterized by geologic mapping and subsurface exploration. The Summer Ditch crosses the crown of the Zaugg Dump landslide. Hepworth-Pawlak noted that proposed snowmaking might increase flows in the ditch during spring runoff. If water seeped into the ground from the ditch in the vicinity of the landslide, it could add ground water to the landslide and destabilize it. They recommended flumes to be installed to assess ditch capacity and seepage losses. Overtopping or sloughing of the ditch bank could be triggered by increased runoff, resulting in uncontrolled discharge, erosion, and slope instability. If needed, mitigation could include lining and stabilizing the ditch. P53 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-17 November 2017 WRC Engineering, Inc., 2001 WRC (2001) prepared a storm drainage master plan for the City of Aspen. WRC (2001) mentioned the historical occurrence of mudflow events off Aspen Mountain, the larger mudflow events in adjacent watersheds, and previous geologic studies that suggested parts of the City of Aspen were prone to mud flood and mudflow events. For their storm drainage plan, they modeled the drainages on Aspen Mountain above town as three basins: Spar Gulch, Vallejo Gulch, and Pioneer Gulch. The only geologic hazard data in their report is from Bryant (1972a). WRC described three development alternatives for the mountain slopes above town. On- mountain Alternative 1 involved a boulder-lined channel and drain system in the bottom of major gulches on Aspen Mountain. Their on-mountain Alternative 2 consisted of a series of buried concrete walls in the bottom of the major gulches above town. On-mountain Alternative 3 would regulate new construction so it would not increase the mudflow hazards in the City. P54 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-18 November 2017 P55 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-19 November 2017 P56 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-20 November 2017 P57 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-21 November 2017 P58 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-22 November 2017 P59 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-23 November 2017 P60 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-24 November 2017 P61 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-25 November 2017 P62 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-26 November 2017 P63 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-27 November 2017 P64 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-28 November 2017 P65 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-29 November 2017 P66 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-30 November 2017 P67 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX B) 161-012.000 Wright Water Engineers, Inc. Page B-31 November 2017 P68 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 November 2017 Appendix C. Technical Memorandum on Red Butte Hydrology To: April Long, P.E. and Mike Horvath, EIT City of Aspen From: Wright Water Engineers, Inc. Date: January 24, 2017 Re: Hydrology and Mudflow Considerations for Red Butte Area, Aspen, Colorado Wright Water Engineers, Inc. (WWE) has prepared this technical memorandum to present results of hydrologic analysis for the Red Butte area in Aspen, Colorado. Red Butte is a prominent geologic feature in the City of Aspen. Silver King Drive runs along the base of the southern and southeastern limits of the butte and Cemetery Lane runs along the base of the butte on the eastern side (Figure 1). In the original scope of work for the Aspen Mud and Debris Flow Study, the Project Team led by WWE, planned to evaluate the hydrology of Re were being using to evaluate Aspen Mountain and the commercial core of the City, including development of a FLO-2D model. Following the field investigation and after obtaining additional information on geologic Butte area does not have alluvial fans and the primary geologic hazards in the area relate to rockfall and water-related erosion and sediment transport from the steep slopes of the butte. As a result, the City and Project Team agreed to perform simplified hydrologic analysis using the Rational Method to quantify peak flow rates and sediment transport potential for this area. The following sections provide a summary of the geology and soils in the area, rainfall calculations and recommendations for hydrologic design criteria to apply in the Red Butte area. Red Butte Geology Bryant (1971) mapped the surficial deposits on the southeast side of the butte as talus. The surficial deposits on the southwest side of the butte were mapped as colluvium, except for an area on southernmost part of the southwest flank which was mapped as On the geologic hazard map by Bryant (1972), the southeast side of Red Butte is denoted as a rockfall hazard area. No geologic hazards were indicated on the southwest side of the butte. Based on field observations, the Project Team concurs that a southeast side of Red Butte. The project team also observed evidence of possible minor City of Aspen Mud & Debris Flow Assessment (APPENDIX C) Wright Water Engineers, Inc. Technical Memorandum on Red Butte Hydrology MEMORANDUM April Long, P.E. and Mike Horvath, EIT Wright Water Engineers, Inc., Andrew Earles and Catherine Berg Hydrology and Mudflow Considerations for Red Butte Area, Aspen, Colorado nc. (WWE) has prepared this technical memorandum to present results of hydrologic analysis for the Red Butte area in Aspen, Colorado. Red Butte is a prominent geologic feature in the City of Aspen. Silver King Drive runs along the base of the southern and southeastern limits of the butte and Cemetery Lane runs along the base of the butte on the eastern In the original scope of work for the Aspen Mud and Debris Flow Study, the Project Team led by WWE, planned to evaluate the hydrology of Red Butte using similar analytical methods that were being using to evaluate Aspen Mountain and the commercial core of the City, including 2D model. Following the field investigation and after obtaining additional information on geologic hazards in the Red Butte area, it was apparent that the Red Butte area does not have alluvial fans and the primary geologic hazards in the area relate to related erosion and sediment transport from the steep slopes of the butte. As a sult, the City and Project Team agreed to perform simplified hydrologic analysis using the Rational Method to quantify peak flow rates and sediment transport potential for this area. The following sections provide a summary of the geology and soils in the area, rainfall calculations and recommendations for hydrologic design criteria to apply in the Red Butte area. Bryant (1971) mapped the surficial deposits on the southeast side of the butte as talus. The surficial deposits on the southwest side of the butte were mapped as colluvium, except for an area on southernmost part of the southwest flank which was mapped as talus. On the geologic hazard map by Bryant (1972), the southeast side of Red Butte is denoted as a rockfall hazard area. No geologic hazards were indicated on the southwest side of the butte. Based on field observations, the Project Team concurs that a rockfall hazard exists on the southeast side of Red Butte. The project team also observed evidence of possible minor Page C-1 Technical Memorandum on Red Butte Hydrology Andrew Earles and Catherine Berg Hydrology and Mudflow Considerations for Red Butte Area, Aspen, Colorado nc. (WWE) has prepared this technical memorandum to present results of hydrologic analysis for the Red Butte area in Aspen, Colorado. Red Butte is a prominent geologic feature in the City of Aspen. Silver King Drive runs along the base of the southern and southeastern limits of the butte and Cemetery Lane runs along the base of the butte on the eastern In the original scope of work for the Aspen Mud and Debris Flow Study, the Project Team led by d Butte using similar analytical methods that were being using to evaluate Aspen Mountain and the commercial core of the City, including 2D model. Following the field investigation and after obtaining hazards in the Red Butte area, it was apparent that the Red Butte area does not have alluvial fans and the primary geologic hazards in the area relate to related erosion and sediment transport from the steep slopes of the butte. As a sult, the City and Project Team agreed to perform simplified hydrologic analysis using the Rational Method to quantify peak flow rates and sediment transport potential for this area. The following sections provide a summary of the geology and soils in the area, rainfall-runoff calculations and recommendations for hydrologic design criteria to apply in the Red Butte area. Bryant (1971) mapped the surficial deposits on the southeast side of the butte as talus. The surficial deposits on the southwest side of the butte were mapped as colluvium, except for an On the geologic hazard map by Bryant (1972), the southeast side of Red Butte is denoted as a rockfall hazard area. No geologic hazards were indicated on the southwest side of the butte. rockfall hazard exists on the southeast side of Red Butte. The project team also observed evidence of possible minor P69 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX C) 161-012.000 Wright Water Engineers, Inc. Page C-2 November 2017 sediment-laden flood hazards in localized areas on the southwest side of the butte, which was confirmed using Google Earth imagery. Major soil groups associated with Red Butte are shown on Figure 2 (NRCS Web Soil Survey). The predominant soil group for Red Butte is identified as map unit 104, which is classified as Torriorthents-Camborthids-Rock outcrop complex, 6 to 65 percent. The Web Soil Survey classifies this complex as Hydrologic Soil Group C with high runoff potential. The erosion potential for these soils is high given the steep slopes of the butte. Rock outcrop comprises approximately 15% of the area. Runoff Analysis Given the relatively small size of the drainage basins, and the relatively steep slopes of the butte, the flow can be well-represented using one-dimensional hydrologic analyses. The Rational Method was used to calculate peak discharge rates for a range of return periods from the 2- to 500-year event. The Rational Method calculates runoff based on a specified runoff coefficient and rainfall intensity corresponding to a duration equal to the time of concentration, using the following equation: Q = CIA Where: Q = peak discharge rate (cfs) I = rainfall intensity (in/hour) for a specified return period and duration equal to the time of concentration A = drainage area (acres). Four sub-basins, two on each side of the butte, were delineated using the 1-foot interval contour data provided by the City (Figure 1). Sub-basin Characteristics Physical sub-basin characteristics were developed based on the 1-foot interval contour mapping, that include area, overland flow length, shallow channelized flow length, overland flow slope, channelized flow slope, and overall watershed slope (Table 1). P70 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX C) 161-012.000 Wright Water Engineers, Inc. Page C-3 November 2017 Table 1. Red Butte Sub-basin Characteristics Sub-basin ID Area (acres) Overland Length (ft) Shallow Concentrated Length (ft) Overland Slope (ft/ft) Shallow Concentrated Slope (ft/ft) 1 4.07 207 569 78% 40% 2 7.06 197 840 76% 36% 3 3.43 197 511 76% 63% 4 5.17 204 664 49% 66% Runoff Coefficients Runoff coefficients for the 2- through 100-year peak rainfall events were determined from the City of Aspen’s Urban Runoff Management Plan (URMP) based on assumed imperviousness of 15% (rock outcrop fraction of soil complex) and Hydrologic Soils Group C (Table 2). Table 2. Runoff Coefficients for Red Butte Analysis (15% Impervious Area, HSG C) Return Period Runoff Coefficient, C 2-yr 0.13 5-yr 0.22 10-yr 0.32 25-yr 0.42 50-yr 0.48 100-yr 0.53 Time of Concentration The time of concentration for each sub-basin was calculated based on overland flow and shallow channelized flow lengths using equations 3-4 through 3-6 in the URMP. In accordance with procedures in the URMP, the 5-year runoff coefficient of 0.22 was used to calculate the overland flow time for the 2- through 100-year rainfall events. The overland flow distance was estimated as the distance from the ridge of the butte to the downgradient point where rills appear to be present based on aerial imagery. Due to the similarity in basin sizes, the overland flow distances were similar for the four sub-basins, approximately 200 feet. Shallow channelized flow lengths were calculated using GIS as the distance from the end of overland flow paths to the sub-basin outlets at the bottom of the butte. A conveyance coefficient of 10 feet/second was used, representing bare soil conditions. The time of concentration was calculated for each sub-basin by adding overland and shallow channelized flow times (Table 3). P71 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX C) 161-012.000 Wright Water Engineers, Inc. Page C-4 November 2017 The calculated times of concentration are all greater than the 5-minute minimum value from the URMP. Table 3. Red Butte Time of Concentration Sub-basin Overland Flow Time (min) Shallow Channelized Flow Time (min) Time of Concentration (min) 1 5.4 1.5 6.9 2 5.3 2.3 7.7 3 6.1 2.3 8.4 4 7.2 2.4 9.6 Rainfall Intensity-Duration-Frequency Rainfall intensity-duration-frequency data were obtained for the Red Butte area from NOAA Atlas 14 (Table 4). Table 4. Intensity-Duration-Frequency Data for Red Butte Duration (min) Rainfall Intensity (inches/hour) Return Period (year) 2 5 10 25 50 100 5 2.04 3.00 3.72 4.80 5.64 6.36 10 1.56 2.22 2.76 3.54 4.14 4.68 15 1.24 1.80 2.24 2.88 3.32 3.80 30 0.78 1.10 1.38 1.74 2.00 2.28 Rainfall Intensity for each sub-basin was determined from Table 4 by interpolating between the 5- and 10-minute durations based on the time of concentration for each sub-basin. Rainfall intensity values for each return period for each sub-basin are included in the calculations attached to this memorandum. Runoff Calculations Peak runoff rates for the 2- through 100-year events were calculated using the Rational Method, Q = CIA, where Q is the peak discharge in cubic feet per second, C is the runoff coefficient for the specified return period, I is the rainfall intensity in inches per hour, and A is the sub-basin area in acres. (Table 7) Runoff calculations are summarized in Table 5 in terms of the peak discharge rate for each sub-basin and as peak unit runoff normalized to sub-basin area. Unit rates of runoff for all four sub-basins are relatively consistent for each rainfall event (Table 5). This is because land use, slopes and other characteristics are similar across the area evaluated. P72 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX C) 161-012.000 Wright Water Engineers, Inc. Page C-5 November 2017 The average values are suitable for planning purposes for the Red Butte area, given adjustments for sediment transport described below (Table 6). Table 5. Unit Rates of Runoff for Return Periods from Rational Method Analysis Return Period Peak Discharge – Average of Four Sub-basins (cfs) Range of Peak Discharges from Four Sub-basins (cfs) 2-yr 0.2 0.2 – 0.2 5-yr 0.6 0.6 – 0.6 10-yr 1.1 1.0 – 1.1 25-yr 1.8 1.7 – 1.9 50-yr 2.4 2.3 – 2.5 100-yr 3.0 2.9 – 3.1 Mudflow Considerations The Rational Method results above account for clear water flows only. To account for sediment in the runoff, we recommend applying a bulking factor of 45% for the 10- and 25-year peak discharges and a bulking factor of 20% for the 2-, 5-, 50- and 100-year peak discharges (Table 6). Bulking factors are not recommended for the 2- and 5-year event. Table 6. Unit Rates of Runoff including Bulking Factor Return Period Clear Water Peak Discharge (cfs) Bulking Factor Bulked Peak Discharge (cfs) 2-yr 0.2 20% 0.3 5-yr 0.6 20% 0.7 10-yr 1.1 45% 1.5 25-yr 1.8 45% 2.6 50-yr 2.4 20% 2.9 100-yr 3.0 20% 3.6 Recommendations Based on this analysis, we recommend using unit rates of runoff from this analysis as the basis for planning in the Red Butte area. This will enable engineers to plan for conveyance simply using the tributary area above their design points. For final design, the engineer should independently perform Rational Method analysis following the procedures and assumptions outlined in this technical memorandum. P73 I. City of Aspen Mud & Debris Flow Assessment (APPENDIX C) 161-012.000 Wright Water Engineers, Inc. Page C-6 November 2017 P74 I. 161-012.000 Wright Water Engineers, Inc. Page C-7 November 2017 P75I. DRAFT 161-012.000 Wright Water Engineers, Inc. Page C-8 November 2017 P76I. DRAFT 161-012.000 Wright Water Engineers, Inc. Page C-9 July 2017 Table 7. Runoff Calculations for Red Butte Sub-basins (does not account for sediment bulking) Sub-basin 1 Return Period Runoff Coefficient, C Overland Flow Time, To (Min) Channel Flow Time, Tf (Min) Time of Concentration, Tc (min) Rainfall Intensity, I (in/hr) Peak Flow, Qp (cfs) Peak Flow, Qp (cfs/acre) 2-yr 0.13 5.4 1.5 6.9 1.86 1.0 0.2 5-yr 0.22 5.4 1.5 6.9 2.70 2.4 0.6 10-yr 0.32 5.4 1.5 6.9 3.35 4.4 1.1 25-yr 0.42 5.4 1.5 6.9 4.32 7.4 1.8 50-yr 0.48 5.4 1.5 6.9 5.07 9.9 2.4 100-yr 0.53 5.4 1.5 6.9 5.72 12.3 3.0 Sub-basin 2 2-yr 0.13 5.3 2.3 7.7 1.78 1.6 0.2 5-yr 0.22 5.3 2.3 7.7 2.58 4.0 0.6 10-yr 0.32 5.3 2.3 7.7 3.21 7.2 1.0 25-yr 0.42 5.3 2.3 7.7 4.13 12.2 1.7 50-yr 0.48 5.3 2.3 7.7 4.84 16.4 2.3 100-yr 0.53 5.3 2.3 7.7 5.46 20.4 2.9 Sub-basin 3 2-yr 0.13 5.3 1.1 6.4 1.90 0.8 0.2 5-yr 0.22 5.3 1.1 6.4 2.78 2.1 0.6 10-yr 0.32 5.3 1.1 6.4 3.45 3.8 1.1 25-yr 0.42 5.3 1.1 6.4 4.45 6.4 1.9 50-yr 0.48 5.3 1.1 6.4 5.22 8.6 2.5 100-yr 0.53 5.3 1.1 6.4 5.89 10.7 3.1 Sub-basin 4 2-yr 0.13 6.3 1.4 7.6 1.79 1.2 0.2 5-yr 0.22 6.3 1.4 7.6 2.59 2.9 0.6 10-yr 0.32 6.3 1.4 7.6 3.21 5.3 1.0 25-yr 0.42 6.3 1.4 7.6 4.13 9.0 1.7 50-yr 0.48 6.3 1.4 7.6 4.85 12.0 2.3 100-yr 0.53 6.3 1.4 7.6 5.47 15.0 2.9 P77I. DRAFT City of Aspen Mud & Debris Flow Assessment (APPENDIX D) 161-012.000 Wright Water Engineers, Inc. Page D-1 November 2017 Appendix D. Clean Up Cost Quotes for Buildings P78 I. DRAFT City of Aspen Mud & Debris Flow Assessment (APPENDIX D) 161-012.000 Wright Water Engineers, Inc. Page D-2 November 2017 P79 I. DRAFT City of Aspen Mud & Debris Flow Assessment (APPENDIX D) 161-012.000 Wright Water Engineers, Inc. Page D-3 November 2017 P80 I. i TABLE OF CONTENTS Page 1.0 Introduction ............................................................................................................... 1 2.0 History & Background ................................................................................................. 3 2.1 History of Mudflow Events in Aspen ...................................................................................3 2.2 Summary of Prior Geologic Hazard Studies & Observations and Interpretations from September 2016 Field Investigation ................................................................................................6 3.0 Hydrology ................................................................................................................ 13 3.1 Previous Hydrologic Studies ............................................................................................. 13 3.1.1 Aspen (WRC, 2001)................................................................................................................ 13 3.1.2 Aspen (WWE, 2014) .............................................................................................................. 17 3.1.3 Breckenridge (Tetra Tech, 1993) ........................................................................................... 17 3.1.4 Glenwood Springs (USACE, 1997) ......................................................................................... 17 3.1.5 Milton Creek (USGS, 2011) .................................................................................................... 18 3.2 Precipitation .................................................................................................................... 20 3.3 FLO-2D Model Development ............................................................................................ 23 3.3.1 FLO-2D Grid ........................................................................................................................... 23 3.3.2 Channel and Overbank Roughness Values ............................................................................ 24 3.3.3 Infiltration Parameters .......................................................................................................... 25 3.3.4 Other Model Components .................................................................................................... 25 3.4 Comparisons between FLO-2D Hydrology and CUHP/SWMM Hydrology ............................ 26 3.5 Red Butte Mudflows and Mud Floods ............................................................................... 32 3.6 Summary and Recommended Hydrologic Approach .......................................................... 33 4.0 FLO-2D Mudflow Analysis ......................................................................................... 34 4.1 Mudflow Characterization and Processes ......................................................................... 35 4.2 Flood Hazard Index .......................................................................................................... 38 4.3 Mudflow Hydrographs ..................................................................................................... 41 4.4 Existing Conditions Model Results .................................................................................... 47 4.4.1 Flood Inundation and Hazard Mapping ................................................................................ 47 5.0 Damage assessment analysis .................................................................................... 65 P81 I. ii 5.1 Impacts to Infrastructure ................................................................................................. 65 5.2 Impacts to Private Property .............................................................................................. 69 6.0 Wildfire Risk Assessment .......................................................................................... 71 6.1 City of Aspen Post Wildfire Mudflow Assessment ............................................................. 71 6.2 Post Wildfire Mudflow Assessment Results ...................................................................... 74 6.2.1 Post Wildfire Mudflow 2-Year Peak Rainfall Event ............................................................... 74 6.2.2 Post-fire Mudflow 25-Year Peak Rainfall .............................................................................. 79 7.0 Mitigation Alternatives............................................................................................. 84 7.1 Reduced Sediment Supply ................................................................................................ 84 7.2 Routing Mudflow thru City Streets and Parks .................................................................... 84 7.3 Warning System ............................................................................................................... 88 7.4 On-Mountain Mitigation Structures .................................................................................. 89 7.4.1 On-Mountain Mitigation Structure Analysis Results ............................................................. 90 8.0 Summary and Recommendations ............................................................................. 94 9.0 Acknowledgements .................................................................................................. 96 10.0 References ............................................................................................................... 98 10.1 Other Potential References (Identified by reference in reviewed documents – documents exist based on other references but could not be located) .......................................................... 101 10.2 Personal Communications by Bob Kirkham ..................................................................... 102 TABLES Table 1. Comparison of Average Unit Rates of Runoff from Studies in Aspen and Vicinity ...................... 15 Table 2. Rainfall Depths and Durations for Studies in Aspen and Nearby ................................................ 15 Table 3. Horton’s Infiltration Parameters for Different Hydrologic Soil Groups ......................................... 17 Table 4. Land Classifications and Overland Manning’s n-Values ............................................................. 25 Table 5. Comparison of Unit Peak Discharges from SWMM and FLO-2D at Key Locations in Spar, Vallejo and Pioneer Gulches for 100-year Return Period .................................................................... 30 Table 6. Comparison of Modeled Volumes from SWMM and FLO-2D for 100-year Return Period .......... 30 Table 7. Red Butte Unit Rates of Runoff including Bulking Factor ............................................................ 33 P82 I. iii Table 8. Mudflow Behavior as a Function of Sediment Concentration ...................................................... 37 Table 9. Hazard Level Descriptions ........................................................................................................... 39 Table 10a. Event Intensities for Mudflows .................................................................................................. 39 Table 10b. Event Intensities for Water Floods ............................................................................................ 39 Table 11. Summary of Peak Clearwater Discharges, Clearwater Volumes and Bulked Sediment Volumes at the Four Debris Flow Locations for the 2-, 25-, 100-Year 2-Hour Rainfall Events and the 100-Year 12-Hour Rainfall Event ......................................................................................................................... 45 Table 12. Estimated Sediment Volume Available for Transport in Lower Spar Gulch ............................... 45 Table 13. Summary of Area (acres) of Low, Moderate and High Hazard Potential for the 2-Year, 25-Year, 100-Year 2-Hour and 100-Year 12-Hour Events .................................................................................. 48 Table 14. Summary of Sediment Volume Deposited in Streets and Parks at End of Simulation ............... 64 Table 15. Sediment Volume Deposited in Streets at End of the 25-Year 2-Hour Event ............................ 67 Table 16. Summary of Peak Flows at the Four Debris Flow Sources under Existing Conditions and the Four Wildfire Scenarios for the 2- and 25-Year Peak Flow Events ...................................................... 74 Table 17. Summary of Area (acres) of Low, Moderate and High Hazard Potential under Existing Conditions and for the Wildfire Scenarios for the 2-Year, 25-Year Events .......................................... 74 Table 18. Mudflow Storage Volume ............................................................................................................ 89 FIGURES Figure 1. General Location Map, Major Drainageways and Sub-basins used for FLO-2D Modeling ........ 16 Figure 2. Sub-basin and Routing Elements used in Previous Studies by WRC (2001) and WWE (2014) 19 Figure 3. Storm Days Recorded at Aspen Climate Stations versus Snow Water Equivalent (SWE) Recorded at the Schofield SNOTEL Site ............................................................................................. 20 Figure 4a. Hyetographs for 2-hour, 2-, 5-, 10-, 25-, 50- and 100-Year Rainfall Events.............................. 21 Figure 4b. 12-Hour, 100-Year Rainfall Hyetograph .................................................................................... 23 Figure 5. Land Use Zones Used to Assign Overland Flow Roughness Parameters in FLO-2D ............... 27 Figure 6. Hydrologic Soil Group Mapping of Project Area used to Assign Infiltration Parameters ............ 28 Figure 7. Locations of FLO-2D Channels and Comparison Points with 2014 SWMM from Master Plan Update .................................................................................................................................................. 29 Figure 8. Comparison on SWMM and FLO-2D Hydrographs for Node 114div, Copper and Spar Gulch . 31 Figure 9. Comparison on SWMM and FLO-2D Hydrographs for Node 116div, Vallejo Gulch .................. 31 Figure 10. Classification of Hyperconcentrated Sediment Flows ............................................................... 38 Figure 11. Clearwater (Q), Sediment Concentration (Cv) and Bulked Sediment (BF) Hydrographs in Lower Spar Gulch (Point 4) for the 25-Year Rainfall Event .................................................................. 43 Figure 12. Clearwater (Q), Sediment Concentration (Cv) and Bulked Sediment Hydrographs (BF) in Vallejo Gulch (Point 6) for the 25-Year Rainfall Event ......................................................................... 43 P83 I. iv Figure 13. Clearwater (Q), Sediment Concentration (Cv) and Bulked Sediment Hydrographs (BF) in Pioneer Gulch (Point 8) for the 25-Year Rainfall Event ........................................................................ 44 Figure 14. Clearwater (Q), Sediment Concentration (Cv) and Bulked Sediment Hydrographs (BF) in Pioneer Gulch (Point 19) for the 25-Year Rainfall Event ...................................................................... 44 Figure 15. Predicted Maximum Depth for the 2-Year Rainfall Event, with 20% Sediment Concentration under Existing Conditions ..................................................................................................................... 50 Figure 16. Predicted Maximum Velocities for the 2-Year Rainfall Event, with 20% Sediment Concentration under Existing Conditions ..................................................................................................................... 51 Figure 17. Predicted Flood Hazard Index for the 2-Year, 2-Hour Rainfall Event, with 20% Sediment Concentration under Existing Conditions ............................................................................................. 52 Figure 18. Predicted Maximum Depth for the 25-Year, 2-Hour Peak Rainfall Event, with 45% Sediment Concentration under Existing Conditions ............................................................................................. 53 Figure19. Predicted Maximum Velocity for the 25-Year, 2-Hour Rainfall Event, with 45% Sediment Concentration under Existing Conditions ............................................................................................. 54 Figure 20. Predicted Flood Hazard Index for the 25-Year, 2-Hour Rainfall Event, with 45% Sediment Concentration under Existing Conditions ............................................................................................. 55 Figure 21. Predicted Maximum Depth for the 2-Hour, 100-Year Event with Sediment Volume Equal to 25- Year Event under Existing Conditions .................................................................................................. 56 Figure 22. Predicted Maximum Velocity for the 100-Year, 2-Hour Rainfall Event with Sediment Volume Equal to 25-Year Event under Existing Conditions .............................................................................. 57 Figure 23. Comparison of the Predicted Maximum Depth from the WRC (2001) Study and the FLO-2D Model for the 100-Year, 2-Hour Rainfall Event with Sediment Volume Equal to 25-Year Event under Existing Conditions ............................................................................................................................... 58 Figure 24. Predicted Flood Hazard Index for the 100-Year, 2-Hour Rainfall Event with Sediment Volume Equal to 25-Year Event under Existing Conditions .............................................................................. 59 Figure 25. Combined Flood Hazard Index for the 2-, 25-, and 100-Year, 2-Hour Rainfall Event under Existing Conditions ............................................................................................................................... 60 Figure 26. Predicted Maximum Depth for the 100-Year, 12-Hour Rainfall Event with Sediment Concentration of 20-percent ................................................................................................................. 61 Figure 27. Predicted Maximum Velocity for the 100-Year, 12-Hour Rainfall Event with Sediment Concentration of 20-percent ................................................................................................................. 62 Figure 28. Predicted Flood Hazard Index for the 100-Year, 12-Hour Rainfall Event with Sediment Concentration of 20-percent ................................................................................................................. 63 Figure 29. Mudflow Depths in the Streets at the End of the 25-Year Peak Flow Event ............................. 66 Figure 30. Mudflow Depths Resulting from the 25-Year Peak Flow Event with Case Study Properties .... 69 Figure 31. Wildfire Scenarios ...................................................................................................................... 72 P84 I. v Figure 32. Predicted Maximum Depth for the 2-Year Rainfall Event under Wildfire Scenario 3 Conditions ............................................................................................................................................. 75 Figure 33. Predicted Increase in Depth between Wildfire Scenarios 3 and Existing Conditions for the 2- Year, 2-Hour Peak Rainfall Event ......................................................................................................... 76 Figure 34. Predicted Flood Hazard Index for the 2-Year, 2-Hour Rainfall Event under Wildfire Scenario 3 Conditions ............................................................................................................................................. 77 Figure 35. Predicted Increase in Depth between Wildfire Scenario 1 and Existing Conditions for the 25- Year, 2-Hour Peak Rainfall Event ......................................................................................................... 79 Figure 36. Predicted Increase in Depth between Wildfire Scenario 2 and Existing Conditions for the 25- Year, 2-Hour Peak Rainfall Event ......................................................................................................... 80 Figure 37. Predicted Increase in Depth between Wildfire Scenario 3 and Existing Conditions for the 25- Year, 2-Hour Peak Rainfall Event ......................................................................................................... 81 Figure 38. Predicted Increase in Depth between Wildfire Scenario 4 and Existing Conditions for the 25- Year, 2-Hour Peak Rainfall Event ......................................................................................................... 82 Figure 39. Location of Debris Flow Catchment Basins Evaluated in the Alternatives Analysis ................. 90 Figure 40. Reduction in Flow Depth at the 2-Year Peak Rainfall Event under the Alternatives Condition ............................................................................................................................................... 91 Figure 41. Reduction in Flow Depth at the 25-Year Peak Rainfall Event under the Alternatives Condition ............................................................................................................................................... 92 APPENDICES A Summary of Field Observations, September 2016 B Chronologic Summary of Geologic, Geologic Hazard, Engineering Geology, and Geotechnical Data Sources C Technical Memorandum on Red Butte Hydrology D Stutsman-Gerbaz Earthmoving, Inc. Estimate Appendices are provided in companion document due to size of overall report, including appendices P85 I. 161-012.000 Wright Water Engineers, Inc. Page 1 November 2017 City of Aspen Mud & Debris Flow Assessment 1.0 INTRODUCTION This report presents the results of a mud and debris flow hazard delineation study for the City of Aspen, Colorado. The commercial core of the City is located at the base of Aspen Mountain with more than 800 acres of steep terrain draining into and through the City. Historically, the City has experienced mud and debris flows originating from Aspen Mountain (WWE, 2014), with documented events occurring in 1919 and 1964 (Chen and Associates, 1985). In addition to geologic hazards from mud and debris flows from Aspen Mountain, other areas in the City have produced mudflows and mud floods including Red Butte and massive gulley erosion in a drainage near the Smuggler Mine draining into Hunter Creek. In 2001, WRC Engineering, Inc. (WRC) developed a Surface Drainage Master Plan for the City of Aspen (WRC, 2001). This study included Stormwater Management Model (SWMM) analysis to quantify hydrology and FLO-2D analysis to define flow paths and depths through the City. This analysis forms the basis of the City’s current mudflow hazard mapping in the Aspen Urban Runoff Management Plan (URMP). After 15 years, updated rainfall data and refined topographic mapping provides the basis for this updated study. Photo 1. Annotated Google Earth aerial image of Aspen Mountain. View is to south. P86 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 2 November 2017 The City retained a team of consultants led by Wright Water Engineers (WWE) to perform this Mud and Debris Flow Assessment to evaluate: (1) the potential risks to the City of Aspen from mud and debris flows originating from Aspen Mountain, (2) representative municipal cleanup costs based on mudflow frequency analysis and estimates of cleanup costs for private property in the mudflow hazard area, (3) the potential impacts of wildfire on mud and debris flows and (4) potential mitigation strategies. WWE worked closely with team members Bob Kirkham, P.G. to investigate the associated geologic hazards, and Tetra Tech, Inc. and Dr. Jim O’Brien, P.E. on FLO-2D modeling, damage assessment and mitigation alternatives. The FLO-2D analysis conducted by the Project Team focused on Aspen Mountain and the commercial core of the City. For the Red Butte Area, simplified methods were applied to understand the potential hydrologic hazards. This report is organized to present results from major tasks in the scope of work including: · History & Background – This section synthesizes results of an extensive literature review and field observations related to mudflow and mud floods in the City. · Hydrology – The starting point for hydrologic analysis in this study was the 2014 update to the Drainage Master Plan performed by WWE. This study integrated updated rainfall data from the National Oceanic and Atmospheric Association (NOAA) Precipitation- Frequency Atlas of the United States, Atlas 14, Volume 8. Because the hydrology is the primary driving factor for mud and debris flow rates and volumes, the project team conducted analysis to compare runoff rates with other recent studies conducted in the high country. This work provides confidence that the underlying hydrology for projecting mud and debris flows in Aspen is realistic and reasonable. · FLO-2D Analysis – Tetra Tech led efforts for FLO-2D modeling. This involved developing a grid for the City based on the most recent topographic information, determining appropriate peak sediment concentrations for a range of events, and developing mapping to define mudflow and mud flood depths in the City. · Damage & Financial Impacts – To evaluate potential damages and financial impacts from mudflow and mud flood events, the project team used several methods to estimate public and private cleanup costs. Public cleanup costs were calculated for frequency analysis events based on the modeled depths and volumes of mud accumulated in streets and unit costs to remove material, clean out storm sewers, and restore streets to a working condition. Financial impacts for private development were characterized using several case studies of specific buildings. For selected buildings, the Project Team worked with a remediation contractor to estimate cleanup and restoration costs based on predicted mudflow depths near the building. · Wildfire Risk Assessment –Burned watersheds have the potential to significantly increase runoff and generate mud and debris flows instead of water flooding. The project team evaluated a range of wildfire scenarios on Aspen Mountain to evaluate the increased risk. P87 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 3 November 2017 · Mitigation Alternatives – A final component of this study was to identify conceptual mitigation measures that can reduce the impacts of mud and debris flows in the City. Flood and mudflow mitigation is challenging given the limited opportunities for potential storage on the mountain or conveyance through the urban area, which is constrained by lot-line-to-lot-line development in the commercial core. The project team evaluated potential mitigation alternatives including storage/detention and flow conveyance to park areas to reduce downgradient impacts. 2.0 HISTORY & BACKGROUND The Project Team conducted research to identify the geologic and land use conditions in study areas including Aspen Mountain above the City (Photo 1) and the Red Butte area (Photo 3) and to find available documentation of past mudflow events. The Project Team reviewed available geologic hazard studies, met with the Colorado Geological Survey (CGS), and conducted outreach to colleagues who have worked on mudflows in the upper Roaring Fork Valley. Field investigations were conducted on September 1 and 2, 2016 to assess geologic conditions in the project area. The following sections summarize this information1. 2.1 History of Mudflow Events in Aspen In the mountainous drainage basins above the City, sudden and severe thunderstorms can produce heavy rains and flash floods capable of transporting large boulders, trees, as well as clay, silt and sands (NRC 1982, p. 13). Events like these have been reported in the Aspen area historically. To develop a history of mudflow events in and around the City of Aspen, the Project 1 Please note that in this report we use the geographic term 'Aspen Mountain' to describe the basins on the mountain that drain into the City, not just the ridgeline labeled Aspen Mountain on the U.S. Geological Survey's Aspen 7.5- minute topographic map. Key Definitions Flood: An infrequent stormwater runoff event, which results in runoff quantities that exceed the capacity of available drainage facilities. Clearwater Flood: A stormwater runoff event with conventional riverine sediment load (bedload material load) that contains a limited concentration of sediment and debris. The flood acts as a low viscosity fluid with a peak sediment concentration by volume that will typically be less than 20%. Mud flood: A stormwater runoff event that contains a concentration of sediment and debris but whose flow and fluid properties (i.e. density, viscosity, and yield stress) result in a flood event that behaves like water. The peak sediment concentration by volume will typically vary from 20% to 40%. Mudflow: A stormwater runoff event that contains a high concentration of sediment and debris and acts as a high viscosity, hyperconcentrated fluid. The peak sediment concentration by volume will typically vary from 40 to 55% with a behavior similar to wet cement. P88 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 4 November 2017 Team first completed a thorough search of the local newspaper archives to document recorded mudflow events. Project Team members also reached out to City staff, the Aspen Skiing Company, engineers and geologists in the area, local news reporters, and others with knowledge of past mudflows. Through this effort, existing mud and debris flow studies were identified, as well as other sources of mudflow information. The table below lists the people interviewed. "Mudslides" (a term describing mudflow events) in the Aspen area have been reported in the local newspapers as early as 1906 (Aspen Daily Times 1906). Early reports documented the mud events that affected travel through the Roaring Fork Valley (Aspen Daily Times 1906, 1908, 1930). In all of the early reports, rainstorms were identified as the source of the events; a “cloudburst” (Aspen Daily Times 1928) and torrential rains (Aspen Daily Times 1936, 1937, 1952) were described as the causes of the mudflow events. Historical reports document the use of horses to avoid “losing the family car in the mud” (Aspen Daily Times 1941) and Aspen being without train service for days due to “one of the largest rock and mud slides that has run in many years” (Aspen Daily Times 1938) near Hanging Rock at Slaughterhouse Falls. “Mudslides” were historically reported in the Crystal River and Frying Pan River Valleys as well. Chen & Associates (1984) researched Aspen's old newspapers for evidence of historical sediment-laden floods on the Pioneer and Little Nell alluvial fans. They found articles describing these types of flood events in the September 5, 1919 and August 7, 1964 issues of the Aspen Times. The 1964 flood event occurred on August 5. The exact date of the 1919 flood event apparently was not stated in the newspaper article. It probably occurred on or shortly before September 5. Both flooding events were the result of intense thunderstorms (Chen & Associates 1984). The September 1919 event was described as a cloudburst, resulting in large volumes of mud and water, but caused little property damage. Chen & Associates (1984) includes the following quote from the newspaper article: "The gulches at the head of Galena, Mill and Monarch Streets sent forth large volumes of water.” It was reported that large areas of town were covered with “yellow clay mud from the mountain” (Chen & Associates 1984). The August 1964 event was reported to be similar to the 1919 event; a cloudburst on Aspen Mountain triggered a mudflow down Pioneer Gulch that flooded Monarch, Mill, and Galena Streets. Chen & Associates documented that 1.29 inches of rain fell over the 24 hours during this storm, with 1.13 inches falling in a one-hour period; this rainfall data was verified by NOAA’s Photo 2. Historical photograph of the Tourtelotte Park area in the 1890s (courtesy of Aspen Historical Society). Note the dense concentration of mine dumps, mine buildings, and access roads. P89 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 5 November 2017 historical rainfall data (Chen & Associates 1984 p. 10) (NOAA 1964). The mud resulting from this event caused damage to the Aspen Inn on Mill Street, generated by mud, rocks, and logs covering the property. The mud in Pioneer Gulch was reported to be up to 5 feet in depth (Chen & Associates 1984). Individuals Interviewed Regarding Aspen Mudflow Events and Studies Name Affiliation Adeh, Nick City of Aspen, City Engineer (former) Berry, Karen Colorado Geological Survey Bryant, Bruce U.S. Geological Survey (retired) Elliot, Jennifer Aspen Music School Gardner-Smith, Brent Aspen Journalism Gerdin, Victor Aspen Ski Company Hynes, Jeffery Colorado Geological Survey (retired) King, Peter Aspen Ski Company Mears, Art Mears and Wilbur Mock, Ralph Hepworth-Pawlak Geotechnical, Inc. Parker, Jay Smuggler Mine Pawlak, Steve Hepworth-Pawlak Geotechnical, Inc. Spitzer, Roy Deere & Ault Consultants, LLC Stover, Bruce Colorado Division of Reclamation, Mining, and Safety William, Rogers "Pat" Colorado Geological Survey (retired) A landslide occurred on Aspen Mountain on June 6, 1984 (UPI 1984 and Russakoff 1984) that resulted in many studies of the slope stability and mudflow hazards associated with Aspen Mountain. This slope failure has commonly been referred to as the Strawpile Landslide. In June 1984, a series of “landslide fissures” (Chen & Associates 1985a) were observed on Aspen Mountain, above the Strawpile Run. As a result, much of downtown Aspen was evacuated in fear of a “major mud slide” (Russakoff 1984). Monitoring of the slide began soon after it was identified; however, the movement of the hillside quickly slowed and monitoring discontinued in July of 1984, when the Colorado Geological Survey and the United States Geological Survey determined there was no longer a risk to the City of Aspen (Chen & Associates 1985a). The landslide was identified as 28 to 62 feet deep, and covering approximately 15 acres on Aspen Mountain. Resulting studies concluded that “adverse groundwater conditions” in the hillside were partially responsible for the slide (Chen & Associates 1984). The Chen reports indicate that this landslide originated in a mine dump area above Dago Road, approximately 1,000 to 1,500 feet upgradient of the area where material settled on the Strawpile ski run. Scarps are still evident on aerial mapping in this area. The winter snowfall during the 1983-1984 season was approximately twice the normal snowfall amount, which was followed by a cool spring, and a quick summer warming period beginning in early May (Chen & Associates 1985b). Since this event, long-term monitoring of the Strawpile Landslide shows the hillside is creeping at a depth of 40 to 60 feet, at approximately 0.24 to 1 inch per year, with some larger displacements during larger snowpack years (Hepworth-Pawlak 1998). P90 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 6 November 2017 Although it did not affect the City of Aspen, a 1997 mud and debris flow event on the western flank of Aspen Mountain, down Keno Gulch, caused extensive damage to the Aspen Music School (Burns 1998, McCalpin 1997). This event was determined to be the result of “rapid melting of an above average snowpack” (Hepworth-Pawlak 1998). During this event, a landslide initiated, which then mobilized into a debris flow event (Wright & Rold 1996). The study concluded, “the Keno-Gulch debris flow is a natural phenomena having a high risk of reoccurrence with above-average precipitation” (Wright & Rold 1996). Based on these historical events, in 2003, a "mudslide" warning system was proposed for Aspen Mountain (Urquhart 2003). The effort was to be a collaboration between the City, Pitkin County, and the Aspen Skiing Company. As part of this effort, WRC Engineering, Inc. identified three potential mudflow routes affecting the City of Aspen: Pioneer, Vallejo, and Spar gulches. To date, a flood and mudflow warning system has not been implemented. While mudflows and mud floods are infrequent by their nature, the geology and hydrology of the Aspen area creates conditions that are favorable for the occurrence of these phenomena. The history presented in this section identifies at least several large events of this type in the past 100-years or so in the Aspen area and is an indication that such events can be expected to occur in the future. Mudflow and mud flood events in the Aspen area most commonly occur due to heavy rainfall, often in combination with a high snowmelt year, rapid spring melting of the snowpack, and saturated soil conditions. 2.2 Summary of Prior Geologic Hazard Studies & Observations and Interpretations from September 2016 Field Investigation This project evaluated two locations in the City of Aspen: 1) areas at the base of Aspen Mountain that were susceptible to future mud and debris flows that were generated on Aspen Mountain; and 2) the southwest and southeast flanks of Red Butte. To evaluate the hazards at the base of Aspen Mountain, it was also essential to study current conditions on Aspen Mountain. A third area near the Smuggler Mine with massive gulley erosion draining down to Hunter Creek was evaluated at a reconnaissance level. The initial phase of the project involved reviewing available geologic hazard literature and reports and interviewing people regarding their knowledge of the project area. Please refer to the memorandum titled “History and Background on Mudflow Events around Aspen" for a list of those interviewed. This phase of Photo 3. Overview of the south end of Red Butte. Photograph taken from Aspen Mountain. P91 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 7 November 2017 the project also included field investigations, which were conducted on September 1 and 2, 2016. Please refer to Appendix A for a summary of observations made during the field investigation. Aspen Mountain is a steep north-facing mountain on the south side of the City of Aspen. There is approximately 3,200 feet of vertical topographic relief on the mountain, ranging from about 11,200 feet above mean sea level at the top of the mountain to an average altitude of approximately 8,000 feet at the base of the mountain. Four deeply incised 'gulches' or valleys carry runoff from the mountain and into the City of Aspen, as shown in Figure 1. Spar Gulch is the largest basin on the mountain, starting at the top of Aspen Mountain and extending through the central part of the area, covering approximately 462 acres, or 56.5% of the entire area on Aspen Mountain that drains into the City of Aspen. Copper Gulch is in the eastern part of Aspen Mountain. It drains the area on the east side of Bell Mountain, which extends across approximately 118 acres (14.4%) of entire basin that drains into the City of Aspen. Copper Gulch flows into Spar Gulch at the north end of Bell Mountain. For the purposes of this report, we refer to the section of Spar Gulch above the confluence as Upper Spar Gulch, and the section below the confluence is called Lower Spar Gulch. Pioneer Gulch is in the northwest part of Aspen Mountain. It covers approximately 131 acres or 16.0% of the basin above the City of Aspen. At approximately 107 acres, Vallejo Gulch is the smallest of the four valleys (13.1% of the mountain above town). Vallejo Gulch is situated between Pioneer Gulch and the lower part of Spar Gulch. The 'gulches' on Aspen Mountain have distinctly different topographic configurations. Copper and Spar Gulches are V-shaped in cross section. They have steep valley walls and narrow valley floors. Both are eroded into hard, well-indurated bedrock that weathers to granular sediment. Pioneer Gulch has a relatively wide valley floor that locally is undulatory or hummocky. Its valley walls are less steep than those on Copper and Spar Gulches. The Belden Formation underlies much of Pioneer Gulch and also the uppermost part of Spar Gulch. The Belden Formation is easily eroded compared to the other erosion-resistant hard bedrock formations on Aspen Mountain, and it tends to weather to surficial soil that contains significantly more clay than the soils formed from hard indurated formations in Copper and Spar Gulches. All landslides identified on Aspen Mountain by Hepworth-Pawlak (1998) originated in the Belden Formation or in soil derived from it. In contrast, Vallejo Gulch is U- shaped, with very steep valley walls and a broad valley bottom. It formed when a large area of bedrock on the mountain apparently broke loose and slid downhill as the Vallejo Gulch Photo 4. Rocky, unconsolidated steep slope. P92 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 8 November 2017 landslide. The landslide may have been a sudden release of material that moved rapidly. The Belden Formation crops out on the floor of Vallejo Gulch. The basal slip plane for the Vallejo Gulch landslide probably was within the Belden Formation. The runout zone for the Vallejo landslide is the elongate, steep-sided ridge that protrudes northward beyond the base of Aspen Mountain. The Gondola Plaza, Little Nell ski lift, and Little Nell ski run are located on the Vallejo Gulch landslide. The literature documents evidence of repeated, episodic, prehistoric sediment-laden flooding events at the base of Aspen Mountain, in addition to the previously discussed September 1919 and August 1964 events. Two historical sediment-laden floods on Pioneer Gulch also are known have affected the City of Aspen. Chen & Associates (1984, 1985b) identified four debris-flow source areas (areas with sufficient available sediment and flows significant enough to mobilize the sediment) on Aspen Mountain based on topographic mapping. They considered Pioneer Gulch, Vallejo Gulch, Spar Gulch, and an interbasin area between Pioneer and Vallejo Gulches as potential debris-flow source basins. Debris flows originating in Pioneer Gulch could deposit sediment on the Pioneer Gulch fan, whereas flows originating in the interbasin area would deposit sediment in the area between the Pioneer Gulch fan and the Vallejo Gulch landslide. As pointed out by Chen & Associates (1984, 1985b) and supported by our investigation, the flow paths and depositional areas for mud floods and mud flows in Vallejo Gulch and Lower Spar Gulch are less predictable. Vallejo Gulch, and to a lesser extent Spar Gulch, has a shallow, poorly defined channel where it discharges onto the Vallejo Gulch landslide. As a consequence, mud floods and mudflows in Vallejo Gulch and Lower Spar Gulch potentially could be deposited on the Vallejo Gulch landslide or they could spill off onto either side of it. WRC (2001) modeled the drainages on Aspen Mountain for their storm drainage master plan as three basins: Pioneer Gulch, Vallejo Gulch, and Spar Gulch. Small future floods in Lower Spar Gulch may be contained in the shallow channel with small sediment catchment basin located at the lower end of the Bell Mountain ski lift, which is on the east side of the Vallejo Gulch landslide. However, large future floods in Lower Spar Gulch may plug the shallow existing channel, spill out across the Vallejo Gulch landslide and run into the area between the Vallejo landslide and Pioneer Gulch fan. The relatively large Pioneer Gulch fan was studied by Chen & Associates (1984, 1985a, b) to better understand the character of prehistoric floods that deposited sediment on the fan. They Photo 5. Well-vegetated slopes on Aspen Mountain. P93 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 9 November 2017 concluded that the frequency of debris flows on the fan may be "on the order of hundreds of years,” and that what they termed mud floods "have occurred much more frequently and may be the major concern for the proposed Top of Mill project." Chen & Associates estimated a recurrence interval of about 25 years for the mud floods. Note that they did not distinguish between mud floods and mudflows but stated that the mud floods could have a solids concentration of about 40% to 50% by volume, which now is considered more typical of mudflows. Grain-size analyses for the mud flood and mudflow deposits exposed in the test pits by Chen & Associates are available in their reports. A very small, rather subtle depositional landform exists where Lower Spar Gulch runs off the steep slopes on the east side of the Vallejo landslide and onto the less steep ground in the vicinity of South Alps Road and South Ute Road. None of the previous studies discovered during our project evaluated the sediments in this area, which is disturbed by human activities, and its natural topographic configuration is difficult to discern. What is apparent is that Lower Spar Gulch has deposited very little sediment in this area during the recent geologic past. The volume of sediment in the Pioneer Gulch fan is much greater than that in the small, subtle depositional area associated with Lower Spar Gulch. However, the combined drainage area of Copper and Spar Gulches far exceeds that of Pioneer. Also, the volume of rock that has been removed from Copper and Spar Gulches by long-term erosion during geologic time far exceeds what has been removed from Pioneer Gulch. Based upon the relative sizes of the drainage basins and the volume of rock eroded from them over the long-term, the fan associated with Lower Spar Gulch should be much larger that the fan of Pioneer Gulch, yet the opposite is true today. As is discussed later in this section of the report, this apparent contradiction may relate to the large landslide that formed Vallejo Gulch sometime since the end of the last major glaciation, some 13,000 to 15,000 years ago. During the field investigation, potential sources of sediment available for mobilization during future floods were evaluated. Large volumes of unconsolidated soil exist on the valleys walls and valley bottoms in most areas visited during the field investigation (Photo 4). Under certain conditions, such as when saturated, these soils may be mobilized during flood events and incorporated into the floodwaters, creating mud floods or mudflows. In many parts of the mountain, these soils appeared to have well-established vegetative cover, which reduces the likelihood of their mobilization during storm events (Photo 5). Where these soils are unvegetated or poorly vegetated, they are more prone to mobilization during storm events. Photo 6. Unvegetated mine dump in Spar Gulch viewed during the gondola ride. Note the rills eroded into the steep slopes on the dump. P94 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 10 November 2017 Unvegetated mine dumps on steep slopes were observed from the gondola in an area referred to as “The Dumps” by the Aspen Ski Company (Photo 6). The area at and immediately below the top of the mountain is heavily trafficked, which has impacted the vegetative cover. Aspen Ski Company is revegetating this area as part of their mountain silviculture program. Several unvegetated or poorly vegetated hillslopes also were noted in the Tourtelotte Park area. These areas may be associated with reclaimed or partially reclaimed mine dumps that once were widespread in Tourtelotte Park (Photos 2 [historic] and 7 [summer 2017]). In contrast, the valley floors in the uppermost part of Spar Gulch and upper and middle Pioneer Gulch lacked incised channels and appeared to be fairly well vegetated, except for gravel roads and adjacent borrow ditches. Incised, unvegetated channels were noted in the lower parts of Copper Gulch, Spar Gulch, and Pioneer Gulch, although the channels were relatively shallow and the channels were approximately 1 to 2 feet deep. Considerable unvegetated soil and small mine dumps exist in the vicinity of the Commodore Mine portal. Exposed rock and sediment also available in channels and along road cuts in the lower part of the mountain (Photos 8 and 9), and a small slump with unvegetated head scarp was observed in a road cut near the mine. These could be significant sources of material for mudflows. Another potential source of sediment during flood events can be landslides, especially if the landslides are active or recently active and have features like open cracks, bulging toes, and areas with raw, exposed soil. Landslides such as the Vallejo Gulch landslide and others described above could be potential sources of sediment during flood events. Hummocky topography was observed in several areas in the upper part of Spar Gulch and in Pioneer Gulch during our field investigation. These geomorphic features are evidence of landslides. During the 1980s and 1990s several small landslides on Aspen Mountain were active, including the Strawpile, Roch Run, Tourtelotte, Snow Bowl, Spring Pitch, Corkscrew, Old Pioneer Dump landslides, as well as two small, unnamed landslides in the lower part of Pioneer Gulch (Hepworth-Pawlak, 1998). In June 1984 concerns that the Strawpile landslide could catastrophically fail and impact the City of Aspen prompted temporary evacuation of part of the city. No evidence of recent landslide activity, such as ground cracks, fresh head scarps, or actively bulging landslide toes, was observed during the field Photo 7. Some hillslopes in the Tourtelotte Park area are poorly vegetated and are potential sources of sediment during future flooding events. Photograph taken during gondola ride. P95 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 11 November 2017 investigation. Landslide monitoring and limiting mitigation were performed on Strawpile, Roch Run, and Old Pioneer Dump landslides (Hepworth-Pawlak, 1998). Monitoring of the Strawpile landslide with inclinometers began in 1988. Inclinometers, piezometers, and test drains were installed in Strawpile landslide. In a 1998 snowmaking study, HP Geotech reported annual displacements ranging from 0.24 to 1.0 inch based on long term monitoring of the Strawpile landslide with the higher displacements corresponding to wetter years. In general, these were characterized as minor displacements, not affecting ski area operations. The HP report also noted that Aspen Skiing Company had implemented some remedial measures to minimize hydrologic factors affecting movement of landslide deposits. An array of sub-horizontal toe drains was observed in the toe of the landslide during our field investigation. A small flow of water discharged from one drain; other drains were dry but appeared to be functional. Several geologic and geotechnical studies recognized the landform referred to as the Vallejo Gulch landslide. Brunton (1888) was the first to recognize the "land-slip," as he called it. Chen & Associates (1984) originally called this landform the Little Nell fan, but attributed it to an earthflow that originated in Vallejo Gulch. Chen & Associates (1985b) called the landform the Little Nell earthflow, and McCalpin (1997) named it the Vallejo Gulch landslide. Either term may be appropriate because evidence indicates the landform is a result of a single large slope failure, not repeated episodic sediment-laden floods. Test pits dug into the Vallejo Gulch landslide by Chen & Associates (1984, 1985b) exposed a single thick deposit of sediment suggestive of deposition during a single event, not multiple relatively thin beds of sediment as was encountered in test pits excavated into the Pioneer Gulch fan. As reported by McCalpin (1997) and observed during our field investigation, the Vallejo Gulch landslide has very steep sides and a Photo 8. Roadcut with drainage channel to right of road. Photo 9. Small drainage channel with rocks that would mobilize in large event. P96 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 12 November 2017 steep front, which also indicates the landform probably was a result of a single slope failure. The time and rate of movement of the Vallejo Gulch landslide is unknown. The steep slopes on the landslide suggest it probably was a fairly rapid failure. The landslide deposits overlie glacial outwash from the most recent major ice age (called the Pinedale Glaciation); this indicates the landslide formed sometime during the past 13,000 to 15,000 years. The Vallejo Gulch landslide appears to have blocked or dammed Spar Gulch. There is a small, relatively flat area in Spar Gulch upstream of the landslide blockage where sediment was deposited after the landslide blocked Spar Gulch. The landslide also may have buried or destroyed a post-glacial fan deposited by Lower Spar Gulch, which carries runoff from both Copper and Spar Gulches. This may explain why the modern fan associated with Lower Spar Gulch is so small relative to the Pioneer Gulch fan. Lower Spar Gulch may have deposited large volumes of sediment in a fan at the base of Aspen Mountain since the last major glaciation, only to have the fan buried or destroyed by the Vallejo Gulch landslide. The evidence also suggests the Vallejo Gulch landslide potentially may be much younger than the 13,000 to 15,000-year-old glacial outwash it overlies. The May 1996 Keno Gulch debris flows occurred on the steep west-facing valley wall of Castle Creek west of the project area. The debris flows apparently involved two events on consecutive days (McCalpin, 1997). The first debris flow reportedly resulted from a liquefied landslide dam that blocked outflow from Summer Ditch. The second event may have initiated in a tributary drainage above the discharge from Summer Ditch. Although the Keno Gulch debris flows occurred on the valley wall of Castle Creek on the west side of Aspen Mountain beyond the limits of the project area, it is likely that at least some of the sediment in the fans at the base of the mountain in the city also initiated in a similar manner as liquefied landslides on the mountain's slopes. Other sediment in the fans likely was eroded from hillslopes, channel walls, and channel floors. Bryant (1971) prepared the only previous investigation addressing the Red Butte area. Bryant mapped the surficial deposits on the southeast side of the butte as talus, and those on the southwest side as colluvium. The southernmost part of the southwest flank of Red Butte was mapped as talus. On the geologic hazard map by Bryant (1972) the southeast side of Red Butte is denoted as a rockfall hazard area. No geologic hazards were indicated on the southwest side of the butte on Bryant's map. Based on our fieldwork, we concur that a rockfall hazard exists on the southeast side of Red Butte. Minor sediment-laden flood hazards were observed during the field investigation in local areas on the southwest side of the butte, which was confirmed by viewing the hillslope in Google Earth. Photo 10. Half culvert that carries flow in Summer Ditch across the Zaugg Dump landslide. P97 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 13 November 2017 A chronologic summary of previous studies and data sources with additional details is provided in Appendix B. 3.0 HYDROLOGY This section presents an overview of past hydrologic studies in the area, analysis of timing of large rainfall events, and development of rainfall and runoff parameters for the FLO-2D modeling. Previous studies (discussed below) used the Colorado Urban Hydrograph Procedure (CUHP) and the Stormwater Management Model (SWMM) to simulate rainfall-runoff. For this study, FLO-2D was used to calculate runoff using the same rainfall hyetographs used in recent CUHP/SWMM studies. The goals of this hydrologic analysis were to confirm that the unit rates of runoff from the most recent CUHP/SWMM analysis (WWE 2014) are reasonable in the context of other regional studies and to “calibrate” the rainfall-runoff response in FLO-2D to be similar to the rainfall-runoff response from CUHP/SWMM. In addition to the 2-hour, 2- through 100-year rainfall events, a 12-hour 100-year event was also simulated to evaluate a long duration storm event. Unless specified otherwise, the rainfall frequencies and depths referred to in this report are related to the 2-hour rainfall duration. A 2-hour temporal storm distribution was used for most of the model simulations to represent conditions present during a summertime convective storm. This storm distribution is “front loaded” with over 80% of the precipitation in the first hour and 25% of the 1-hour point rainfall depth in a 5-minute interval. Mudflow events can occur under varying circumstances from rain during snowmelt to summer thunderstorms; however, the largest total rainfall depths and greatest rainfall intensities are associated with storms in late-spring and summer months, which is why the 2-hour temporal storm distribution was applied. To accomplish this analysis, a FLO-2D model was developed that covers the Aspen Mountain watersheds and the City between the base of the mountain and the Roaring Fork River. Figure 1 shows the area modeled using FLO-2D, including major drainages from the mountain and sub- basins applied to the SWMM. 3.1 Previous Hydrologic Studies Five studies located in Aspen and similar mountainous areas have included rainfall-runoff modeling. The following sections summarize these studies and compare the rainfall and runoff characteristics used in these existing studies with the FLO-2D model results. Table 1 presents a comparison of average unit rates of runoff from studies discussed below. 3.1.1 Aspen (WRC, 2001) WRC (2001) performed hydrologic and debris flow analyses as part of the Surface Drainage Master Plan for the City of Aspen (WRC, 2001). As part of the hydrologic analysis, they developed a Storm Water Management Model (SWMM) that is a dynamic rainfall–runoff- subsurface model used for simulating surface/subsurface hydrology mostly in urban/suburban areas. A SWMM is comprised of subbasins, with each subbasin assigned representative physical properties including impervious/pervious areas, depression areas and infiltration characteristics. Runoff can be routed through the subbasins in open channels, closed pipes and water storage features. SWMM computes the flow rate and volume generated within each subbasin as well as P98 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 14 November 2017 within the pipe network during a simulation period composed of multiple fixed or variable time steps. The WRC (2001) SWMM was developed using the older NOAA Atlas 2 rainfall estimates and the Horton infiltration method (Table 2 and Table 3). The 1-hour 100-year peak rainfall was 1.69 inches (Table 2). The SWMM model area, which included Aspen Mountain and the City, was divided into 25 subbasins (Figure 2). The subbasin naming convention applied to the WRC (2001) model was also used in the WWE (2014) study and in this study (Figure 2). Each subbasin was assigned Horton soil infiltration rates (Table 3) based on Natural Resources Conservation Service (NRCS) hydrologic soil class identified in soil class mapping. In general, the upper part of the Aspen Mountain (Subbasins 22, 23 & 24) and western subbasins (6, 9 and 20) were assigned Hydrologic Soil Class C, and the remaining lower basins and the city area where assigned Hydrologic Soil Class B (Figure 2). Spar and Copper Gulches form the largest drainage basin area upstream from the City with a combined area of approximately 580 acres at Junction 114div in the SWMM (Figure 2). At Junction 114div, the predicted runoff for the 100-year rainfall using the NOAA Atlas 2 rainfall values event is 477 cfs, which equates to 0.82 cfs per acre (Table 1). WRC performed debris flow analysis using the FLO-2D software to estimate the maximum mudflow depths for the 10- and 100-year peak rainfall events. The WRC FLO-2D model was not available for this study. A description of the model in the Master Plan indicates the model covered Aspen Mountain and the area of the City between the base of the mountain and the Roaring Fork River. The aerial extent of the WRC model was similar to the FLO-2D model project area used in this study. Flow hydrographs and sediment concentration hydrographs were applied as input to the debris flow model. WRC also modeled the effects of the streets and building using the modeling options in FLO-2D. The model output was used to develop 1-foot depth inundation mapping for the 100-year peak flow event. The WRC 100-year depth inundation mapping was compared to mapping developed from this study. Table 1. Comparison of Average Unit Rates of Runoff from Studies in Aspen and Vicinity Study Location NOAA Atlas Duration Area (acre) Average unit runoff by return period storm (cfs/acre) 2 5 10 25 50 100 WRC (2001) Aspen* 2 2-hour 580 0.82 WWE (2014) Aspen* 14 2-hour 580 <0.01 0.02 0.04 0.10 0.23 0.42 USACE (1997) Glenwood Springs 2 2-hour ~14,000 0.77 1.12 1.77 USGS (2011) Milton Creek 2 2-hour ~3,000 1.6 Tetra Tech (1993) Breckenridge 2 24-Hour ~35,000 <0.01 0.01 0.01 0.04 0.12 0.17 *Average unit runoff from Pioneer Gulch, Vallejo Gulch, and Spar Gulch Table 2. Rainfall Depths and Durations for Studies in Aspen and Nearby P99 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 15 November 2017 Study Location NOAA Atlas Duration Recurrence Interval (years) 2 5 10 25 50 100 WRC (2001) Aspen 2 2-hour 0.64 1.00 1.20 1.40 1.60 1.69 WWE (2014) Aspen 14 2-hour 0.47 0.64 0.77 0.95 1.09 1.23 USACE (1997) Glenwood Springs 2 2-hour 1.65 USGS (2011) Marble 2 2-hour 0.90 1.40 Tetra Tech (1993) Breckenridge 2 24-Hour 1.33 1.77 1.99 2.44 2.88 3.10 P100 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 16 November 2017 Figure 1. General Location Map, Major Drainageways and Sub-basins used for FLO-2D Modeling P101 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 17 November 2017 Table 3. Horton’s Infiltration Parameters for Different Hydrologic Soil Groups Hydrologic Soil Group Initial Rate (in/hour) Final Rate (in/hour) Decay Coefficient (1/second) A 5.0 1.0 0.0007 B 4.5 0.6 0.0018 C 3.0 0.5 0.0018 D 3.0 0.5 0.0018 3.1.2 Aspen (WWE, 2014) WWE performed rainfall-runoff and detention analysis for the City of Aspen in 2014. As part of the study, WWE updated the WRC SWMM to include the updated NOAA Atlas 14 rainfall values in Table 2. The NOAA Atlas 14 1-hour, 100-year peak rainfall was 1.23 inches, approximately 27 percent less than the NOAA Atlas 2 value in Table 2. WWE did not make any other changes to the baseline SWMM in this analysis. In this study, the modeled peak flow for the 100-year rainfall draining from the Spar and Copper Gulches (Junction 114div) is 243 cfs, which is 0.42 cfs per acre (Table 1); this is approximately a 50 percent reduction in peak flow at this node, compared to the original WRC study (2001). 3.1.3 Breckenridge (Tetra Tech, 1993) Tetra Tech developed a HEC-1 rainfall-runoff model as part of the Master Plan developed for the Town of Breckenridge. Breckenridge is located approximately 50 miles north east of Aspen at an elevation of approximately 9,600 feet. In this analysis, the 24-hour 100-year rainfall event was determined to be 3.1 inches (Table 2). Over the 24-hour rainfall event, approximately 1 inch of rainfall occurred over a 1.75-hour period, which is less than 1.23 inches determined by WWE for the City of Aspen in 2014. The HEC-1 model included 22 sub-basins ranging in size from 180 to 7,100 acres, with a total area of 35,000 acres. The runoff values varied significantly due to the variation in basin elevation, topography and geology. The sub-basin runoff for the 100-year rainfall event ranged from 0.10 to 0.36 cfs per acre and averaged 0.17 cfs per acre (Table 1). 3.1.4 Glenwood Springs (USACE, 1997) The U.S. Army Corps of Engineers (USACE) conducted a hydrologic analysis using the HEC-1 software as part a flood study in Glenwood Springs (USACE, 1997). The USACE hydrology analysis is reported in the City of Glenwood Springs Master Plan (Matrix, 2003). Glenwood Springs is located approximately 40 miles northwest of Aspen at an elevation of approximately 5,700 feet. The model included 53 sub-basins ranging in size from 1.9 to over 7,000 acres, with a total area of approximately 14,000 acres (21.9 mi2). NOAA Atlas 2 rainfall data were used in this study. The 1-hour rainfall for the 100-year event was 1.65 inches; the rainfall depths for the other recurrence intervals were not reported in the City of Glenwood Springs Master Plan (Matrix, 2003) (Table 2). The sub-basin unit runoff for the 100-year rainfall event ranged from 0.49 to 3.13 cfs per acre and averaged 1.77 cfs per acre P102 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 18 November 2017 (Table 1). The higher end of the range of unit rates of runoff typically corresponds to urbanized sub-basins with significant impervious cover. 3.1.5 Milton Creek (USGS, 2011) The Unites States Geological Survey (USGS) performed hydrologic analysis to evaluate the impacts of wildfire on debris flow in the vicinity of Marble, Colorado (USGS, 2011). Marble is located approximately 20 miles southwest of Aspen at an elevation of approximately 7,900 feet. The USGS evaluated debris flows in four drainage basins: Carbonate, Slate, Raspberry ad Milton Creeks. The USGS applied NOAA Atlas 2 rainfall values for the 1 -hour, 5- and 25-year of 0.9 and 1.4 inches, respectively. The Soil Conservation Service (SCS) Runoff Curve Number (RCN) methodology was used to estimate the infiltration losses. Only the 25-year peak flow for Milton Creek was reported, which was approximately 4,700 cfs and corresponds to 1.6 cfs per acre of runoff, as indicated in Table 1. This appears to be high for the 25-yr event because the 100-yr event would be significantly higher, however, the 4,700 cfs may include sediment, which may be bulked for 30 to 60% sediment concentration by volume. P103 I. DRAFT 161-012.000 Wright Water Engineers, Inc. Page 19 November 2017 Figure 2. Sub-basin and Routing Elements used in Previous Studies by WRC (2001) and WWE (2014) P104I. City of Aspen Mud & Debris Flow Assessment 161-012.000 November 2017 3.2 Precipitation To understand when the large precipitation events are occurring in the City of Aspen, the daily precipitation data recorded at the Aspen Climate Station (1900 Station (1980-2016) was analyzed 1935; therefore, the period of record from recorded precipitation events containing more than an inch of moisture (relatively large events) were identified and characterized as r number of days with precipitation events greater than one inch during each mo of record as well as the average monthly snow water equivalent (SWE) recorded at the Schofield Pass SNOTEL station from 1986 through 2016. The majority of large rain events occur in the summer, and late summer when the SWE is low, if not zero. Therefore, the analysis on rain on snow events. It should be noted that a lesser frequency event (e.g. 10 occurs on snow-covered or saturated ground event with drier antecedent conditions. with less infiltration but also not all of the snow pack will melt off during the rainfall event. general, the rain-on-snow events are of smaller magnitude. Figure 3. Storm Days Recorded at Aspen Climate Stations versus Snow Water Equivalent (SWE) City of Aspen Mud & Debris Flow Assessment Wright Water Engineers, Inc. understand when the large precipitation events are occurring in the City of Aspen, the daily precipitation data recorded at the Aspen Climate Station (1900-1979) and Aspen 1 SW Climate was analyzed. Daily precipitation data was not consistently available until the period of record from 1935 through 2015 was used. From this data set, all recorded precipitation events containing more than an inch of moisture (relatively large events) were identified and characterized as rain, snow, or a mix of rain and snow. Figure 3 number of days with precipitation events greater than one inch during each mon of record as well as the average monthly snow water equivalent (SWE) recorded at the Schofield TEL station from 1986 through 2016. The majority of large rain events occur in the summer, and late summer when the SWE is low, if will focus on the effects of thunderstorm events versus, a t should be noted that a lesser frequency event (e.g. 10 covered or saturated ground might have a similar runoff response to a larger event with drier antecedent conditions. Also, a rain on snow event may also have frozen gr with less infiltration but also not all of the snow pack will melt off during the rainfall event. snow events are of smaller magnitude. torm Days Recorded at Aspen Climate Stations versus Snow Water Equivalent (SWE) Recorded at the Schofield SNOTEL Site Page 20 understand when the large precipitation events are occurring in the City of Aspen, the daily 1979) and Aspen 1 SW Climate sistently available until From this data set, all recorded precipitation events containing more than an inch of moisture (relatively large events) Figure 3 shows the nth for the period of record as well as the average monthly snow water equivalent (SWE) recorded at the Schofield The majority of large rain events occur in the summer, and late summer when the SWE is low, if focus on the effects of thunderstorm events versus, and not t should be noted that a lesser frequency event (e.g. 10-year event) that have a similar runoff response to a larger a rain on snow event may also have frozen ground with less infiltration but also not all of the snow pack will melt off during the rainfall event. In torm Days Recorded at Aspen Climate Stations versus Snow Water ecorded at the Schofield SNOTEL Site P105 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 21 November 2017 Previous studies including the 2001 WRC study and the 2014 master plan update have applied a 2-hour storm duration rainfall for the 100-year design storm. Only local daily rainfall data are available, so more detailed rainfall duration analysis is not feasible within this scope of work. A one-hour point value is used to develop a 2-hour hyetograph for the return period storms. Using this rainfall duration is consistent with existing master planning in the City and with the current requirements for mudflood and mudflow analysis in Pitkin County. Although weather varies from the top of Aspen Mountain to the City, orographic variations are not accounted for in design storms, and average rainfall values are applied to all sub-basins without adjustment for elevation or drainage basin size. For consistency with previous studies and with the City’s past master planning, the 2-hour duration hyetograph based on the 1-hour point precipitation depth is applied in this study. Figure 4a shows rainfall hyetographs developed for this study using the 1-hour point precipitation depths from NOAA Atlas 14. The process for developing 2-hour hyetographs from one-hour point rainfall depths is described in the Rainfall Chapter of the Aspen Urban Runoff Management Plan (City of Aspen, 2014). 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0 0.5 1 1.5 2 2.5Rainfall Depth (in.)Time (hours) 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year Figure 4a. Hyetographs for 2-hour, 2-, 5-, 10-, 25-, 50- and 100-Year Rainfall Events P106 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 22 November 2017 For comparison purposes, a 12-hour, 100-year peak rainfall event was also simulated to evaluate a longer duration rainfall event. These longer duration events typically generate greater flood volumes than the short and intense thunderstorm events, which typically are associated with mudflow events. For the 12-hour, 100-year event, the SCS Type II hyetograph was applied (Figure 4b). Selection of a 2-hour duration storm for the 100-year design has important ramifications if this is used for flood hazard mapping and mudflow mitigation. The 2-hour duration has been used historically in Aspen because it mimics a convective summer thunderstorm and has relatively high, short-duration rainfall intensities (25% of 1-hour depth in 5-minute interval) that produce higher runoff peaks than longer duration storm events. It is also a duration that is of a similar scale to the hydrologic response time of the mountain watersheds above Aspen. A 2-hour storm duration is also consistent with the City’s Urban Runoff Management Plan and is similar to the design storm used by UDFCD but adjusted for Aspen precipitation frequency. The 2-hour duration storm will typically have higher peak discharges at the watershed points of concentration associated with the more intense rainfall. A longer duration storm will have more volume and result in large areas of urban flood inundation. A short duration storm floodwave with a high peak discharge will attenuate quickly when as peak flow progresses as unconfined flooding through the urban area below the fan apex or basin point of concentration. Mitigation measures that are focused on conveying peak discharges through developed areas can be conservatively designed with a 2-hour high peak flow. Storage-based mitigation to reduce the flood inundation or the mudflow hazard area should be based on conservative estimates of volumes in the hydrographs, as the peak discharges are not as important in this case. P107 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 23 November 2017 3.3 FLO-2D Model Development FLO-2D is a two dimensional (2-D) hydraulic model that was developed to perform both clear- water and hyperconcentrated sediment flow routing in channels and/or on alluvial fans with an unconfined flow path. It is also a hydrologic model simulating rainfall and infiltration. The model uses a finite volume numerical algorithm that simulates both sub- and super-critical flows using a full dynamic wave approximation of the momentum equation. A central difference routing scheme with eight potential flow directions is used to simulate the progression of the flood wave hydrograph over a system of square grids. The FLO-2D model contains several components that are used to represent and model the complex topography and processes including: channel-floodplain flow exchange, loss of storage due to buildings, flow obstructions, hydraulic structures, street flow and sediment transport in addition to mudflows. 3.3.1 FLO-2D Grid A FLO-2D flow domain grid system was assigned to cover Aspen Mountain and the area of the City of Aspen project area between the base of the mountain and the Roaring Fork River (Figure 1). The model has 165,214 grid elements that are 20 feet wide (400 ft2). For comparison, the 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0 2 4 6 8 10 12Rainfall Depth (ft)Time (hours) Figure 4b. 12-hour, 100-Year Rainfall Hyetograph P108 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 24 November 2017 WRC (2001) FLO-2D model had a grid resolution of 100 feet (10,000 ft2); therefore, the updated model has 25 times resolution (10,000ft2/400ft2=25) of the WRC (2001) model. Improvements in the FLO-2D model and computer processing speed allows significant increases in grid resolution, however, there is always a balance between grid size, efficiency in developing the model and evaluating the model output and runtime. The 20-foot grid size applied to the updated model provided an excellent balance. The Grid Developer System (GDS), a FLO-2D pre-processor program, was used to assign elevations to each of the grid elements by interpolating a Digital Terrain Model (DTM) developed from topographic mapping provided by the City. The topographic mapping consists of 1- and 10-foot interval contour mapping. The 1-foot interval contour mapping data covers the City and the lower portion of Aspen Mountain, while the 10-foot interval contour mapping covers the upper part of the mountain. The 1- and 10-foot mapping was used to create a DTM with a 3-foot resolution. The City of Aspen is at an elevation of approximately 7,900 feet, and the elevation at the top of Aspen Mountain is approximately 11,200 feet. The topography and coordinate system is referenced to the NAD1983 State Plane Coordinate System (Colorado Central) and the National Geodetic Vertical Datum (NGVD) of 1929. 3.3.2 Channel and Overbank Roughness Values Rainfall runoff from the watershed is simulated as shallow overland flow, which is typically slow because the roughness elements are on the order of the flow depth. To correctly model the time of concentration to locations of flow collection, the “rill and gully” channel component was applied in FLO-2D. Multiple small channels were modeled in the major sub-basins, including Copper, Spar, Vallejo and Pioneer Gulches. The channel sizes and alignments were determined based on aerial photography and field observations. A uniform channel roughness value of 0.06 was assigned to represent all of the channels in the model. The longest gully channels are located on Spar and Copper Gulches. The Spar Gulch channel extends from near the top of the mountain to the junction with Copper Gulch, a distance of approximately 6,400 feet. Copper Gulch channel extends 4,800 feet upstream from its intersection with Spar Gulch. Downstream from the junction of Spar and Copper Gulches, the sub-basin is referred to as Lower Spar Gulch. The Bell Mountain lift is located approximately 2,500 feet downhill from the junction of Copper and Spar Gulches; the area between the Bell Mountain lift and the junction is known as Spar Narrows. The ground in the vicinity of the base of the Bell Mountain Lift is relatively flat, and a well-defined channel extends approximately 150 feet upstream of the lift base. Downstream from the base of the Bell Mountain lift, flow is conveyed down Lower Spar Gulch and emerges in a residential neighborhood near Powder Lane. The Summer Ditch is located in the upper area of Vallejo Gulch. The purpose of the ditch is to collect and convey flows originating in the upper basin into the neighboring Keno Gulch. Flow into Keno Gulch enters Castle Creek, which flows in a northerly direction along the western edge of the City. The Summer Ditch is intended to reduce the flows in Vallejo Gulch. Summer ditch is approximately 5 to 10 feet wide and 0.5 to 2 feet deep based on field observations. For the P109 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 25 November 2017 purposes of this analysis and to be conservative, it was assumed that Summer Ditch was blocked and flow remains in Vallejo Gulch and is conveyed into urban area. Rainfall runoff from the watershed is simulated as shallow overland flow, which is typically slow because the roughness elements are on the order of the flow depth. Manning’s roughness n- values were established by delineating polygons from the 2015 aerial photography that represent zones with similar roughness characteristics, using ArcGIS, a Geographic Information Software (GIS) program. The individual zones were visually identified based on vegetation type and density, and land-uses that include grassland/ski areas, forested areas, mine tailings, and urbanized areas. The specific values were assigned based on guidance in the FLO-2D manual (FLO-2D, 2015). Land use mapping for the study area is shown in Figure 5. The n-values assigned to these areas ranged from 0.02 for roads which are relatively smooth, to 0.40 for dense forest and Mine Tailings that typically have a very rough surface. Land classifications and associated n-values are provided in Table 4. Table 4. Land Classifications and Overland Manning’s n-Values Land Use n-value Urban/Structures 0.04 Roads 0.02 Mine Tailing 0.40 Grassland/Ski Runs 0.20 Light Forest 0.30 Medium Forest 0.35 Dense Forest 0.40 3.3.3 Infiltration Parameters Soil infiltration losses in FLO-2D were modeled using the same parameters that WWE applied in CUHP when SWMM models were updated in 2014 (WWE, 2014). CUHP uses Horton’s soil- infiltration (Horton, 1933), which consists of an initial soil loss (fi), a final soil loss (f0), and a decay coefficient (a) (Table 3). The CUHP program lists the Horton parameters for a range of NRCS Hydrologic Soil Groups (UDFCD, 2005). In general, the Horton parameters vary with soil type (for example clay, loam, sand, bedrock) and degree of saturation (dry, moist). The soil infiltration properties applied to each sub-basin in the 2014 SWMM were applied, as best as possible, to the elements in the FLO-2D model based on Hydrologic Soil Group Mapping shown in Figure 6. 3.3.4 Other Model Components Other model parameters typically applied to FLO-2D models include width reduction factors (WRFs), area reduction factors (ARFs), street elements and levees. The WRF’s and ARF’s are assigned to the FLO-2D grid elements to represent the blockage of flow paths and reduction in P110 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 26 November 2017 storage that mostly occur due to the presence of buildings. Width reduction factors are applied to represent the blockage of flow through the side of an element, while ARFs represent the loss of floodplain storage volume due to the buildings. For example, a wall might obstruct 40 percent of the flow width of a grid element side and a building could cover 75 percent of the same grid element. The City provided an ArcGIS shapefile of the building outlines and streets. The building shapefile was used to interpolate the ARF’s and WRF’s to grid elements using the FLO- 2D GDS. Shallow street shallow flow can be simulated with FLO-2D using the grid element elevation and roughness. The elements representing the streets were assigned Manning’s n-values of 0.02 and no infiltration. Flow can be supercritical in the steep streets of Aspen. The levees component of FLO-2D can be used to represent walls, barriers and embankments as well as berms and levees. Levees were assigned to represent a wall protecting a house on the southern end of South Mill Street and walls on either side of the channel exiting Vallejo Gulch, located to the east of the Mountain Queen Condos and north of Summit Street. The walls can be overtopped or can collapse by assigning the levee failure option. The model outflow from the grid system for overland flow was assigned to a reach along the Roaring Fork River. FLO-2D has a storm drain component. The updated SWMM (WWE, 2014), incorporates a pipe network to convey flows between subbasins, with the majority of the pipes representing the storm water drainage system in City. The storm component was not considered in the updated FLO-2D Aspen model, presuming that the storm drain inlets would be plugged by the mudflow and debris frontal waves. 3.4 Comparisons between FLO-2D Hydrology and CUHP/SWMM Hydrology The FLO-2D model was run for the 2-hour duration, 2-, 5-, 10-, 25-, 50- and 100-year rainfall return period events over a 6-hour simulation time. The FLO-2D model output was compared to the updated SWMM results at key locations (concentration points) located along the major drainage paths, specifically at locations where mudflows may originate, indicated on Figure 7. The three major drainage basins that debris flows likely originate from are Spar Gulch, Vallejo Gulch, and Pioneer Gulch. Table 5 provides comparisons of peak discharges from SWMM and FLO-2D at key locations in Spar, Vallejo, and Pioneer Gulches, and Table 6 provides volume comparisons. In general, the FLO-2D model predicted peak discharges and volumes that were very comparable to the peak discharges from the 2014 CUHP/SWMM modeling. These differences are explained by physical factors as well as differences between one- and two-dimensional modeling approaches. P111 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 27 November 2017 Figure 5. Land Use Zones Used to Assign Overland Flow Roughness Parameters in FLO-2D P112 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 28 November 2017 Figure 6. Hydrologic Soil Group Mapping of Project Area used to Assign Infiltration Parameters P113 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 29 November 2017 Figure 7. Locations of FLO-2D Channels and Comparison Points with 2014 SWMM from Master Plan Update P114 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 30 November 2017 Table 5. Comparison of Unit Peak Discharges from SWMM and FLO-2D at Key Locations in Spar, Vallejo and Pioneer Gulches for 100-year Return Period Location SWMM Node FLO- 2D Node Drainage Area (acre) SWMM Peak Flow (cfs) FLO- 2D Peak Flow (cfs) SWMM Unit Flow (cfs/acre) FLO-2D Unit Flow (cfs/acre) Copper + Spar (Sub-basins 14,22,23,24) 114div 5 580 243 234 0.42 0.40 Vallejo (Sub-basin 16) 116div 6 107 65 45 0.61 0.42 Pioneer (Sub-basins 20,21) 120div 8 131 78 109 0.60 0.83 Average 0.54 0.55 Table 6. Comparison of Modeled Volumes from SWMM and FLO-2D for 100-year Return Period Location SWMM Node FLO-2D Point Drainage Area (acre)* SWMM Vol (AF) FLO-2D Vol (AF) Differen ce Copper + Spar (Sub-basins 22,23,24) 122 3 461 17.1 20.8 21% Copper + Spar (Sub-basins 14,22,23,24) 114div 4 580 21.3 23.2 9 Copper + Spar (Sub-basins 14,22,23,24) 114div 5 580 21.3 16.5 -23% Vallejo (Sub-basin 16) 116div 6 107.1 4.5 2.7 -39% Pioneer (Sub-basin 21) 121 7 79.9 2.1 3.3 56% Pioneer (Sub-basins 20,21) 120div 8 131.3 4.0 5.0 24% *Drainage area reported at the SWMM node. The area at FLO-D Point 4 is approximately 30 acres smaller compared to the area at Point 5. The FLO-2D model predicts flow transfer between Points 4 and 5, hence the difference in predicted volume. The timing of the hydrographs and the shape of the hydrographs show good agreement between the SWMM and FLO-2D model in Copper, Spar and Pioneer Gulches. Figure 8 shows an example comparison of 100-year hydrographs for Copper and Spar Gulch. As Figure 9 shows, in Vallejo Gulch, the hydrograph predicted by the FLO-2D model lags the SWMM hydrograph by approximately 0.5 hours. The peak flow rate and volume are also much lower using the FLO-2D model. Some of the differences in timing, rates, and volume can be attributed to the differences between the 1-D and 2-D models. The SWMM model routs flows from CUHP, which calculates flows from subbasins using representative area, slope, length to centroid, and basin roughness. FLO-2D routes the flow across each element and applies spatially varied Manning’s n roughness values. These differences likely effect the timing of the runoff, P115 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 31 November 2017 and the longer travel time in FLO-2D allows for more infiltration to occur, lowering the peak response and reducing the overall volume of runoff. Figure 8. Comparison on SWMM and FLO-2D Hydrographs for Node 114div, Copper and Spar Gulch for the 100-year rainfall event Figure 9. Comparison on SWMM and FLO-2D Hydrographs for Node 116div, Vallejo Gulch for the 100-year rainfall event 0 50 100 150 200 250 300 0 1 2 3 4Flow (cfs)Time (hours) SWMM Node 114div FLO-2D Point 5 0 10 20 30 40 50 60 70 0 1 2 3 4Flow (cfs)Time (hours) SWMM Node 116div FLO-2D Point 6 P116 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 32 November 2017 Some of the factors resulting in differences between FLO-2D and SWMM include: 1. There is some flow transfer that occurs from Spar Gulch to Vallejo Gulch in the vicinity of the Bell Mountain lift. The topography in the vicinity of the lift base and nearby roads is reasonably flat; therefore, at higher modeled flow depths, the FLO-2D model transfers flow from Spar Gulch into the adjoining sub-basins in Vallejo Gulch. 2. In Vallejo Gulch, the FLO-2D model predicts significantly lower flows compared to the SWMM. At the 100-year event, the FLO-2D model predicts peak flow of 45 cfs at Point 6 compared to 65 cfs from SWMM. There are a couple of factors that explain why the Vallejo Gulch runoff is lower compared to the other sub-basins. The Vallejo Gulch sub- basin is located entirely within Soil Class B, and therefore has reasonably high infiltration losses. Other factors contributing to lower runoff values in Vallejo Gulch are: (1) the sub- basin has relatively greater proportion of dense forest areas compared to the other sub- basins, which increases the runoff time, and correspondingly, increases infiltration, and (2) unlike the other sub-basins, there is no main flow path in the upper part of the sub- basin to concentrate the flows. For example, the Spar Gulch and Pioneer Gulch sub- basins have well defined flow paths, which tend to concentrate flow, increase the peak discharge and causes the peak flow to occur sooner. The absence of a well-defined flow path in Vallejo Gulch, particularly in the upper part of the basin, increases the runoff time and reduces the peak flows. 3. The FLO-2D model applies spatially varied Manning’s n roughness values that affect the timing of the runoff. 4. SWMM and FLO-2D model are fundamentally different rainfall-runoff models; SWMM is a two-plane kinematic wave model, typically best suited for urban areas. FLO-2D is based on a dynamic wave versus SWMM’s use of a kinematic wave, which leads to longer travel times in FLO-2D, and more infiltration. There is no floodwave attenuation in the SWMM kinematic wave model because there is no diffusive term (pressure term) in the momentum equation. The shape of the hydrograph cannot vary from one point of concentration to the next without floodplain storage. It will only vary with losses. The FLO-2D model provides a more detailed representation of the rainfall-runoff process and likely produces a more realistic representation of runoff peak rates and volumes for both water flooding and mudflows. 3.5 Red Butte Mudflows and Mud Floods In the original scope of work for the Aspen Mud and Debris Flow Study, the Project Team planned to evaluate the hydrology of Red Butte using similar analytical methods that were used to evaluate Aspen Mountain and the commercial core of the City, including development of a FLO-2D model. Following the field investigation and after obtaining additional information on geologic hazards in the Red Butte area, it was apparent that the Red Butte area is not an alluvial fan. As a result, the City and Project Team agreed to perform simplified hydrologic analysis P117 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 33 November 2017 using the Rational Method to quantify peak flow rates and sediment transport potential for this area. The Project Team performed Rational Method calculations for a range of return periods for multiple sub-basins in the Red Butte area. The steep terrain results in steep, rocky rills and shallow gullies running down the butte toward developed areas below in milder sloped areas. The hydrologic response is rapid given the steep slopes and material on steep slopes is erodible during intense rainfall and runoff. The Project Team assigned sediment bulking factors from 20% to 45% based on rainfall-runoff potential and sediment limitations in larger flood events. Table 7 provides recommended unit rates of runoff, clearwater and bulked for sediment, for the Red Butte area. Table 7. Red Butte Unit Rates of Runoff including Bulking Factor Return Period Clearwater Peak Discharge (cfs) Bulking Factor Bulked Peak Discharge (cfs) 2-Year 0.2 20% 0.3 5-Year 0.6 20% 0.7 10-Year 1.1 45% 1.5 25-Year 1.8 45% 2.6 50-Year 2.4 20% 2.9 100-Year 3.0 20% 3.6 The unit rates for bulked peak discharges in Table 7 can be used by planners and engineers to calculate conveyance requirements based on the tributary area above their design points. For final design, the engineer should independently perform Rational Method analysis following the procedures in the Aspen Urban Runoff Management Plan and apply the bulking factors in Table 7 in the analysis. A technical memorandum documenting the Rational Method analysis for Red Butte is provided as Appendix C. 3.6 Summary and Recommended Hydrologic Approach While there are some notable differences, comparison of the modeled peak flows, hydrograph timing and hydrograph shape indicate reasonable agreement between the FLO-2D and SWMM in Spar Gulch and Pioneer Gulch. In Spar Gulch, the SWMM and FLO-2D models predicted peak flows of 243 and 234 cfs, respectively for the 100-year rainfall event. The differences are due to the FLO-2D model predicting flow transfer between sub-basins that occurs near the base of the Bell Mountain Lift. In Pioneer Gulch, the SWMM and FLO-2D models predict 78 and 109 cfs for the 100-year event. The peak flow comparisons for the 2- through 50-year peak rainfall events show similar trends, in good agreement between the SWMM and FLO-2D model results. In Vallejo Gulch, the FLO-2D model predicts peak discharge of 45 cfs compared to 65 cfs from the SWMM for the 100-year event; the differences in Vallejo Gulch is likely because: (1) the sub-basin is located in area with comparatively high infiltration losses, (2) there is a P118 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 34 November 2017 proportionately high amount of dense forest in the sub-basin, and (3) unlike the Pioneer and Spar Gulches, there is no clearly defined flow path, and (4) differences between the 1- and 2-D modeling procedures. In general, differences between the two models arise from predicted volumes and the conceptual differences flood routing in the flood routing approach. The volumetric differences can be attributed to runoff losses over different sub-basin surface areas. The flood routing discharges are different because the flow velocity equations have conceptual difference methods. The SWMM model hydraulics are based on a lumped parameter approach based on average values of data input such as slope and roughness. FLO-2D uses a discretized solution in time and space to predict floodwave progression. As a result, the SWMM travel times to points of concentration tend to be shorter because kinematic wave routing does not predict floodwave attenuation whereas the FLO-2D full dynamic wave momentum equation computes floodplain storage and attenuation. In addition in the SWMM model the average roughness conditions are typically underestimated and the flow is numerically conveyed as conceptual concentrated flow resulting in higher velocities that also reduce the infiltration losses. The FLO-2D model has a physically based approach with a higher proportion of shallow overland flow. It also has checks and adjustments using a limiting Froude number to ensure that the floodwave velocities are reasonable. Overall the unit runoff predicted by the FLO-2D model tends to fall within the range of results determined in the other regional studies. Based on this analysis, the Project Team and City agreed that it was reasonable to model hydrology in FLO-2D for the purpose of developing hydrology for the mudflow and mud flood analysis. 4.0 FLO-2D MUDFLOW ANALYSIS The FLO-2D Aspen rainfall-runoff model was expanded to perform mudflow simulations for the existing conditions and to evaluate the potential effects of a wildfire in the drainage basin. The modifications included applying the clearwater flow and sediment hydrographs at four locations where mudflow would be generated. The mudflow model was run using the same parameters as the clearwater simulations, including the rainfall and infiltration parameters to simulate the overland runoff. Rainfall and the subsequent overland runoff were simulated to account for potential dilution of the mudflow. The rainfall was only applied to the areas downstream from the four inflow hydrograph locations to prevent double accounting of the overland runoff. The mudflows simulations were conducted for the following scenarios: 1. Existing conditions for the 2-, 25- and 100-year peak rainfall events and the 12-hour, 100- year peak rainfall event. 2. Four wildfire scenarios for the 2- and 25-year peak flow events. The model output for the existing conditions was used to develop hazard mapping that depicts the depth and extent of flooding, the maximum flow velocities and the hazard potential for each event. The mudflow mapping for the 100-year Existing conditions event was compared to the mapping developed by WRC (2001). The model output for the 2- and 25-year existing conditions events was used to perform an economic analysis to assess the costs associated with: (1) P119 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 35 November 2017 removing debris from the streets, cleaning out the storm drain system and (3) damage to buildings. The model output for the four wildfire scenarios was used to develop depth inundation and hazard potential mapping for the 2- and 25-year return period events. 4.1 Mudflow Characterization and Processes Hyperconcentrated sediment flows (mudflows and mud floods) are part of a continuum in the “physics of flowing water and sediment movement that ranges from clear water flow to mass wasting processes (landslides)” (SLA and O’Brien, 1989). In general, the sediment transport characteristics in the continuum range from suspended and bed load transport in water floods to mass wasting in landslide events. The National Research Council Committee (NRC, 1982) proposed four categories to delineate this continuum: water floods, mud floods, mudflows, and landslides (Table 8). The bounds of each of these categories can be approximated based on the fluid properties, and in particular by the sediment concentration (by volume) of the fluid (Figure 10). The sediment concentration of fluid, CV, is defined as the ratio of the sediment volume to the water volume and is given by: CV = Volume of Sediment / (Volume of Water + Volume of Sediment) The continuum indicates that water floods are mostly comprised of water with some sediment (low concentration of sediment), whereas, landslides are mostly comprised of bulk sediment with some water (high concentration of sediment). The concentration of the sediment is an important component in determining the physical processes that govern the behavior of the fluid-sediment mixture in each of these categories. For example, the flow characteristics of a mud flood are dominated by the turbulent and viscous forces within the fluid matrix, whereas, movement of a landslide is dominated by the dispersive stresses and particle friction. This study focuses on the sediment transport characteristics of the mud flood and mudflow categories; however, a brief description of the sediment transport characteristics of the water floods and landslide events is presented for the purpose of describing the bounding categories. Flood flows generally have sediment concentrations of less than 20-percent (by volume). They are essentially water floods with high bed load and suspended loads where the bed load may be affected by the high concentration of suspended load (i.e. fine sediment wash load). The sediment transport characteristics of water floods are modeled using conventional bed-load and suspended load formulas and methodologies. Landslides generally have sediment concentrations greater than 55 percent (by volume) and are considered as bulk solid movement as opposed to fluid motion. Landslides may range from slow moving earth flow and creeping soil masses to rapid rotation or slippage failures. Hyperconcentrated sediment flows are defined as flood events with sediment concentrations that range between approximately 20 and 55 percent by volume: however, the sediment concentration for a given event is generally considered to be between 20 and 45 percent (O’Brien, 2004). The fine sediment concentration (silt, clay and fine sands in the fluid matrix) controls the properties P120 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 36 November 2017 of the fluid, including, viscosity, density, and yield stress. Mudflows are non-Newtonian and they have much higher viscosities and densities compared to water flows. These properties result in mudflows having significantly slower velocities compared to water floods on the same slope. The fine sediments increase the density of the fluid matrix, which increases the buoyancy of sediments thereby creating conditions that allow gravel to boulder-sized material to be transported near the flow surface by mudflows. The yield stress is a measure of the internal fluid resistance to flow and affects both the initiation and cessation of flows. The sediment matrix of a hyperconcentrated flow is non-homogeneous and the sediment properties change significantly as they flow down steep watershed channels or across alluvial fans. As the mudflow moves over the alluvial fan, dewatering of the fluid matrix can occur by infiltration and separation from the surface flow. This may further increase the concentration of the hyperconcentrated sediment flows and alter the transport characteristics of the flow. “Almost all hyperconcentrated sediment flows are fully turbulent, unsteady and non-uniform, and are characterized by surging, flow cessation, blockage, and roll waves” (SLA and O’Brien, 1989). During a mudflow event, the average sediment concentration over the duration of the hydrograph generally ranges between 20 and 35 percent by volume with peak concentrations approaching 45 percent (Table 8 and Figure 10). Large flood events such as the 100-year flood likely contain too much water to produce a viscous mudflow event, and therefore, are generally considered to be mud floods. Lower recurrence interval rainfall events such as the 10- or 25-year return period storm may have a greater propensity to create viscous mudflows. Most watersheds with a history of mudflow events will eventually develop a sediment supply in the channel bed such that even relatively small rainfall-runoff storms may generate mudflow surges. In general, mudflows have a distinct pattern of flood evolution. Initially, clear water flows from the basin rainfall-runoff may arrive at the fan apex. This may be followed by a surge or frontal wave of mud and debris (40- to 55-percent concentration by volume). When the peak arrives, the average sediment concentration generally decreases to the range of 30 to 40 percent by volume. On the falling limb of the hydrograph, the sediment concentration decreases due to the reduced availability of sediment, however, surges of higher sediment concentration may occur. P121 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 37 November 2017 Table 8. Mudflow Behavior as a Function of Sediment Concentration (FLO 2D Manual, 2015) Sediment Concentration Flow Characteristics by Volume by Weight Landslide 0.65 - 0.80 0.83 - 0.91 Will not flow; failure by block sliding 0.55 - 0.65 0.76 - 0.83 Block sliding failure with internal deformation during the slide; slow creep prior to failure Mudflow 0.48 - 0.55 0.72 - 0.76 Flow evident; slow creep sustained mudflow; plastic deformation under its own weight; cohesive; will not spread on level surface 0.45 - 0.48 0.69 - 0.72 Flow spreading on level surface; cohesive flow; some mixing Mud Flood 0.40 - 0.45 0.65 - 0.69 Flow mixes easily; shows fluid properties in deformation; spreads on horizontal surface but maintains an inclined fluid surface; large particle (boulder) setting; waves appear but dissipate rapidly 0.35 - 0.40 0.59 - 0.65 Marked settling of gravels and cobbles; spreading nearly complete on horizontal surface; liquid surface with two fluid phases appears; waves travel on surface 0.30 - 0.35 0.54 - 0.59 Separation of water on surface; waves travel easily; most sand and gravel has settled out and moves as bed load 0.20 - 0.30 0.41 - 0.54 Distinct wave action; fluid surface; all particles resting on bed in quiescent fluid condition Water Flood < 0.20 < 0.41 Water flood with conventional suspended load and bed load P122 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 38 November 2017 Figure 10. Classification of Hyperconcentrated Sediment Flows (Modified from FLO-2D Manual, 2015) 4.2 Flood Hazard Index A flood hazard index (FHI) was used to identify potential hazard areas for each of the modeled scenarios. The FHI contains three hazard levels (High, Medium and Low) that were categorized based on event intensity. A description of the danger to people and potential structural damage for each Hazard Level is defined in Table 9. P123 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 39 November 2017 Table 9. Hazard Level Descriptions (from FLO-2D Guidelines on Hazard Maps, 2006) Two methods are applied for calculating the FHI, one for mudflows and the other for clearwater floods and mud floods. The mudflow intensity criteria are applied to the 25-year debris flow event and the clearwater flood intensity criteria are applied the 2-year and 100-year mud flood events. Both methods define the event intensity (EV) in terms of a combination of the sum of the maximum water depth (h) and the product of depth (h) and velocity (v) (Table 10a and Table 10b): EV = h+vh where: h = flow depth v = velocity Both methods enable high intensities to be related in terms of depth independently of velocity. Delineation of the three intensity categories was conducted for the floodplain elements using the HAZARD module in the FLO-2D Mapper program and the FLO-2D model output. The range of hydraulic intensities are user-selected. The FHI values were used to develop hazard mapping. Table 10a. Event Intensities for Mudflows (Modified from OFEE et al., 1997) Mudflow Hazard Index Maximum Depth (h) (ft) Logical Operator Maximum Depth (h) times maximum velocity (v) (ft2/s) High h >= 3.3 OR vh >= 3.3 Medium 0.7 <= h <3.3 AND 0.7 <= vh <3.3 Low 0.7 <= h <3.3 AND vh < 0.7 Table 10b. Event Intensities for Water Floods Mudflow Hazard Index Maximum Depth (h) (ft) Logical Operator Maximum Depth (h) times maximum velocity (v) (ft2/s) High h >= 4.9 OR vh >= 4.9 Medium 1.6 <= h <4.9 OR 1.6 <= vh <4.9 Low 0.3 <= h <1.6 AND 0.3 <= vh <4.9 Mudflow Hazard Index Map Color Description High Red People are in danger both inside and outside their houses. Buildings can be destroyed. Medium Orange People are in danger outside their houses. Buildings may suffer damage and possible destruction depending on construction materials. Low Yellow Danger to persons is low or non-existent. Buildings may suffer little damage, but flooding or sedimentation may affect building interiors. P124 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 40 November 2017 P125 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 41 November 2017 4.3 Mudflow Hydrographs Determination of the recurrence interval of hyperconcentrated sediment flow events is challenging due to the complex relationships between rainfall (duration and intensity), antecedent soil moisture conditions and availability of sediment. Antecedent moisture conditions can influence whether a mudflow event will occur for a given rainfall event. For example, a 25- year rainfall event may initiate a mudflow if the soils are saturated, whereas the same 25-year rainfall event will not initiate a mudflow if the soils are not saturated. Antecedent moisture conditions can also effect soil erosion, bank and slope stability, and the magnitude and timing of runoff. The availability of stored sediment in the basin affects the magnitude and characteristic of the hyperconcentrated sediment flow events. In addition, the time elapsed since the last major flow event also affects the quantity of sediment stored in the channel and watershed, which is available to produce a mudflow. Given that the last major event was in 1964 (approximately 50 years ago), the erodible nature of the sediment in the channels, gullies, along roads and in mine tailings as well as field observations of the basin, indicates there is sufficient sediment stored in the basin to create another event, given the right localized rainfall event and antecedent soil conditions. Given the complexity of assigning a return interval to mudflow events, the frequency of a mudflow was conservatively assumed to be equal to the probability of the exceedance for the rainfall. This assumption is reasonable considering that the probability of exceedance of mudflows can be no greater than that for the rainfall. It is possible that a 25-year return period flood or rainfall may generate a 100-year return period mudflow event. The clear-water inflow hydrographs are input to the FLO-2D model at four specified locations identified as being likely starting locations of debris flows. The four locations include Lower Spar Gulch (Point 4), Vallejo Gulch (Point 6) and two locations in Pioneer Gulch (Points 8 and 19) (Figure 7). The clearwater hydrograph at each of these locations (discussed in Section 3) is bulked with sediment using a representative sediment concentration (by volume, CV) hydrograph. The total volume of the water and sediment in a mudflow can be determined by multiplying the clearwater volume by the bulking factor, where the bulking factor is defined by: BF = 1/(1 - CV) For example, a sediment concentration of 10 percent (CV=0.10) creates a bulking factor of 1.11, indicating the flood volume is 11 percent greater than if the flood was considered to be only water. The sediment concentration hydrograph was developed to represent the likely variation in sediment concentration at four source locations and for the 3 events based on field observations, recommendations provided in the FLO-2D manual and following discussion with Dr. Jim O’Brien (personal communication November 2016). For the 2-year event, the sediment concentration was set at 20 percent at the four source locations. This sediment concentration is intended to represent “nuisance” flooding to the City. For the 25-year event, sediment P126 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 42 November 2017 concentration hydrographs were developed for Lower Spar Gulch, Vallejo Gulch and the eastern location in Pioneer Gulch have the following characteristics (Figures 11 through 14): 1. The initial rising limb and the last part of the recessional limbs of the hydrographs have a sediment concentration of 20 percent, which corresponds to the minimum concentration for a mudflow. 2. The steep rising limb of the hydrograph is bulked to a maximum concentration of 45 percent to simulate the frontal wave of the mudflow. The peak of the sediment concentration hydrograph occurs 6 minutes before the peak of the clearwater hydrograph. 3. The sediment concentration at the peak of the clearwater hydrograph is less than the peak sediment concentration in order to simulate water dilution. 4. The average sediment concentration over the period of the hydrograph is approximately 30 percent. At the western location in Pioneer Gulch (Point 19), a constant concentration of 0.20 was applied. This location has a relatively small contributing basin and is well vegetated, and therefore, has a low chance of developing a high concentration mudflow event (Figure 14). Lower Spar Gulch generates the largest sediment volumes of the four locations with a total sediment volume of 5.6 acre-feet (AF) (Table 11). A volume estimate was made to confirm the potential for generating the sediment volume from the water shed. Three sources of sediment contribute to the debris flow: channel erosion along Lower Spar Gulch, side slope (bank and hillslope failure) and sediment from upstream basins (Table 12). The channel length along Lower Spar Gulch from the junction to its terminus is approximately 4,000 feet. Assuming the debris flow erodes a channel with average width of 15 feet and depth of 3 feet, this would result in approximately 4.1 AF of sediment. The hillslope sloughing volume was estimated assuming a wedge of sediment 550 feet long – based on aerial photography measurements – and a width and height of 5 feet. The hillslope failure erosion contributes approximately 0.2 AF. The sediment supply from the upstream drainage basins was assumed to be 5-percent of the clearwater volume (0.8 AF). The total estimated volume of available sediment is 4.0 AF, which is slightly higher than 5.1 AF applied to the mudflow hydrograph for Lower Spar Gulch (Table 11). Therefore, the sediment hydrograph developed for Lower Spar Gulch is reasonable compared to the available sediment volume. The total sediment volume applied as input for the 25-year rainfall event at the Vallejo Gulch (Point 6) was 0.56 AF. At Pioneer Gulch, 1.22 AF of sediment was applied at Point 8 and 0.01 AF at Point 19 (Table 11). Large flood events such as the 100-year event may contain too much water to produce a viscous mudflow event. During infrequent flood events, the clear water volume is larger than during smaller events, but the available volume of sediment remains approximately the same. Therefore, the average sediment concentration is lower during large flood events compared to the smaller ones. P127 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 43 November 2017 For the 100-year event, the sediment concentrations in Lower Spar Gulch (Point 4) was set so the total sediment volume matched the 25-year event, which resulted in an average sediment concentration of 0.17 (Table 11). A constant sediment volume of 0.2 was applied to Vallejo Gulch, Pioneer Gulch and the eastern Pioneer Gulch (Table 11). In the FLO-2D model, the fluid properties of the mudflow are represented by yield stress and viscosity parameters. To select the fluid properties, an analysis of 7 source materials (soils) was conducted. The 7 materials are listed in the FLO-2D manual and were developed based on research conducted by Dr. O’Brien in the Aspen area. Based on the results of the analysis and discussion with Dr. O’Brien (personal communication, November 2016), the Aspen Natural Soil was selected to represent the yield stress and viscosity parameters (FLO-2D Manual, 2015 P128 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 44 November 2017 Figure 11. Clearwater (Q), Sediment Concentration (Cv) and Bulked Sediment (BF) Hydrographs in Lower Spar Gulch (Point 4) for the 25-Year Rainfall Event Figure 12. Clearwater (Q), Sediment Concentration (Cv) and Bulked Sediment Hydrographs (BF) in Vallejo Gulch (Point 6) for the 25-Year Rainfall Event 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 50 100 150 200 250 300 0 0.5 1 1.5 2 2.5 3 Sediment Conc. (by Vol.)Flow (cfs)Time (hrs) Q BF Cv 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 5 10 15 20 25 30 0 0.5 1 1.5 2 2.5 3 Sediment Conc. (by Vol.)Flow (cfs)Time (hrs) Q BF Cv P129 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 45 November 2017 Figure 13. Clearwater (Q), Sediment Concentration (Cv) and Bulked Sediment Hydrographs (BF) in Pioneer Gulch (Point 8) for the 25-Year Rainfall Event Figure 14. Clearwater (Q), Sediment Concentration (Cv) and Bulked Sediment Hydrographs (BF) in Pioneer Gulch (Point 19) for the 25-Year Rainfall Event. Note: Constant sediment concentration of 0.2 used for Point 19 due to well-vegetated condition and low-potential for generating significant sediment 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 10 20 30 40 50 60 70 80 90 0 0.5 1 1.5 2 2.5 3 Sediment Conc. (by Vol.)Flow (cfs)Time (hrs) Q BF Cv 0 0.05 0.1 0.15 0.2 0.25 0 0.5 1 1.5 2 2.5 3 3.5 0 0.5 1 1.5 2 2.5 3 Sediment Conc. (by Vol.)Flow (cfs)Time (hrs) Q BF Cv P130 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 46 November 2017 Table 11. Summary of Peak Clearwater Discharges, Clearwater Volumes and Bulked Sediment Volumes at the Four Debris Flow Locations for the 2-, 25-, 100- Year 2-Hour Rainfall Events and the 100-Year 12-Hour Rainfall Event Table 12. Estimated Sediment Volume Available for Transport in Lower Spar Gulch Event Peak Clearwater Flow (cfs) Clearwater Volume (AF) Bulked Volume (AF) Sediment Volume (AF) Average Conc. Lower Spar Gulch, Point 4 2-Year 4.6 0.3 0.3 0.06 0.20 25-Year 159.0 12.3 17.9 5.56 0.31 100-Year 329.5 23.2 28.9 5.69 0.20 100-Year SCSII 12-Hour 364.3 24.6 29.7 5.10 0.17 Vallejo Gulch, Point 6 2-Year 1.0 0.0 0.0 0.01 0.20 25-Year 17.8 1.2 1.8 0.56 0.32 100-Year 43.0 2.7 3.4 0.68 0.20 100-Year SCSII 12-Hour 47.9 2.8 3.5 0.68 0.19 Pioneer Gulch, Point 8 2-Year 0.8 0.0 0.0 0.01 0.20 25-Year 51.7 2.6 3.8 1.22 0.32 100-Year 109.0 5.0 6.2 1.25 0.20 100-Year SCSII 12-Hour 113.2 5.1 6.4 1.25 0.20 Pioneer Gulch, Point 19 2-Year 0.5 0.0 0.0 0.00 0.20 25-Year 2.5 0.0 0.1 0.01 0.20 100-Year 3.6 0.1 0.1 0.02 0.20 100-Year SCSII 12-Hour 3.8 0.1 0.1 0.02 0.20 Location Sediment Volume (AF) Channel 4.13 Sideslope 0.17 Upstream Drainage Basins 0.81 Total 5.12 P131 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 47 November 2017 4.4 Existing Conditions Model Results The existing conditions mudflow model was run for the 2-, 25- and 100-year peak rainfall events, and the model output was used to map and evaluate the extent, depth, velocity, and hazard potential. 4.4.1 Flood Inundation and Hazard Mapping 2-Year Peak Flow Event The model results for the 2-year peak show no discernable pattern of flooding in the City. The depth mapping shows isolated areas of flooding ranging up to 0.7 foot (Figure 15), which occur mainly near the base of the mountain. Note: the depth mapping shows values equal or greater than 0.1 foot. Depths less than 0.1 feet are not shown on the figures. The maximum flow velocities occur in the channels downstream from the four source locations with maximum values ranging up to approximately 1 ft/s (Figure 16). The flood hazard mapping indicates that approximately 0.11, 0.21 and 0.22 acres are classified as High, Moderate and Low hazard potential areas, respectively (Table 13). The hazard potential areas occur along Lower Spar Gulch at the base of the mountain and near the end of Powder Lane (Figure 17). 25-Year Peak Flow Event The model output for the 25-year peak flow event predicts extensive mudflow inundation across the City. In general, the mudflows follow the street alignment from the base of the mountain across to the Roaring Fork River (Figure 18). The mudflow originating from Lower Spar Gulch creates a significant amount of the total inundation. From the base of Lower Spar Gulch, the mudflow moves in an approximately northwest direction across to the Roaring River. The depth of flooding ranges from approximately 1 to 5 feet near the source area. Near the base of the mountain, the mudflow depths range up to 15 feet in the channel and up to 5 feet on the floodplain, with the maximum depths occurring on the upstream side of buildings and at topographically low points (Figure 18). The maximum flow velocities occur near the source of Lower Spar Gulch and along the channel with velocities ranging up to 15 fps (Figure 19). The velocities near the base of mountain range up to 10 fps, while the velocities in the streets range up to approximately 6 fps. Generally, the velocities in the street are higher than in the overbank areas due to their lower roughness values and concentration of the flow due to the confinement by the buildings. Maximum flow velocities in the overbank areas range from approximately 0.5 to 5 fps, with the majority of the velocities between 1 and 2 fps, which is equivalent to a slow walking speed and is consistent with observed velocities for mudflows. The hazard potential mapping shows that the majority of the high and moderate hazard potential areas are located: near the source areas, along the base of the mountain, along the upstream side of buildings, and in constricted areas generally formed by buildings (Figure 20). P132 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 48 November 2017 The hazard potential mapping indicates that approximately 5.3, 7.4 and 8.8 acres are classified as High, Moderate and Low hazard potential areas, respectively under the 25-year mudflow vent (Table 13). P133 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 49 November 2017 Table 13. Summary of Area (acres) of Low, Moderate and High Hazard Potential for the 2-Year, 25-Year, 100-Year 2-Hour and 100-Year 12-Hour Events Hazard Potential Event 2-Year 25-Year 100-Year, 2- Hour 100-Year, 12- Hour Combined 2-hr, 2-, 25- and 100- Year Low 0.22 8.8 35.6 30.2 37.2 Moderate 0.21 7.4 21.0 18.1 20.8 High 0.11 5.3 4.9 4.4 6.3 Total 0.5 21.5 61.5 52.7 64.3 100-Year, 2-Hour Peak Flow Event The model results for the 100-year, 2-hour peak flow event predicts similar flooding patterns across the City compared to the 25-year event, but with greater depths and larger extents. In general, the mud floods follow the street alignment from the base of the mountain across to the Roaring Fork River (Figure 21). The mud flood originating from Lower Spar Gulch contributes significantly to the total inundation. From the base of Lower Spar Gulch, the flooding moves in an approximately northwest direction across to the Roaring River. The depth of flooding ranges from approximately 1 to 5 feet near the source area. Near the base of the mountain, the flood depths range up to 15 feet in the channel and up to 5 feet on the floodplain, with the maximum depths occurring on the upstream side of buildings and at topographically low points. Some large flooding depths occur in the center of town at locations where the topography may be unusually low due to below ground level shopping areas. In general, the flow depths for the 100-Year, 2-hour event are similar to the 25- year event in the overlapping areas; however, the 100-year, 2-hour has greater flooding extents due to the larger volume and more fluid mixture. The maximum flow velocities occur near the source of Lower Spar Gulch and along the channel with velocities ranging up to 15 fps (Figure 22). The velocities near the base of mountain range up to 10 fps, while the velocities in the streets range up to approximately 6 fps. A comparison of the flooding extents for the 100-year, 2-hour event compared to the WRC (2001) mapping indicates some correlation with both sets of mapping show the areas of greatest depths occur in similar areas (Figure 23). The WRC (2001) mapping shows significantly larger extents of flooding. For example, the 2-foot contour interval from the WRC (2001) study, shown as a blue line, has much larger extents compared with the predicted inundation area from this study. The differences in area of inundation and depths are likely due to the differences in grid resolution, input flow hydrographs and because the WRC (2001) simulated a mudflow, whereas this study evaluated a mud flood event for the 100-year event. The mudflow event would have had more volume associated with the much higher concentrations. The hazard potential mapping shows that high and moderate hazard potential areas occur in similar areas predicted by the 25-year event (Figure 24). The model 100-year, 2-hour event predicts significant extents of low hazard areas extending across town to the Roaring Fork River. P134 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 50 November 2017 The hazard potential mapping indicates that approximately 4.4, 18.1 and 30.2 acres are classified using water flood intensity criteria as High, Moderate and Low hazard potential areas, respectively (Table 13). Figure 25 shows the combined flood hazard areas for the 2-hour, 2-, 25- and 100-year rainfall events. The hazard potential mapping for the combined years is 37.2, 20.8 and 6.3 acres for the High, Moderate and Low hazard potential areas, respectively (Table 13). 100-Year, 12-Hour Peak Flow Event The model results for the 100-year, 12-hour peak flow event predicts similar flooding patterns across the City compared to the 100-year, 2-hour peak flow event, but with greater depths and slightly larger extents (Figure 26). Similar to the 100-year, 2-hour event, the higher depths typically follow the street alignment from the base of the mountain across to the Roaring Fork River and a significant amount of flooding originates from Lower Spar Gulch. The predicted velocity patterns and magnitudes for the 100-year 12-hour event are similar to the 100-year, 2- hour event with maximum flow velocities occurring along Lower Spar Gulch with velocities ranging up to 15 fps (Figure 27). The velocities near the base of mountain range up to 10 fps, while the velocities in the streets range up to approximately 6 fps. The hazard potential mapping shows similar patterns to the 100-year, 2-hour event with hazard areas extending from the base of the mountain to the Roaring Fork River (Figure 28). The hazard potential mapping indicates that approximately 4.4, 18.1 and 30.2 acres are classified using water flood intensity criteria as High and Moderate hazard potential areas, respectively (Table 13). P135 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 51 November 2017 Figure 15. Predicted Maximum Depth for the 2-Year Rainfall Event, with 20% Sediment Concentration under Existing Conditions P136 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 52 November 2017 Figure 16. Predicted Maximum Velocities for the 2-Year Rainfall Event, with 20% Sediment Concentration under Existing Conditions P137 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 53 November 2017 Figure 17. Predicted Flood Hazard Index for the 2-Year, 2-Hour Rainfall Event, with 20% Sediment Concentration under Existing Conditions P138 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 54 November 2017 Figure 18. Predicted Maximum Depth for the 25-Year, 2-Hour Peak Rainfall Event, with 45% Sediment Concentration under Existing Conditions P139 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 55 November 2017 Figure 19. Predicted Maximum Velocity for the 25-Year, 2-Hour Rainfall Event, with 45% Sediment Concentration under Existing Conditions P140 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 56 November 2017 Figure 20. Predicted Flood Hazard Index for the 25-Year, 2-Hour Rainfall Event, with 45% Sediment Concentration under Existing Conditions P141 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 57 November 2017 Figure 21. Predicted Maximum Depth for the 2-Hour, 100-Year Event with Sediment Volume Equal to 25-Year Event under Existing Conditions P142 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 58 November 2017 Figure 22. Predicted Maximum Velocity for the 100-Year, 2-Hour Rainfall Event with Sediment Volume Equal to 25-Year Event under Existing Conditions P143 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 59 November 2017 Figure 23. Comparison of the Predicted Maximum Depth from the WRC (2001) Study (shown as contour lines) and the FLO-2D Model (shown as elements) for the 100-Year, 2-Hour Rainfall Event with Sediment Volume Equal to 25-Year Event under Existing Conditions P144 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 60 November 2017 Figure 24. Predicted Flood Hazard Index for the 100-Year, 2-Hour Rainfall Event with Sediment Volume Equal to 25-Year Event under Existing Conditions P145 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 61 November 2017 Figure 25. Combined Flood Hazard Index for the 2-, 25-, and 100-Year, 2-Hour Rainfall Event under Existing Conditions P146 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 62 November 2017 Figure 26. Predicted Maximum Depth for the 100-Year, 12-Hour Rainfall Event with Sediment Concentration of 20-percent P147 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 63 November 2017 Figure 27. Predicted Maximum Velocity for the 100-Year, 12-Hour Rainfall Event with Sediment Concentration of 20-percent P148 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 64 November 2017 Figure 28. Predicted Flood Hazard Index for the 100-Year, 12-Hour Rainfall Event with Sediment Concentration of 20-percent P149 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 65 November 2017 5.0 DAMAGE ASSESSMENT ANALYSIS A damage assessment analysis was performed to quantity the potential impacts to public and private property associated with the various return period flood events. Given the complexity of real estate and property values in Aspen, it was beyond the scope of this assessment to conduct a structure-by-structure or aggregated flood damage assessment for the hazard area. Initially, the City’s real estate specialists were contacted to evaluate the feasibility of estimating the structure value on a city block level. The objective would be to develop depth-damage tables to apply in the FLO-2D Mapper damage assessment tool. This effort was complicated by a number of factors including the extensive number of buildings, complexities of underground space in Aspen, differences in assessed and market values of properties, and the diverse types of buildings and businesses in Aspen. As an alternative approach, the City and the Project Team decided to prepare two types of estimates to assess damages. The first type of estimate is of municipal cleanup costs and timelines based on FLO-2D modeling scenarios. The second type of estimate was intended to address impacts to private development by using case studies of several buildings located within the mudflow hazard area. While the damage assessment presented in this report is limited in scope, the FLO-2D model results, which show maximum predicted depths and velocities, could be used by owners of buildings in affected areas to assess their own potential for damages in a mudflow or mudflood event. This type of assessment also could be used to evaluate costs and benefits of floodproofing or other structural mitigation measures to increase the level of protection from a mudflow or mud flood at the site or building level. 5.1 Impacts to Infrastructure As part of the FLO-2D output, the volume of sediment deposited in the parks and streets of Aspen was tabulated as shown in Table 14 and Figure 28. Table 14. Summary of Sediment Volume Deposited in Streets and Parks at the End of Simulation Rainfall Event Total Sediment Volume (yds3)1 Total Street Sediment Volume (yds3)2 Total Park Sediment Volume (yds3) Street Sediment Removal Total Cost Park Sediment Removal Total Cost Total Street + Park Sediment Removal Cost (Based on $60.83 (Based on $60.83 per cubic yard) per cubic yard) 2-Year, 2-hour 825 15.3 0 $933 $7 $940 25-Year, 2-hour 31,632 4,154 3,169 $252,681 $192,785 $445,467 100-Year, 2-hour 44,912 7,465 7,157 $454,106 $435,378 $889,485 100-Year, 12-Hour 27,752 3,688 4,347 $224,352 $264,421 $488,772 1 Total sediment volume is the amount that passes through the street during an event; the total street sediment volume is what is left behind, deposited on the street, after the event. 2Reported for elements greater the 0.1 foot. P150 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 66 November 2017 3 Storm drain cleanout costs were calculated for the 25-year 2-hour event. The lengths of storm drain affected by the 2-year, 2-hour, 100-year, 2-hour and 100-year, 12-hour events are similar. The time required for jetting depends on how full the pipes are and how densely packed sediment and debris are in the pipe. It is assumed that the 2-year clean out costs would be approximately 50% of the 25-year clean out costs due to less material in the pipes. For the 100-year mudflood events, the same jetting costs as the 25- year mudflow event is used since the length of pipe affected is similar. As with streets, it is unlikely that all storm drains would be affected by a single mudflow event – the more likely scenario is that a mudflow or mudflood would occur in one of the gulches on the mountain, affecting only a portion of the street and storm drain system in the City. In order to simplify the street cleanup cost estimate, the City of Aspen Streets Department provided targeted estimates for both general mud removal as well as costs for jetting the City’s stormwater system based on the areas of mudflow inundation identified during the 25-year peak flow event (Figure 29). The sediment removal costs were estimated by multiplying the volume of sediment remaining in the streets by a unit cost of approximately $60 that includes the use of front-end loaders, trucks to haul the sediment to the Pitkin County Landfill, as well landfill charges for the material. Similar to the quantification of street surface cleanup costs, the City of Aspen Streets Department provided estimates for the manpower, and vactor truck time required to jet the affected stormwater system. With the 25-year peak flow mudflow depths associated with each street identified, the length of underlying storm drain system for each street was also quantified. The City’s Street Department estimates that 30 feet of pipe can be jetted per hour, at a cost of $240 per hour. This results in a total cleanup cost for the 25-year mudflow event of approximately $88,000. As with streets, it is unlikely that all storm drains would be affected by a single mudflow event – the more likely scenario is that a mudflow or mudflood would occur in one of the gulches on the mountain, affecting only a portion of the street and storm drain system in the City. It is very important to note that the citywide mudflow cleanup cost estimates in Table 14 assume that the entire area modeled in FLO-2D is affected (i.e. 25-year mudflows from Pioneer, Vallejo and Spar Gulches). While it is possible that there could be mudflows or mud floods from all three gulches from the same event, it is unlikely that all three would generate major mudflow events. To provide a more detailed assessment, Table 15 was prepared to provide a breakdown of estimated costs by street. This table reflects higher cleanup costs (greater volume of mud to remove) closer to the urban-mountain interface where the mudflows and mudfloods enter urban areas. P151 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 67 November 2017 Figure 29. Mudflow Depths in the Streets at the End of the 25-Year Peak Flow Event P152 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 68 November 2017 StreetMax Street Depth (ft)Ave. Street Depth (ft)Length of Street Affected (ft)Volume of Material in Street (yd3)Days Required to Clan Up1Street Clean Up Cost2Affected Stormwater Pipe Length (ft)Days Required to Jet Stormwater Pipe3Stormwater System Clean Up Cost4Total Days for Clean UpTotal Cost of Clean UpE Bleeker St 1.46 0.25 360 133.8 0.45 $8,140 00 $00.45 $8,140E Bleeker St - Alley 0.22 0.22 20 3.3 0.01 $202 00 $00.01 $202E Cooper Alley 1.11 0.57 100 50.8 0.17 $3,087 00 $00.17 $3,087E Cooper Ave 0.28 0.15 320 50.1 0.17 $3,050 7473.1 $5,9763.28 $9,026E Dean St 2.86 1.92 140 199.1 0.66 $12,109 00 $00.66 $12,109E Durant Alley 0.37 0.18 130 21.6 0.07 $1,317 00 $00.07 $1,317E Durant Ave 0.25 0.18 310 40.3 0.13 $2,452 7203.0 $5,7603.13 $8,212E Hallam St 0.28 0.18 310 84.4 0.28 $5,132 00 $00.28 $5,132E Hopkins Ave 0.66 0.21 160 118.5 0.40 $7,208 3671.5 $2,9361.92 $10,144E Hyman Alley 1.43 0.32 430 139.3 0.46 $8,474 00 $00.46 $8,474E Hyman Ave 0.27 0.15 460 74.5 0.25 $4,534 1,7647.4 $14,1127.60 $18,646E Main St 0.25 0.16 440 66.7 0.22 $4,056 00 $00.22 $4,056Founders Pl 0.97 0.62 200 100.4 0.33 $6,106 00 $00.33 $6,106Gilbert St 0.82 0.82 20 12.1 0.04 $738 00 $00.04 $738Hopkins Alley 0.18 0.14 60 10.1 0.03 $612 00 $00.03 $612Lake Ave 0.22 0.16 200 28.9 0.10 $1,760 00 $00.10 $1,760N 1st Street 0.22 0.16 340 84 0.28 $5,108 00 $00.28 $5,108N Aspen St 0.31 0.17 580 199.6 0.67 $12,140 00 $00.67 $12,140N Galena St 0.38 0.18 60 10.8 0.04 $657 2200.9 $1,7600.95 $2,417N Galena St - Alley 0.2 0.18 80 13.3 0.04 $808 00 $00.04 $808N Garmisch St 0.32 0.18 480 119.8 0.40 $7,290 9253.9 $7,4004.25 $14,690N Monarch St 0.17 0.13 100 13 0.04 $791 7413.1 $5,9283.13 $6,719N Spring St 0.15 0.13 110 11.1 0.04 $678 6842.9 $5,4722.89 $6,150Puppy Mill St 0.26 0.16 60 7.2 0.02 $441 00 $00.02 $441Rio Grande Pl 3.48 0.93 500 470.8 1.57 $28,640 00 $01.57 $28,640S 1st Street 0.19 0.16 40 6.9 0.02 $422 00 $00.02 $422S Alps Road 0.84 0.84 20 12.4 0.04 $755 00 $00.04 $755S Aspen St 0.18 0.13 120 11.9 0.04 $724 5632.3 $4,5042.39 $5,228S Galena St 4.35 0.29 700 176.2 0.59 $10,719 3191.3 $2,5521.92 $13,271S Garmisch St 1.93 1.38 250 757.9 2.53 $46,104 4401.8 $3,5204.36 $49,624S Hunter St 0.23 0.15 360 39.2 0.13 $2,387 2501.0 $2,0001.17 $4,387S Mill St 0.96 0.18 380 91.5 0.31 $5,565 8433.5 $6,7443.82 $12,309S Monarch St 0.22 0.15 360 71 0.24 $4,318 00 $00.24 $4,318S Original St 0.15 0.13 90 16.7 0.06 $1,014 8663.6 $6,9283.66 $7,942S Spring St 0.23 0.13 160 31.6 0.11 $1,921 7823.3 $6,2563.36 $8,177S West End St 0.1 0.1 20 1.5 0.01 $94 00 $00.01 $94Summit St 2.01 2.01 20 29.8 0.10 $1,813 00 $00.10 $1,813Ute Ct 0.82 0.82 20 12.1 0.04 $737 00 $00.04 $737W Cooper Ave 0.16 0.14 40 4.2 0.01 $256 00 $00.01 $256W Francis St 0.24 0.17 370 70.6 0.24 $4,293 6172.6 $4,9362.81 $9,229W Hallam St 0.12 0.12 20 1.7 0.01 $105 00 $00.01 $105W Hopkins Ave 0.19 0.18 140 29.5 0.10 $1,797 1910.8 $1,5280.89 $3,325W Hopkins Ave - Alley 0.87 0.57 40 25.2 0.08 $1,530 00 $00.08 $1,530W Hyman Alley 0.29 0.29 20 4.3 0.01 $262 00 $00.01 $262W Main St 0.34 0.19 20 19.8 0.07 $1,206 00 $00.07 $1,206W Smuggler St0.160.132060.02$363 0 0 $0 0.02 $3631. Assumes 300 CY per day can be removed and trcuked to the Pitkin County Landfill.2. Assumes a cost of $60.83 per CY.3. Assumes the removal of 30 feet of pipe per hour, but this may vary depending on how full the pipes are.4. Assumes a cost of $240 per hour.Table 15. Sediment Volume Deposited in Streets at End of the 25-Year 2-Hour Event P153I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 69 November 2017 5.2 Impacts to Private Property The assessment of impacts to private property was difficult to assess due to Aspen’s unique real- estate market. Therefore, the Project Team and City Staff, identified three properties to be analyzed as case studies for mudflow impacts and cleanup. The three properties identified were: · Single Family: 820 Chance Ct. · Hotel/Multifamily: St. Regis, 315 E Dean St. · Commercial: Ink/Ajax Mountain Bldg.: 520 E Durant Ave. Based on the depth of mudflow expected at each of these properties during the 25-year peak flow event, the Project Team worked with a local contractor, Stutsman-Gerbaz Earthmoving, Inc. to develop a cleanup cost estimate for each property, the location of each of these properties are depicted in Figure 30. The cleanup costs were based on mudflow deposits 6 feet of mud and debris at the 820 Chance Ct property, and 3 feet of mud and debris at the St. Regis and Ink/Ajax Mountain Bldg. properties. It was acknowledged that both the St. Regis and Ink/Ajax Mountain Bldg. have courtyards and below grade spaces that will be impacted in a mudflow event, and the intent was to capture the cost of cleaning up these spaces in the estimate. Based on both the Stutsman-Gerbaz Earthmoving, Inc. estimate (Appendix D), 6 feet of mud and debris at the 820 Chance Ct property would take 11 days to clean up, and cost approximately $846,000. The majority of this cost is associated with the dump fees for over 5,400 cubic-yards of mud and debris, and the 550 hours of trucking required to haul the material to the Pitkin County landfill. Clean up costs associated with 3 feet of mud and debris at the St. Regis and Ink/Ajax Mountain Building would take about 50 days and 2 to 3 days, and cost approximately $4,560,000 and $165,000, respectively. The dramatic differences in clean up time and cost for these two properties is due to the difference in the size of the properties (St. Regis is much larger) and the presence of sub-grade courtyards at the St. Regis which fill with mud resulting in much greater cleanout volumes. The high price associated with the St. Regis property includes the removal of nearly 25,000 cubic yards of material estimated to inundate the property, including the underground parking garage. It is estimated that over 2,400 trucking hours would be required to remove the material from the St. Regis site and truck to the Pitkin County Landfill. Similar to the other two properties, the dump fees and hauling costs associated with the over 1,000 cubic yards of material estimated to inundate the Ink/Ajax Mountain Building make up the majority of the total cleanup cost. With these estimates, a large-scale estimate of the cleanup costs associated with a mudflow event can be estimated. If we assume each City block is approximately 65,000 square feet, and between 55 and 75 percent of the block is developed as a structure, then we can assume the cost for cleanup of 3-feet of mud and debris, associated with a full city block would range from $700,000 to $1,000,000. And similarly, for a 6-foot mud and debris event, the cleanup costs associated with a full city block would range from $1,400,000 to $2,000,000. However, this is a very rough order of magnitude projection that would require further analysis and estimation both based on an actual event and on the specifics of each city block. It is very important to note that these cleanup costs are only for mud and debris removal. They do not include the costs of damages to structural elements of buildings, interior damage, damage P154 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 70 November 2017 to contents. A building affected by 2 to 3 feet of mud inundation would require significant interior repairs and restoration that are not included in the Stutsman-Gerbaz cost estimate. In Aspen, restoration costs could easily exceed the mud removal costs, and cleanup and restoration cost for a block in the 2- to 3-foot depth zone could easily exceed $5,000,000. Figure 30. Mudflow Depths Resulting from the 25-Year Peak Flow Event with Case Study Properties P155 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 71 November 2017 6.0 WILDFIRE RISK ASSESSMENT Wildfires in the Colorado high country can increase the frequency and magnitude of mudflows. The loss of vegetative cover and soil sealing in burn areas can significantly increase runoff, and the burned and barren slopes are more prone to erosion, resulting in increased peak discharge and bulking rates (White et al., 2008). Relatively frequent storm events of high intensity, and short durations, have the potential to cause unusually large mudflow events in post wildfire conditions (Rosgen 2013). In 2012, an event such as this was experienced after the Waldo Canyon Fire outside Colorado Springs. Another example is the 1994 debris flows on Storm King Mountain west of Glenwood Springs (Kirkham et al., 2000). In 2009, the USGS initiated a study of the mudflow hazard in the unburned area of Marble, Colorado, just 20 miles from Aspen. Several years of drought, the spread of mountain pine beetle, and years of forest management schemes that suppress wildfires have made many forest lands in the intermountain west vulnerable to wildfires (Stevens et al., 2011). This is one of the most recent and closest studies to Aspen, and was reviewed by the project team for comparability with Aspen results. Assessing mud and debris flow hazards for post-wildfire conditions has been conducted across Colorado. A similar analysis was completed by the USGS for the Fourmile area, in Boulder County after a 2010 wildfire. This report was drafted as part of an emergency response, as the mudflow hazard potential was highest in the 3 years following the wildfire. In this assessment, a 36 mm (1.4 inch) rainfall over 2-hour, with a 25-year recurrence interval was used to estimate the probability of occurrence and volume of debris using the empirical equations developed by Cannon (Ruddy et al., 2010). A similar analysis was completed by the USGS after the 2012 High Park wildfire near Fort Collins, Colorado. In this analysis, 2-year recurrence, 10-year recurrence, and 25-year recurrence storms were modeled (Verdin et al., 2012). 6.1 City of Aspen Post Wildfire Mudflow Assessment A wildfire analysis was conducted to evaluate the impact of debris flows. Following a wildfire, the clearwater runoff peak flows and volumes as well as the sediment yields are generally higher compared to pre-wildfire conditions due a decrease in soil infiltration and roughness. Burning of organic matter in the “duff” layer on the forest floor has the effect of: (1) causing hydrophobicity, which is the tendency of the soil to resist wetting or infiltration of moisture (NRCS, 2015); (2) decreasing flow roughness; and (3) increasing soil erosion. Because a wildfire encompassing all of Aspen Mountain that drains into the City is highly unlikely with the managed ski area, four wildfire scenarios were developed, in partnership with the Aspen Skiing Company that represent more realistic conditions based on the following assumptions: · The study area is comprised of a number of drainage basins that makes it unlikely for a wildfire to move across all of the basins. The four scenarios represent likely areas a wildfire would likely be contained within. P156 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 72 November 2017 · Aspen Skiing Company actively manages the forested areas in cooperation with the U.S. Forest Service. The forest management includes removing woody debris from the forested areas, which is a hazard to skiers, and thinning trees. · Aspen Skiing Company also has snowmaking equipment that could potentially be used to provide water for firefighting. The four wildfire scenarios were developed based on basin topography, coverage and density of forested areas (Table 4) and vegetation management. The four scenarios shown in Figure 31 are: · Scenario 1 – Wildfire in the upper part of Upper Spar Gulch and the eastern side of Copper Gulch. · Scenario 2 - Wildfire along the eastern side of Lower Spar Gulch. This scenario is separated into two areas, 2A and 2B. The higher elevation area (2A) is actively managed by Aspen Skiing Company. The lower elevation area (2B) is located mostly off Aspen Skiing Company property and is not actively managed by Aspen Skiing Company. Because area 2B is not managed, it was assumed the fire would generate higher temperatures on the ground and higher hydrophobicity in the soil compared to area 2A. · Scenario 3 - Represents a wildfire in Vallejo Gulch, western side of Lower Spar Gulch and northern end of Upper Spar Gulch. · Scenario 4 - Wildfire along the western side of Pioneer Gulch. P157 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 73 November 2017 Figure 31. Wildfire Scenarios P158 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 74 November 2017 To represent the wildfire scenario, the existing conditions model was modified to reflect the changes in infiltration and Manning’s n roughness values. For Scenarios 1, 3, and 4, which includes forested areas that are mostly managed by Aspen Skiing Company, the Horton infiltration capacity for the medium and heavily forested area was set to the lowest infiltration group (Hydrologic Soil Group C) (Table 3, Figure 6). For Scenario 2, area 2A was set to the lowest infiltration group (Hydrologic Soil Group C) and area 2B was set to no-infiltration due to the higher likelihood of hydrophobicity in the soil. The Manning’s n roughness values for the medium and heavily forested areas was set to 0.20 to represent bare ground conditions (Table 4). The model was run for the four scenarios over the 2- and 25-year peak rainfall events. The model was not run for the 100-year event because it is likely the vegetation would recover relatively quickly compared to the recurrence interval for this event. Also, at the 100-year event, the differences in peak flow and volume runoff under existing and wildfire conditions are smaller compared to the more frequent events. 6.2 Post Wildfire Mudflow Assessment Results The four, wildfire debris flow models were run for the 2- and 25-year peak rainfall events, and the model output was used to map and evaluate the depth hazard potential. 6.2.1 Post Wildfire Mudflow 2-Year Peak Rainfall Event For the 2-year event, Scenario 3 resulted in the largest increase in flow, with peak flow in Spar Gulch increases from 4.6 cfs under existing conditions and 29 cfs under Scenario 3 (Table 16, Figure 32). This is a large increase, but it is not surprising since the 2-year event produces very little runoff under unburned conditions. This results in a large percentage increase for the most frequent events that produce little runoff under natural conditions – these percentages decrease for less frequent events that have greater amounts of runoff under the unburned conditions. The resulting flood inundation mapping for the other scenarios indicated little change compared existing conditions, and therefore, only inundation mapping for Scenario 2 was developed. The depth inundation mapping for the 2-year event, under wildfire Scenario 3 shows an increase in extents (Figure 32) of flooding compared to the existing conditions (Figure 15) with the flooding extending farther across the residential neighborhood into the commercial area. Under existing conditions, the maximum depths or mud range up to 10 feet in some localized depressions along Lower Spar Gulch. The mudflow depths are up to approximately 1 foot greater under Scenario 2 compared to existing conditions (Figure 33). The hazard potential mapping for the 2-year event, under wildfire Scenario 2 shows that approximately 0.18, 0.41 and 0.42 acres are classified as High, Moderate and Low hazard potential areas, respectively (Table 17, Figure 34), compared to 0.14, 0.21, and 0.22 acres classified as High, Moderate and Low hazard potential areas, respectively under existing conditions. The hazard potential areas are mostly located along the Lower Spar Gulch and along the uphill side of buildings located in the vicinity of the uphill end of Powder Lane. P159 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 75 November 2017 Table 16. Summary of Peak Flows at the Four Debris Flow Sources under Existing Conditions and the Four Wildfire Scenarios for the 2- and 25-Year Peak Flow Events Location Existing Scenario 1 Scenario 2 Scenario 3 Scenario 4 2-Year Event Spar Gulch (PT4) 4.6 12.9 15.4 28.8 4.7 Vallejo Gulch (PT6) 1.0 1.0 1.0 8.9 1.0 Pioneer (PT8) 0.84 0.84 0.84 2.71 14.2 Pioneer (PT19) 0.49 0.49 0.49 0.49 0.67 25-Year Event Spar Gulch (PT4) 159 199 184 204 159 Vallejo Gulch (PT6) 18 18 19 48 18 Pioneer (PT8) 52 52 52 59 87 Pioneer (PT19) 2 2 2 2 3 Table 17. Summary of Area (acres) of Low, Moderate and High Hazard Potential under Existing Conditions and for the Wildfire Scenarios for the 2-Year, 25-Year Events Hazard Potential Existing Conditions (Table 13) Event Scenario 1 Scenario 2 Scenario 3 Scenario 4 2-yr 25-yr 2-yr 25-yr 2-yr 25-yr 2-yr 25-yr 2-yr 25-yr Low 0.22 8.8 0.34 9.0 0.42 8.9 2.17 9.0 1.16 7.3 Moderate 0.21 7.4 0.36 9.0 0.41 9.1 1.25 10.5 0.77 9.9 High 0.11 5.3 0.15 6.7 0.18 6.7 0.38 8.6 0.35 8.6 Total 0.54 21.5 0.84 24.6 1.02 24.7 3.79 28.1 2.28 25.8 P160 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 76 November 2017 Figure 32. Predicted Maximum Depth for the 2-Year Rainfall Event under Wildfire Scenario 3 Conditions P161 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 77 November 2017 Figure 33. Predicted Increase in Depth between Wildfire Scenarios 3 and Existing Conditions for the 2-Year, 2-Hour Peak Rainfall Event P162 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 78 November 2017 Figure 34. Predicted Flood Hazard Index for the 2-Year, 2-Hour Rainfall Event under Wildfire Scenario 3 Conditions P163 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 79 November 2017 6.2.2 Post-fire Mudflow 25-Year Peak Rainfall For the 25-year event, Scenario 3 resulted in the largest increase in flow, with peak flow in Lower Spar Gulch increasing from 159 cfs under existing conditions to 204 cfs under Scenario 3, an increase of 45 cfs (Table 16, Figure 35). Due to the extensive flood inundation for the 25- year event, it is difficult to see the changes compared to existing conditions; therefore, difference in depth mapping is provided to show the changes. The depth increases are generally in the 0.1 to 0.2-foot range and occur on the southern side of the Aspen Alps Condominiums on Ute Ave and adjacent to the building located on the northwest corner of East Durant Ave and South Hunter Street. For Scenario 2, the peak flow in Spar Gulch increased from 108 cfs under Existing conditions to 114 cfs under Scenario 2, an increase of 6 cfs (Table 16). There were no changes in peak flow in the other basins. The largest depth increases of approximately 0.5-feet occurred near the Aspen Black Swan and apartments located to northeast of the Black Swan on the other side of Ute Avenue (Figure 36). Under Scenario 3, the peak flow increased in Lower Spar Gulch by 4 cfs to 112 cfs, increased in Vallejo Gulch by 6 cfs to 17 cfs, and in Pioneer Gulch, increased by 3 cfs to 35 cfs (Table 16). The largest depth increases of approximately 1-foot occurred near the Aspen Black Swan and apartments located to northeast of the Black Swan on the other side of Ute Avenue (Figure 37). Depth increases in the 0.25 to 0.5-foot range were predicted near the corner of South Mill Street and East Dean Street. Under Scenario 4, the peak flow in Pioneer Gulch increased from 32 under Existing conditions to 43 cfs, an increase of 11 cfs, and the peak flow in Pioneer Gulch increases from 2 cfs to 4 cfs. The largest depth increases of up to 1-foot are predicted to occur along South Mill Street between East Dean Street and Summit Street (Figure 38). In addition, depth increases of approximately 0.5 foot are also predicted near the Corner of Gilbert Street and South Monarch Street, located approximately 200 feet northeast of the base of the Shadow Mountain lift. The hazard potential mapping for the 25-year event, under each wildfire scenario shows that the total acreage classified as High, Moderate or Low hazard potential does not significantly increase under the 25-year event (Table 17). P164 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 80 November 2017 Figure 35. Predicted Increase in Depth between Wildfire Scenario 1 and Existing Conditions for the 25-Year, 2-Hour Peak Rainfall Event P165 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 81 November 2017 Figure 36. Predicted Increase in Depth between Wildfire Scenario 2 and Existing Conditions for the 25-Year, 2-Hour Peak Rainfall Event P166 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 82 November 2017 Figure 37. Predicted Increase in Depth between Wildfire Scenario 3 and Existing Conditions for the 25-Year, 2-Hour Peak Rainfall Event P167 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 83 November 2017 Figure 38. Predicted Increase in Depth between Wildfire Scenario 4 and Existing Conditions for the 25-Year, 2-Hour Peak Rainfall Event P168 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 84 November 2017 7.0 MITIGATION ALTERNATIVES Mitigation of watershed flooding or mudflows can be group into two categories: storage or conveyance, or a combination thereof. Due to the steep watershed slope (limited storage) and the City’s proximity to the base of Aspen Mountain, there is no single conceptual approach to mitigate the mudflow hazard. Common mitigation techniques include construction of conveyance channels, confinement berms, diversions, catchment basins, and debris-trapping structures. The Portland Creek flume in the Town of Ouray, Colorado is an example of conveyance mitigation measure. Given the current state of development along the base of the mountain, including on the fan at the base of Lower Spar Gulch, construction of a large debris conveyance flume, diversion structure, or a catchment basin is not feasible. As a result, alternative analyses were conducted to identify and evaluate conceptual mitigation measures to reduce the impacts of mud and debris flows in the City. 7.1 Reduced Sediment Supply Reducing the available sediment by controlling the source areas is considered a plausible mitigation technique. Aspen Mountain is a well-managed ski area and the sediment supply on the mountain is greatly reduced through the Ski Company’s revegetation efforts, stormwater management, and snow management practices. Stabilizing and planting unvegetated slopes will decrease the rainfall runoff. The continued and additional efforts of the Aspen Skiing Company to reduce the sediment supply will help mitigate the potential mudflow impacts on the City. 7.2 Routing Mudflow thru City Streets and Parks As seen in the FLO-2D model results, the City’s streets act as conveyance channels during a mudflow event. In many areas, the mud and water depths are in the streets are typically less than six-inches. Some of the deepest areas depicted in Figures 18, 21, and 25 are the result of shallow flows routing into local low-lying areas that fill during an event. This could be mitigated through the application of either temporary or permanent barriers. Three examples of low-lying areas that could fill during a mudflow event are shown below. In Photos 11 and 13, these low- lying areas could be mitigated with the application of flood barriers that are temporarily raised when a mudflow or mud flood is anticipated. These are entrances to parking areas, so they cannot be permanently obstructed. In other areas near the urban mountain interface, low-lying areas that have inundation depths of several feet could be protected with architectural modifications (with structural reinforcement) to create a higher perimeter around sunken terraces (Photo 12). Temporary barriers could be used to protect openings for stairways if there is adequate time to prepare. Examples of flood barriers are shown in Photos 14 and 15. A number of flood barrier systems are available that are self-rising, relying on hydrostatic pressure from the floodwaters to raise the barrier. P169 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 85 November 2017 Photo 11. Parking garage entrance – if unprotected mudflow and mud floods will enter underground garage. Photo 12. Below grade terrace space – this is an area that has the potential to fill with mud and water due to shallow overtopping of sidewalk. P170 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 86 November 2017 Photo 13. Parking area is lower than street - without protection, shallow overtopping of entrance to parking area leads to deeper flooding off street. Photo 14. Example of flood barrier for parking area – self-rising flood barriers are available as well as those that are raised manually (source: http://www.floodcontrolinternational.com/). P171 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 87 November 2017 Photo 15. Bottom-hinged flood barrier (source: https://www.psfloodbarriers.com). In addition to these floodproofing solutions for areas that lie below the grade of the street and gutter and can be filled with significant mud depths due to only shallow overtopping at the street level, two City-owned parks, Glory Hole Park and Wagner Park, offer opportunities to store some of the flood volume and can help to reduce the volume and impacts downstream. Some of the flow from both Vallejo Gulch and Pioneer Gulch arrive at Wagner Park, before flowing north, out of the park and towards the Roaring Fork River. The mudflow that reaches Wagner Park could be partially contained with the construction of a low berm (possibly less than one-foot), or similar permanent structure, allowing for retention of some of the mudflow. Much of the flow coming out of Lower Spar Gulch is routed towards Ute Avenue, before moving down South Original Street towards the Roaring Fork River. Glory Hole Park is situated at the corner of Ute Avenue and South Original Street (Photo 16). With the building of some permanent structures, such as a raised crosswalk, and low berms or bench wall, the flow currently routed through this intersection could be diverted to Glory Hole Park. Photo 17 illustrates a conceptual sketch of such a feature that would be capable of diverting flows. P172 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 88 November 2017 Photo 16 – Existing Corner of Ute Avenue and South Original Street. Photo 17. Corner of Ute Avenue and South Original Street With Raised Crosswalk Concept Illustrated to Divert Some Flow to Glory Hole Park – Further coordination on feasibility and refinement of concept needed working in conjunction with streets and parks staff. The existing Glory Hole Park contains a pond, which could be dredged and/or a berm could be added to increase the capacity of the existing pond, allowing for a larger volume of mud to be contained in Glory Hole Park, helping to mitigate the impacts of a mudflow event. 7.3 Warning System The purpose of an early warning system is to save lives by providing sufficient time for an individual to reach a safe area by walking (not driving). People need to get off the streets and away from building in flow paths and structure not capable of withstanding dynamic impact P173 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 89 November 2017 pressures of 3-foot diameter or larger boulders traveling at 10 feet per second. Early warning could be associated with rainfall or with flooding. Rainfall intensity or duration could trigger one warning signal. Flood or mudflow impact or pressure could trigger a louder more distinctive siren or signal. Predicting frontal wave travel times for different return period events has been done with the study modeling effort, so the advanced warning that people would have can be calculated based both on rainfall and flooding at different locations on the mountain right now. Currently weather patterns affecting the operations at Aspen Mountain are monitored through weather stations and communication with other weather forecasting resources. If this system were improved to monitor storm intensity, a warning system could be implemented to alert authorities when a storm reached a certain rainfall intensity, as monitored by gauges on the mountain. For example, a warning system could be programmed to issue an alert if more than 0.5 inches of rainfall were to occur in a 15-minute period or if more than 1 inch occurred in 30 minutes. This could allow at-risk properties to be notified when a given storm intensity is reached, resulting in the placement of the temporary flood barriers described above. This study strongly recommends implementation of an early warning system based on rainfall intensity measurements collected on Aspen Mountain. The technology is well established in Colorado and entities such as the Denver Urban Drainage and Flood Control District operate extensive ALERT systems (http://alert5.udfcd.org/). Development of an early warning system also could include development of public information on how to respond in an emergency and emergency access plans for the potentially affected areas. 7.4 On-Mountain Mitigation Structures A third mitigation alternative is the building of catchment basins higher up the mountain. This is not ideal because the steeper slopes on the mountain make it difficult to create sufficient storage facilities that would protect downstream property. In many cases the benefit cost analysis of storage basins in upper watersheds is prohibitive. Three potential catchment basins locations were identified (Figure 39). The three basins are located along the largest channel in Lower Spar, Vallejo and Pioneer Gulches. The basins were conceptually sized based on the topography, and to prevent the dams from being jurisdictional structures. Catchment Basin 1, is located near the Bell Mountain Lift and has a volume of approximately 23,850 ft3 (883 yd3) and a maximum depth of approximately 10 feet. Catchment Basin 2 is located downhill from the Bell Mountain Lift along the divide between the Vallejo and Lower Spar subbasins. Catchment Basin 2 has a volume of approximately 619 yd3 and depth of 5 feet. The sediment into Catchment Basins 1 and 2 originates from Lower Spar Gulch, therefore, these basins were combined and the trap efficiency was calculated for the combined basins (Table 18). The total volume of Catchment Basins 1 & 2 is 1,502 yd3. For Basins 1 and 2, the sediment load resulting from the Lower Spar Gulch mudflow 2-, year event was approximately 100 yd3 and at the 25- and 100-year events sediment load was approximately 8,965 yd3 and 9,180 yd3, respectively. Based on these volumes, Catchment Basins 1 & 2 have sufficient capacity to store the sediment at the 2-year event (assuming 100- percent sediment trapping), and store approximately 16 and 17 percent of sediment at the 25-year and 100-year events, respectively (Table 18). The mitigation structures did not change the P174 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 90 November 2017 timing of the mudflow hydrograph at the base of the mountain under the 25-year or 100-year peak flow events. Catchment Basin 3 is located in the Pioneer subbasin and has a volume of approximately 38 yd3 and depth of approximately 2 feet. The sediment load at the 2-year event was approximately 10 yd3 and at the 25- and 100-year events was 1,970 yd3 and 2,2012 yd3, respectively. Based on these volumes, Basin 3 has sufficient capacity to store the sediment at the 2-year event (assuming 100-percent sediment trapping which is highly unlikely), and approximately 2 percent of the storage capacity at the 25- and 100-year events (Table 18). In Basin 3 check dams could also be installed to spread out the storage. For example, if the slope of the hill was 2.5:1, and 4 foot check dams were installed in a 10 wide channel, each check dam would have a storage volume of 1.2 CY, resulting in a total storage of 12 CY with the installation of 10 check dams. The installation of these check dams would also help with the Skiing Company’s stormwater management requirements. The intent of these check dams would be to store the coarsest material, while allowing the water and wash load to proceed downstream. A series of small check dams or water bars diverting flows into the forest has the potential to contribute to modest reductions in flows; however, diversions into the forest should be carefully evaluated to be sure that they do not create slope instability issues. 7.4.1 On-Mountain Mitigation Structure Analysis Results The alternatives model was run with all three basins over the 2-, 25- and 100-year peak flow events. The model output was used to develop difference in depth mapping to show the reduction in depth (alternative versus existing) with the basins in place. Note: the difference in depth mapping is the opposite compared to the of the wildfire conditions that showed the increase in depth. While the detention basins have storage capacity to store the sediment for a 2-year event, the damage reduction for the less frequent events in minimal (Figure 40). The flow depth reductions along the lower section of Lower Spar Gulch generally occur between Powder Lane and Chance Court with depth reductions ranging from approximately 0.1 to 0.5 foot. The detention basins marginally reduce the mudflow flow depths in the area generally defined as the Lower Spar Gulch fan (Figure 41) for the 25-year event. The flow depth reductions all occur mostly on the upstream side of the buildings in the area between Powder Lane and Ute Avenue. The flow depth reductions range up to approximately 2 feet, with a representative depth reduction of about 0.5 foot. Table 18. Mudflow Storage Volume Catchment Basin Storm Event Volume (CY) Total Mudflow Volume (CY) Total Sediment Volume (CY) Percent Sediment Load Captured 1 & 2 2-Year 883+619 = 1,502 506 101 100% 25-Year 28,843 8,963 17% 100-Year 46,567 9,180 16% 3 2-Year 38 51 10 100% P175 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 91 November 2017 25-Year 6,119 1,970 2% 100-Year 10,059 2,012 2% P176 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 92 November 2017 Figure 39. Location of Debris Flow Catchment Basins Evaluated in the Alternatives Analysis P177 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 93 November 2017 Figure 40. Reduction in Flow Depth at the 2-Year Peak Rainfall Event under the Alternatives Condition P178 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 94 November 2017 Figure 41. Reduction in Flow Depth at the 25-Year Peak Rainfall Event under the Alternatives Condition 8.0 SUMMARY AND RECOMMENDATIONS The updated mudflow and mud flood analysis and hazard mapping presented in this document will serve as the basis for regulation of development in mudflow hazard areas in the City. These areas include much of the southern fringe of the City at the interface between watershed and urban area. To evaluate the debris flow potential, a FLO-2D model was developed that included Aspen Mountain and the area of the City from the base of the mountain to the Roaring Fork River. The model had a grid resolution of 20 feet, and 25-fold increase in resolution compared to the original FLO-2D model developed by WRC (2001). The model was run over the 2-hour, 2- through 100-year rainfall events, and the 12-hour 100-year rainfall event. The model output was used to develop depth, velocity and hazard inundation mapping. The mapping and hydraulic results will be incorporated into the City’s Urban Runoff Management Plan. Key findings and recommendations include the following: · The debris flow results for the 2-year peak rainfall shows relatively minor flooding with isolated areas of flooding ranging up to 0.7 feet in depth and maximum velocities up to 1 feet/s. The flood hazard mapping indicates that approximately 0.11, 0.21 and 0.22 acres are classified as High, Moderate and Low hazard potential areas, respectively. · The 25-year peak flow event from Lower Spar Gulch was predicted to extensively inundate urban areas, with the mudflow mostly following the street alignment from the base of the mountain across to the Roaring Fork River. The depth of flooding ranges from approximately 1 to 5 feet near the source areas. Near the base of the mountain, the mudflow depths range up to 15 feet in the channel and up to 5 feet on the floodplain, with the maximum depths occurring on the upstream side of buildings and at topographically low points. The maximum flow velocities occur near the source of Lower Spar Gulch and along the channel with velocities ranging up to 15 fps. The velocities near the base of mountain range up to 10 fps, while the velocities in the streets range up to approximately 6 fps. Maximum flow velocities in the overbank areas range from approximately 0.5 to 5 fps, with the majority of the velocities between 1 and 2 fps, which is equivalent to a slow walking speed. The hazard potential mapping indicates that approximately 5.3, 7.4 and 8.8 acres are classified as High, Moderate and Low hazard potential areas, respectively. · The model results for the 100-year, 2-hour peak flow event predict similar flood inundation and flow paths compared to the 25-year event, but with greater depths due to the increased flow volume. Similar to the 25-year event, the maximum depths during the 100-year event range from approximately 1 to 5 feet near the source area, range up to 15 feet along Lower Spar Gulch and on the fan and range up to 5 feet on the floodplain, with the maximum depths occurring on the upstream side of buildings and at topographically low points. The maximum flow velocities occur near the source of Lower Spar Gulch and along the channel with velocities ranging up to 15 fps. The velocities near the base of mountain range up to 10 fps, while the velocities in the streets range up to approximately 6 fps. The hazard potential mapping indicates that approximately 5.3, 7.4 and 8.8 acres P179 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 95 November 2017 are classified using water flood intensity criteria as High, Moderate and Low hazard potential areas, respectively · The existing City mudflow regulations require that debris flow analyses be conducted using the 100-year peak rainfall event and maximum sediment concentrations of 45- percent. This criterion is considered to be conservative given that debris flows typically occur in the 10- to 25-year recurrence interval range, and that the 100-year peak flows events more typically behave as mudfloods as opposed to mudflows. For these reasons, it is recommended that the regulatory design events are based on mudflow hydraulics and hazard mapping. · It is recommended that future regulatory design events are evaluated for the following two events and criteria: o The 25-year, 2-hour rainfall event with maximum sediment concentration of 55- percent and an average sediment concentration of 30-percent by volume. o The 100-year, 2-hour rainfall event with an average sediment concentration of 20- percent. · In the previous mudflow study and in the Urban Runoff Management Plan, a “no adverse impact” is taken to redevelopment, requiring that applicants model modifications to properties and design mitigation measures so that the FLO-2D model does not cause a rise in maximum flow depth of greater than 0.00 feet. “No adverse impact” is the intent of the City mudflow regulations; however, requiring applicants to demonstrate a 0.00- foot rise in the FLO-2D model places unrealistic expectations on the accuracy of the model for representing an actual mudflow or mud flood event. A 0.5-foot tolerance is recommended when evaluating effects of a proposed development on adjacent properties that are within the mapped mudflow zone. This tolerance would allow for model results to show up to a 0.5-foot rise on neighboring properties that are already affected by mudflows. This would not allow a property to cause mudflow/mud flood impacts to a neighboring property that is not already affected by this hazard. · The Red Butte area was evaluated as a part of this study using the Rational Method and assumed peak sediment concentrations. These results were used to develop guidance on unit rates of runoff for multiple return periods to use for planning of redevelopment in this part of the City. · Several wildfire scenarios were evaluated for Aspen Mountain to look at potential increases in mudflow depths and volumes in the event of a wildfire. Aspen Skiing Company actively manages the vegetation on Aspen Mountain, and the risk of a major wildfire is considerably lower than other local forested watersheds. Based on the scenarios evaluated, peak runoff rates and volumes would be expected to increase in all of the major drainages following a wildfire. The relative increases in runoff rates and volumes are greatest for the more frequently occurring events, and the differences between burned and unburned scenarios diminish for larger storm events. P180 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 96 November 2017 · Due to the complexities of Aspen’s real estate, and the high property values, it is difficult to accurately quantify the total costs that could be incurred as a result of a mud and debris flow event. The estimates provided by Stutsman-Gerbaz Earthmoving, Inc. provide information on cleanup costs, and FLO-2D results are used to estimate quantities and costs for cleaning streets and storm drains. Costs for structural repairs, restoration, loss of contents and other factors and not included. It is estimated that in areas affected by a mudflow or mud flood with maximum depths of 2- to 3-feet, costs for cleanup and restoration could exceed $5,000,000 per block with damages occurring on lower levels of buildings and sub-grade areas such as sunken terraces and below ground parking garages at the greatest risk. · The time and costs associated with the cleanup of public parks, streets, and the storm drain system following a mud and debris event are significant. The total cost associated with cleaning up the City’s streets and parks could total over $445,000 during the 25-year event, with the jetting of the City’s storm drain system costing over $350,000, for an estimated total of almost $800,000. The time required to remove mud and debris on the impacts to local businesses would be far greater that the cleanup costs. · The most viable mitigation alternative to address some of the areas of greatest mudflow depths is floodproofing at the lot scale. Many of the areas of deepest flooding are those that are below-grade areas (terraces, parking garages, parking lots, etc.) adjacent to streets. The flooding on the adjacent streets is only on the order of a foot or so on many of these streets, but when the water or mudflow elevation exceeds the highpoint between the street and the low-lying area, the low-lying area will fill with mud and water to depths that are much greater than the flooding on the adjacent street. Temporary and/or permanent flood barriers would improve the level of flood protection for many of these areas. These barriers could be structurally and architecturally integrated with the buildings and/or hardscaping and barriers that can be raised and retracted could also be considered. 9.0 ACKNOWLEDGEMENTS WWE would like to thank the many people that assisted with gathering information to complete this report, including significant assistance from the Colorado Geological Survey's State Geologist Karen Berry and Larry Scott, as well as geologists William "Pat" Rogers and Jeff Hynes, who are retired from the Colorado Geological Survey. Roy Spitzer, presently with Deere & Ault Consultants, discussed with us his experience with geologic hazards on Aspen Mountain, and he also loaned copies of the unpublished Chen & Associates reports that he wrote. Bruce Bryant, retired U.S. Geological Survey geologist, shared his knowledge of the geology and geologic hazards in the project area and also contributed an original copy of his published geologic map of the Aspen quadrangle. Art Mears discussed his experiences with debris flows on or near Aspen Mountain. Ralph Mock, with Hepworth-Pawlak Geotechnical, related some of his knowledge of the geologic hazards on Aspen Mountain to us, the Aspen Skiing Company’s Peter King and Victor Gerdin were greatly helpful in tracking down existing and unpublished reports, and Steve Pawlak provided some of their unpublished reports to us. Jay Parker, an Aspen miner, and Bruce Stover, Colorado P181 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 97 November 2017 Division of Reclamation, Mining, and Safety, described the historical underground mine workings on Aspen Mountain and their relationships with geologic hazards. We also thank our colleagues in the Roaring Fork Valley who have shared past experiences and studies with us to help compile the literature review. The field observations made on Aspen Mountain would not have been possible without the cooperation and assistance of several people with Aspen Skiing Company and their geotechnical consultants with CTL Thompson Inc. We thank them for their contributions. P182 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 98 November 2017 10.0 REFERENCES Brunton, D.W., 1888, Aspen Mountain: Its ores and their mode of occurrence: The Engineering and Mining Journal, v. 46, p. 22-23 (July 14, 1888) and p. 42-45 (July 21, 1888). Bryant, Bruce, 1971, Geologic map of the Aspen quadrangle, Pitkin County, Colorado: U.S. Geological Survey, Geologic Quadrangle Map GQ-933, scale 1:24,000. Bryant, Bruce, 1972a, Map showing areas of selected potential geologic hazards in the Aspen quadrangle, Pitkin County, Colorado: U.S. Geological Survey, Folio of the Aspen quadrangle, Colorado, Map I-785-A, scale 1:24,000. Bryant, Bruce, 1972b, Map showing ground-water potential in the Aspen quadrangle, Pitkin County, Colorado: U.S. Geological Survey, Folio of the Aspen quadrangle, Colorado, Map I- 785-B, scale 1:24,000. Bryant, Bruce, 1972c, Map showing types of bedrock and surficial deposits in the Aspen quadrangle, Pitkin County, Colorado: U.S. Geological Survey, Folio of the Aspen quadrangle, Colorado, Map I-785-H, scale 1:24,000. Bryant, Bruce, 1979, Geology of the Aspen 15-minute quadrangle, Pitkin and Gunnison Counties, Colorado: U.S. Geological Survey, Professional Paper 1073, 146 p. Bryant, Bruce, and Martin, P.L., 1988, The geologic story of the Aspen region--Mines, glaciers and rocks: U.S. Geological Survey, Bulletin 1603, 53 p. Bussone, P.S., 1989, Re: Aspen Skiing Company--Keno Gulch drainage: unpublished letter report by Wright Water Engineers, Inc., Denver, Colorado, to Arthur Ferguson, Holland & Hart, Aspen, Colorado, dated June 29, 1989, job number 871-057.020, 3 p., 1 figure, 9 photographs. Chen & Associates, Inc., 1984, Geologic and geotechnical evaluation; Debris flow study; Aspen Mountain Planned Unit Development; Top of Mill complex; Aspen, Colorado: unpublished report prepared by Chen & Associates for Commerce Realty Corporation, San Antonio, Texas, dated November 14, 1984, job number 4 392 84, 20 pages and an appendix with numerous figures and tables. Chen & Associates, Inc., 1985a, Preliminary report, Geotechnical study of 1984 landslide, Above Top of Mill condominiums and Aspen Mountain Lodge sites, Strawpile Run, Aspen Mountain, Aspen, Colorado: unpublished report prepared by Chen & Associates, Glenwood Springs, Colorado, for Commerce Realty Corporation, San Antonio, Texas, dated January 4, 1985, job number 4 385 84, 20 p., 5 figures, 2 appendices. Chen & Associates, Inc., 1985b, Debris flow and landslide investigations; Top of Mill site; Aspen Mountain Planned Unit Development; Aspen, Colorado: unpublished report prepared by Chen & Associates for The Aspen Mountain PUD Project, Aspen, Colorado, dated September 20, 1985, job number 4 385 84, volume I is 8 pages with 4 figures and a separate reference section, volume II includes 5 appendices with numerous figures and tables, the copy of the report made available to WWE was missing Figure B-1, a plate showing surficial geology. P183 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 99 November 2017 Chen-Northern, 1989, Reconnaissance of recent landslide, Roch Run below Ruthie's Restaurant, Aspen Mountain, Colorado: unpublished report prepared by Chen-Northern for Pitkin County Engineering Department, Aspen, Colorado, dated May 26, 1989, job number 4 361 89, cited and described in Hepworth-Pawlak (1996). Chen-Northern, 1991, Reconnaissance of landslide movement, Roch Run below Ruthie's Lift, Aspen Mountain, Colorado: unpublished report prepared by Chen-Northern for Aspen Skiing Company, Aspen, Colorado, dated July 26, 1991, job number 4 361 89 1, cited and described in Hepworth-Pawlak (1996). Chen-Northern, 1992, Reconnaissance of landslide movement, Roch Run below Ruthie's Restaurant, Aspen Mountain, Colorado: unpublished report prepared by Chen-Northern for Aspen Skiing Company, Aspen, Colorado, dated June 23, 1992, job number 4 361 89 1, cited and described in Hepworth-Pawlak (1996). Colorado Geological Survey Archives, 2016, Landforms-hazards-aggregate sources map of the Aspen 7.5-minute quadrangle: unpublished House Bill 1041 geologic hazard map, provided by Colorado Geological Survey on April 6, 2016, map probably was prepared by a consultant in the mid to late 1970s. F.M. Fox and Associates, Inc., 1974, Roaring Fork and Crystal Valleys; An environmental and engineering study; Eagle, Garfield, Gunnison, and Pitkin Counties, Colorado: Colorado Geological Survey, Environmental Geology 8, 4 plates, scale 1:48,000. Hepworth-Pawlak Geotechnical, Inc., 1996, Geotechnical Engineers Study; 1995 slope movements in the Roch Run and Spring Pitch areas; Aspen Mountain, Colorado: unpublished report prepared by Hepworth-Pawlak Geotechnical, Inc. for Aspen Skiing Company, Aspen, Colorado, job number 294 150, dated May 13, 1996, 17 pages and 5 figures, WWE's copy is missing the appendices and tables. Hepworth-Pawlak Geotechnical, Inc., 1998, Review of potential slope stability impacts; proposed new snowmaking on Aspen Mountain, Pitkin County, Colorado: unpublished report prepared by Hepworth-Pawlak Geotechnical, Inc. for Aspen Skiing Company dated January 26, 1998, job number 197 461, 16 p. plus 5 figures and 3 tables. Horton, R.E., 1933. The role of infiltration in the hydrologic cycle. Trans. Am. Geophys. Union. 14th Ann. Mtg: 446–460. Laing, David, and Lampiris, Nicholas, 1980, Aspen high country-The Geology-A pictorial guide to roads and trails: Thunder River Press, Aspen, Colorado, 132 p. McCalpin, J.P., 1997, Geologic and hydrologic impacts of proposed snowmaking on upper Aspen Mountain, Pitkin County, Colorado: unpublished report by GEO-HAZ Consulting, Inc., Estes Park, Colorado, for Aspen Skiing Company, Aspen, Colorado, dated December 5, 1997, job number 2047, 52 p., 22 figures. WWE's copy is missing plate 1 P184 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 100 November 2017 Mears, A.I., 1983, Letter report on the geologic hazards at the Music Association of Aspen: unpublished letter report prepared by Arthur I. Mears, P.E., Inc., Gunnison, Colorado to Mr. Ford Schumann, Music Association of Aspen, Colorado, dated October 6, 1983, 3 p., 1 figure. Mears, A.I., 1992, Geologic hazard analysis, Aspen Country Day School, Pitkin County, Colorado: unpublished letter report from Arthur I. Mears, P.E., Inc., Gunnison, Colorado to Mr. Joe Wells, Wells Land Planning, dated November 10, 1992, 3 p., 2 figures. Mears, A.I., 1996, Re: Debris flow hazard: unpublished letter report prepared by Arthur I. Mears, P.E., Inc., Gunnison, Colorado, for David Pearcy, Aspen Music Festival and School, Aspen, Colorado, dated May 15, 1996, 3 pages, 1 figure. Mineral Systems, Inc., 1989, Re: Pioneer Gulch slope failure: unpublished letter report on the 1989 slope failure in Pioneer Gulch, submitted by Charles S. Robinson, Mineral Systems, Inc., Golden, Colorado, to Mr. Beryl Eylar, Pitkin County Engineer, dated May 26, 1989, 3 p. National Oceanic and Atmospheric Administration (NOAA) 1973. Precipitation-Frequency Atlas of the Western United States, Volume III Colorado (NOAA Atlas 2). National Oceanic and Atmospheric Administration, Washington, D.C.: U.S. Department of Commerce, National Weather Service. NOAA, 2013. Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 8, Colorado. National Oceanic and Atmospheric Administration, Washington, D.C.: U.S. Department of Commerce, National Weather Service. Schmueser and Associates, 1984, Aspen Mountain landslide: unpublished report prepared by Schmueser and Associates, project number B4379A, cited and summarized in Chen & Associates, 1985a and 1985b. Spurr, J.E., 1898, Geology of the Aspen mining district, Colorado, with atlas: U.S. Geological Survey Monograph, v. 31, 260 p., 30 atlas sheets. Tetra Tech, 1993. Drainage Master Plan, Town of Breckenridge Colorado. Prepared for Town of Breckenridge Colorado. April, 1993. United States Army Corps of Engineers (USACE), 1997. Glenwood Springs, Garfield Count, Colorado Flood Insurance Study – Hydrology, Volume 1, December 1997 (unpublished). United States Environmental Protection Agency (USEPA), 2015. Stormwater Management Model (SWMM), Version 5.1. United States Geological Survey (USGS), 2011. Estimated Probabilities, Volumes and Inundation Area Depths of Potential Postwildfire Debris Flows from Carbonate, Slate, raspberry, and Milton Creeks, near marble, Gunnison County, Colorado. Scientific Investigations Report 2011-5047. Urban Drainage and Flood Control District (UDFCD), 2014. CUHP User Manual, Version 1.4.3. Released January 24, 2014. P185 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 101 November 2017 WRC Engineering, Inc., 2001, Storm drainage master plan for the City of Aspen, Colorado: unpublished report prepared by WRC Engineering, Inc., Denver, Colorado, for the City of Aspen, Colorado, dated November, 2001, project number 1963-20, 22 p. plus 21 tables, 11 figures, 25 drawings, and executive summary. Wright, K.R., and Rold, J.W., 1996, Report on the Aspen Music School debris flow investigation (draft): unpublished report prepared by Wright Water Engineers, Inc., Denver, Colorado, for Fireman's Fund Insurance Company, Denver, Colorado, dated May 31, 1996, job number 961 062.000, 17 p., 5 figures, and several appendices. Wright Water Engineers (WWE), 2014. City of Aspen Detention Analysis. Prepared for the City of Aspen. 59p. September. 10.1 Other Potential References (Identified by reference in reviewed documents – documents exist based on other references but could not be located) Unavailable Aspen Times, 1919, Cloudburst scatters mud over city: newspaper article on page 1 of the September 5, 1919 issue, cited and described in Chen & Associates, 1984 and 1985. Unavailable Aspen Times, 1964, Worst cloudburst in years floods Aspen on August 5: newspaper article on page 16 of the August 7, 1964 issue, cited and described in Chen & Associates, 1984 and 1985. Unavailable Chen & Associates, Inc., 1983, Engineering geology reconnaissance for the proposed Top of Mill Condominium Complex, Aspen, Colorado: unpublished report by Chen & Associates, job number 2615A. Unavailable Chen & Associates, Inc., 1984, Geotechnical investigation for proposed Top of Mill Development, Aspen Mountain Lodge Project, Aspen, Colorado: unpublished report by Chen & Associates, job number 4 392 84. Unavailable Hepworth-Pawlak Geotechnical, 2003, Long-Term Monitoring of Strawpile Slope, Aspen Mountain, Pitkin County, Colorado: unpublished report prepared for the Aspen Ski Company, Aspen, Colorado, job number 294 150, dated April 7, 2003. Unavailable Hepworth-Pawlak Geotechnical, Inc., 1997, Summary of 1996 Strawpile, Roch Run, and Spring Pitch slope monitoring, Aspen Mountain, Pitkin County, Colorado: unpublished report prepared for the Aspen Ski Company, Aspen, Colorado, job number 294 150, dated April 24, 1997. Unavailable McCalpin, J.P., 2010, Geologic and hydrologic impacts of 2010 proposed improvements on Aspen Mountain, Pitkin County, Colorado: unpublished report prepared by GEO-HAZ Consulting, Inc., Crestone, Colorado, for SE Group, Frisco, Colorado, dated November 26, 2010, job number 2132, 50 p. Unavailable Rea Cassens and Associates, Inc., 1983, Top of Mill and Spar Complex, storm water drainage report: unpublished report by Rea Cassens and Associates. P186 I. City of Aspen Mud & Debris Flow Assessment 161-012.000 Wright Water Engineers, Inc. Page 102 November 2017 Unavailable Rea Cassens and Associates, Inc., 1984, Master drainage study, Aspen Mountain: unpublished report by Rea Cassens and Associates. Unavailable Rea Cassens and Associates, Inc., 1985a, Aspen Mountain Lodge, 700 South Galena Street, Top of Mill and Ute City Place, storm water drainage report: unpublished report by Rea Cassens and Associates. Unavailable Rea Cassens and Associates, Inc., 1985b, Aspen Mountain, 100 year unit and storm hydrographs: unpublished report by Rea Cassens and Associates. Unavailable Shannon and Wilson, Inc., 1984, Letter report on apparent instability on the north face of Aspen Mountain: unpublished report prepared by Shannon and Wilson. Unavailable U.S. Geological Survey, 1928, Mines of Aspen, Pitkin County, Colorado: U.S. Geological Survey, Field Records File RQ-34. Currently unavailable because the library is moving. Unavailable U.S. Geological Survey, 1980, Mine maps and cross-sections of the mines at Aspen, Pitkin County, Colorado: U.S. Geological Survey, Geologic Division Field Records File RO-24, maps and sections by R.P. Rohling 1928 and 1943. Currently unavailable because the library is moving. 10.2 Personal Communications by Bob Kirkham William 'Pat' Rogers, retired, Colorado Geological Survey, Chief of Engineering and Environmental Section, February 25, 2016. Jeffrey Hynes, retired, Colorado Geological Survey, Senior Geologist who assisted Pitkin County and the City of Alamosa with geologic hazards, February 25, 2016. Bruce Stover, Chief of the Inactive Mine Program of the Colorado Division of Reclamation, Mining, and Safety, familiarity with the abandoned underground mine works on Aspen Mountain, March 1, 2016. Ralph Mock, Engineering Geologist with H-P Geotech, conducted several investigations of the slope stability and debris flow hazards on Aspen Mountain in the 1990s, March 2, 2016. Roy Spitzer, Engineering Geologist with Deere & Ault Consultants, conducted landslide and debris flow studies on Aspen Mountain in the 1980s, March 2 and 9, 2016. Bruce Bryant, USGS emeritus, who published geologic maps and reports on Aspen area, March 24, 2016. Jay Parker, Aspen resident with knowledge of the old mines and mine drainage, March 25, 2016. P187 I. 1 MEMORANDUM TO: Mayor and City Council FROM: John D. Krueger Director of Transportation THRU: Barry Crook, Assistant City Manager DATE OF MEMO: January 25, 2018 DATE OF MEETING: February 5, 2018 RE: RFTA Integrated Transportation System Plan (ITSP) Stage III-Alternatives Analysis Update and Funding Discussion SUMMARY Parsons, the RFTA consultant for the Integrated Transportation System Plan (ITSP) will present an update to City Council on Stage III of the ITSP study including an analysis of alternatives and funding discussion. PREVIOUS COUNCIL ACTION City Council was previously updated on Stage I of the ITSP at a work session on May 17, 2016 and on Stage II on February 28, 2017. BACKGROUND There are four stages of the ITSP. Stage I of the ITSP began in March of 2016. The plan has progressed through Stage II into Stage III. Stage III is an analysis of options and funding strategies. Stage IV will begin in 2018. The RFTA board, the EOTC and individual member boards of RFTA have received updates on the study as it has progressed. DISCUSSION Stage III has identified over 70 improvements to the RFTA system. More than ten will benefit the City of Aspen. Four of these improvements are highlighted in the presentation. These improvement alternatives have been categorized as short-term, medium-term and long-term. P188 II. 2 RFTA will need to secure additional, long-term funding to satisfy public demands for safe reliable transportation. A preliminary funding analysis will be presented for the improvement options including existing RFTA revenue sources, potential local revenue sources and mill levy scenarios. A mill levy sensitivity analysis will be presented for a 1, 3 and 5 mill levy to fund the improvements. FINANCIAL IMPLICATIONS There may be financial implications if a mill levy is approved by the voters. ATTACHMENTS Attachment A: RFTA ITSP stage III Presentation P189 II.