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HomeMy WebLinkAboutminutes.apz.19991122ASPEN PLANNING & ZONING COMMISSION NOVEMBER 22, 1999 This Special meeting of the Aspen Planning & Zoning Commission convened at 11:30 a.m. with the following members present: Bob Blaich, Steve Buettow, Ron Erickson, Roger Hunt, and Tim Mooney. Roger Haneman, and Jasmine Tygre were excused. Council member, Jim Markalunas, also attended. Staff in attendance were Nick Adeh, City Engineer; Nick Lelack, Commuhity Development; Joyce Ohlson, Community Development; Phil Overeynder, Water; Jack Reid, Streets; Chuck Roth. Engineenng; Lonnie Whitmire, Water; JeffWoods. Parks: John Worcester, City Attorney; and Joy Homing, Engineering Administrative A~sistant. Bruce Curtis and Alan Leak. with WRC Engineering,/mc, John Sarpa, Savanah Limited Partnership, and John Colson, staff writer for The Aspen Times,'were also in attendance. ASPEN MOUNTAIN DRAINAGE BASIN MASTER PLAN Nick Adeh, City Engineer, opened the meeting with a brief overview of the activities that have occurred during the last 18 months involving the Aspen Mountain Dra'mage Basin Master Plan. As part of this analysis, debris flow and mudflow models were developed. Scenarios involving snow melt and rain-on-snow were examined: soil conditions were considered as well. He also talked about a university designed model that shows the impact ora mudflow event on developed communities. In line with running the model and defirfing the master plan for the drainage basin, the City will develop eight criteria. The consultants will provide an overview of some solutions and the options they believe are feasible for Aspen Mountain and the City of Aspen. The boundaries of the study are defined by the Roaring Fork River on the north, by Cooper Avenue on the east, by Shadow Mountain on the west, and extend to the summit of Aspen Mountain. The Aspen Mountain Dra'mage Basin Master Plan was prepared as part of the City's agreement with Savanah Limited Partnership to identify potential impacts for their development. Nick introduced Alan Leak and Bruce Curtis from WRC Engineering, Inc. Alan is President of WRC engineering and the principal in charge of this project. Bruce is the project manager. The purpose of this presentation wasto provide an update on the issues coming out of the tuaster plan so as m facilitate a more informed decision with regard to developing criteria, rules, regulations, and law that might affect development within the Aspen community. Bruce Curtis provided an overview of the results obtained from Phase I. During Phase I, a drainage facility analysis was conducted, which included the City's drainage facilities, the storm sewers, and the street flow that can carry runoff. Runoff events, such as snow melt and ram-on- snow events were also examined. A rain-on-snow event occurs when a heavy rainstorm combines with a small amount of snow left on the mountain resulting in substantially more runoffthan would happen with rain only. A mudflow analysm was also done, and some conclusions were reached based on that analysis. As part of the drainage facility analysis, WRC looked at the hydrology, which involves determining the amount and path of the flow. Flow versus capamty was examined to determine if the existing facilities are sufficient to carry the flow. Location and size of the storm sewers, the material used in their construction, and the amount of flow they can carry was considered.. The study area was divided up into 25 different sub-basins, and the flow coming off each of those sub-basins was examined. This resulted in a series of hydrographs, which show flow over time. For example, a rainstorm starts at time equals zero and after about 45 minutes, it reaches peak flow. Then the flow gradually subsides. The purpose is to look at routing the peak flows. The hydrographs from all 25 sub-basins were examined collectively to determine flow down the mountain to the Roaring Fork River. Ideally, this will show what the flow will be at critical points and times. In addition, W'RC examined a 2-year storm through a 100-year storm. A 100-year storm occurs on an average of once every 100 years. And a 2-year storm occur~ on the average of once every 2 years. As such, a storm that is going to occur once every 100 years is much more severe than one that occurs every 2 years. Then next step was to look at the major mink lines and the major storm sewer lines. The area was divided into reaches, and each was given a letter designation. For example. Reach A is Original Street from Ute down to Main. The actual flow for a 2-year storm is 18 CFS, whereas the flow for a 100-year storm is 528 CFS. The existing storm sewer has a capacity to carry 50 CFS. Combined with the street capacity where the water would be flowing to the top of the curb (1/2 foot of flow depth in the flow line of the gutter), the system would be able to handle 116 CFS. As such, Reach A has the current capacity to pass about a 20-year storm. A 50-year storm with 345 CFS will cause flooding. The general recommendation is to design for a 100-year storm, and to have storm sewer capacity for a 5-year storm. WRC then examined runoff events, such as snow melt with natural snow, snow melt with both natural and manmade snow, and rain on snow events to ascertain the effect on peak runoff ~rwo different locations were studied, Spar Gulch at Original Street and Vallejo Gulch. The results for both locations were essentially the same. According to the study, the inclusion of manmade snow does not increase the peak runoff; however, it may extend the nmoffperiod. Rain on snow. however, increases the runoff substantially - 1 Vz to 2 times in some cases. WRC conducted a mudflow analysis using a program called FLOW-2D developed by Dr. O'Brien. FLOW-2D is a finite difference model. It models the runoff of the mudflow down the mountain through the city. As part of this analysis, they examined water only, mudflow, and then calculated in the effects of development. FLOW-2D is a relatively new model; however, it compared very favorably to other models. Examples of recent mudflow events include the Aspen Music School and Keno Gulch. Briefly, the FLOW-2D methodology is as follows. For water only, they input rainfall and some additional factors, run the model, and get a depth of flow. For mudflow, they input the rainfall, mn the model, then add sediment to the hydrographs and nm the model again. The FLOW-2D analysis provided the following results for water only. From Spar Gulch down to Ute, Vallejo Gulch, and Pioneer Gulch, the flow proceeds down the mountain into the town, where you get ½ foot to 1 ½ foot of flow depth. This model does not take into account the effect of the buildings and streets. At Glory Hole Park, the depth exceeds 5 feet. The analysis provided the following results for mudflow. Again, this analysis does not take into account the effect 6f streets or buildings, which would increase the depth of flow greatly. The flow comes down in two different locations. At the top of Mill area, the mudflow depth will be 1-3 feet. To determine the effect of buildings on the flow, a hypothetical development is added. An 80% reduction in area due to developmem was assumed. The flow depth then jumps to 5 foot in depth. Development allows flow to go through it, but it's impeded. The analysis indicates that development at the base of the mountain going to have an significant impact on a mudflow if nothing is done to address it. The conclusions of Phase I.are summarized as follows: 1. There is an existing lack of drainage facility capacity, and storm sewers will need to be built for the City to carry a 100-year storm without flooding of the city. 2. Snowmelt isn't a flood problem, it is more ora maintenance problem. Every year snowmelt brings down a small mount of sediment, but it is not going to cause flooding. 3. Manmade snow has little effect on runoff. 4. Rain on snow is a possible problem. If that event occurs, a lot more water will come down the mountain than would occur with a rainfall runoffevent. However, rain on snow has been examined in the context of a 100-year rainstorm, and the probability is much less likely that a 100-year rainstorm will occur at precisely the right time during the spring to create a rain on snow event. If it does happen, it will be a major problem. 5. Mudflow increases the depth of flow. 6. Development can increase the flow depth. 7. FLOW-2D can be a useful tool. It can help a city detennine the effect of flows, and the effect development will have. Alan Leak discussed the scope of work and Phase II of the Aspen Mountain Drainage Basin Master Plan. Although the probability ora rain on snow event is fairly rare, the January 1998 event on the Turkey River was a rain on snow event which increased the flows from the river from 200-500 CFS upwards to 20,000-25,000 CFS. Mr. Leak distributed an article from a geological journal that outlines debris flows that have occurred in Colorado in the recent past. One of the key items noted, was that at the same time the debris flow occurred that closed 1-70 near Georgetown, 80 or more additional'debris flows occurred around the state. Mr. Leak addressed the difference between a landslide and a debris flow. A debris flow is typically created by a rainstorm event that erodes and picks up material as it continues down the mountain. A landslide occurs when the ground is highly saturated, and the moisture content of the ground will allow it to start to slide. The two kinds of events are intertwined from the standpoint that moisture has a major impact on both of them. Sometimes a landslide will start a mudflow event. Mr. Leak stipulated that the FLOW-2D model does not deal with landslides. Because of the uncertainties of the data that go into the FLOW-2D model, the flow depths are estimations. A 5 - 6 foot estimated mudflow depth could in fact range fi.om 2 - 3 foot upwards to 7 - 8 foot. Mr. Leak indicated that the schedule for Phase II is approximately 3 months. Some Phase II tasks have akeady been completed. The plan is to obtain detailed information regarding the existing storm sewer systems. The piping has already been included in the alternatives analysis. The topography the resulted fi.om the re-grading at the base of the mountain has also been incorporated. In addition, they have reviewed the ski company's master plan for the mountain itself so they have some understanding of what's being proposed in the future. Based on the new topography, the hydrologic modeling has been revised. The FLOW-2D analysis has also been revised by adding in the impact of the City's buildings and streets on the mudflow events. They have almost finished revising the report of Phase I to include these items, and they have started the FLOW-2D criteria preparation. They have also begun examining the alternatives. After today's meeting, they plan to delve into conceptual design of an alternative acceptable to the City, compile the final report of the Aspen Mountain Drainage Basin Master Plan, and develop the draft drainage and design criteria, which will impact subdivision and zoning regulations. Mr. Leak stated that they have revised the flow capacity analysis based on the existing storm sewer systems, and they have revised the flood hydrology to include the effect of adding streets and buildings to the model. This results in approximately 80% blockage of flow coming through the city and shows flow depths of 8 - 10 feet and higher. So, there's def'mitely a potential problem fi.om a mudflow event within the City as this time. even without development occurring anyplace throughout the City or at the base of the mountain. Most of it is focused in two relaf~vely wide bands going through the City. Typically, a mudflow comes down steep out of the mountain and falls out once it hits the flatter area. So, It is unlikely that very much mud will make it to the river. Mr. Leak posed the following questions: How do we solve it and what is it going to cost to solve it? Or do we live with it? How do we regulate problems that may occur fi.om this type of event? WRC presented three possible draft alternative systems. In general, the existing system within the City itself does not have the capacity m carry a 100-year storm event through the city. Nor is there capacity in many of the storm sewer systems to carry a 5-year event. The alternatives show approximate size, draft locations, and alignments of storm sewer systems needed to convey the runoff down the mountain, through the city, and on to the Roaring Fork River. Also included in the alternatives are water quality extended detention basins. With the EPA regulating water quality fi.om storm water runoff, the City of Aspen will eventually need to address this. So, one of the suggestions is to put in some water quality basins, create a park area, and utilize the park area and basins to treat and store excess runoff. This is accomplished by slowing down the flow before it gets into the river, so it can drop out the sediment and other contaminants attached to the sediment itself. This system would consist of 22" to 84" reinfomed concrete pipe, and as such, is fairiy costly. The need to shut down certain areas as you build through them is also costly. Bob Blaich asked about the extent of the new piping. The answer was that a great deal of the proposal is new piping. Mr. Leak indicated that this is just one concept of the kind of system it would take to bring the City of Aspen up to a 5-year storm sewer system capacity and 100-year total system capacity. This is looking fi.om the base of the mountain on down to the Roaring Fork River. Mr. Leak indicated that this system does not address mudflow. If the City opted to build just the storm sewer system and a mudflow condition occurred, the mudflows themselves would plug the storm sewers and eliminate the capacity. Them are three options for dealing with mudflow. One is to do nothing, simply leave it as it is today. Two is to build facilities that keep the mudflow or debris flow fi.om occurring. Three is to build facilities to convey the mudflow through the City. Mr. Leak showed a diagram of a rock-lined channel with an under drain system and armoring on the top to prevent the water from moving the sediment. This is seen as one possible means to reduce the risk of a mud/debris flow. This option is very conceptual. Basically, the rock-lined channels would go all the way up the main channels within the ski area. The channels would contain boulders with a mean diameter of 3 ft, underlined by some smaller riprap, river rock, some gravel, and granular material with an 18" diameter pipe going down the middle to convey the underground water off the mountain. The purpose is to keep the surrounding soils from becoming saturated with moisture, protecting the stream itself as you get the flows coming down. This option is based on gradation of filters and is fairly expensive. In response to a question from the audience, Mr. Leak indicated that as yet they have not included any fabric. Until the technical meeting, this is simply a concept. It will require some refinement and stability analysis to make sure it can't slide. It may require some different materials to keep it from sliding. The second alternative system looks similar to the first one within the City itself. The City would still have major storm sewer systems with a couple of alternative alignments for the storm drain systems. Again, proposed water quality facilities would alleviate the water quality impacts on the river. The primary difference with this alternative is the placement cutoffconcrete wails spaced every 100 feet all the way up the mountain as a means of keeping debris from coming down the mountain. The walls would be buried below the surface and would not affect skiing. Again, this is very expensive. Conceptually, trying to build these walls may be extremely difficult. Mr. Leak indicated that a few buffed cutoffwalls akeady exist in Spar Gulch along with some riprap in a channel. The risk with these walls is that there are some very steep sections, and between the walls you can get some erosion of material. In Mr. Leak's opinion, the buried concrete walls have more risk of not working than the rock-lined channels. In response to a question from the audience, Mr. Leak indicated that there is no drainage plan in effect with these walls. They will retain whatever water is in the ground. Basically, the purpose is to prevent the head cutting going up, so all you have to create a debrisflow is the volume of material between the two walls. Location of bedrock is a key item in both of these first alternatives. Bob Blaich asked ifWRC had analyzed the existing tunneling from the mines and possible down shafts in these areas. He wondered if it would be possible to divert water vertically down into the mineshafis and then carry it all the way through. He also asked if this would be part of the study. Mr. Leak indicated that this was something that would have to discussed ifP & Z wanted to include it. Ownership of the mines, location and size of the shafts, their condition, and the nnpact of using them as a water conveyance was discussed. What it would take to use the shafts, and their physical ability over the long term were also examined. Mr. Adeh stated his concern that introducing more water into the mineshafts might pose another threat. Concern was expressed about lateral movement and possible landslides. Saturating the mountain with additional water may heighten the problem. A member of the audience asked if the cutoffwalls might back up water creating surface pressure. Mr. Leak stated that there would be some water pressure behind the cutoff walls. He suggested the possibility of putting some relief holes in the walls and introducing some material that would allow the water to flow through. Mr. Leak stated that most of the debris flows that have happened in the state have occurred from erosion of the main channel stem itself. Alternatives one and two are seen as the only potentially feasible means of keeping a debris flow from occurring. The other option is to collect and convey the debris flow through the City. This would involve a proposed system of storm sewers and collection boxes. Essentially this would consist of 'V' shaped collection channels coming into very large box sections, approximately 12' wide by 13' high. The feasibility of this is probably fairly remote. Facilities would be needed in basically three places. In many cases, this would not look any different than the existing humps and valleys that you see when you look up the mountain now. Parts of the area would not be able to be seen, other pans may be visible. Jack Reid asked what would happen if a large pine tree got cross wise in the mouth and stopped everything. Mr. Leak indicated that this was one of the risks. As such, the design includes a free board area that would enable potential blockage to move on through. Mr. Reid asked what would be done with the mud afler it had been conveyed into the box sections. Mr. Leak indicated that this alternative calls for the use of large sedimentation basins. These would have to be much larger than the ones discussed previously. A storage volume of about 100-acre feet would be necessary to keep the mudflow from going into the Roaring Fork River. The water quality basins themselves only require about 15-acre feet. As such, this would entail a substantial excavation to try to contain that mudflow. The collection system was included as a component of this alternative because of the potential for liability if the mud were diverted elsewhere and no attempt was made to keep it from damaging other people's property. The alternatives are summarized as follows: 1. The rock lined channels going up the mountain. 2. The concrete cutoffwalls. 3. Conveying mud/debris on through the city. 4. The do nothing alternative (the no construction or no hard improvement alternative), which means the City will have to deal with mudflows or regulations, etc. WRC believes that alternative #1 would have the lowest cost from an annual operation or maintenance standpoint. Alternative #2 and alternative #3 are going to cost about the same. The do nothing alternative is going to be very expensive, especially when a debris flow does occur. The impact on aesthetics is somewhat subjective. Most of the alternatives will have a fairly low impact, although one may show up more than the others. Of course, the do nothing alternative would have no aesthetic impact until a debris flow occurs. In regards to risk of failure, alternative #1 would be low and alternative #3 would be very low. Alternative #2 with the concrete cutoff walls has more risk of failure than the other methods. The risk of failure for doing nothing is high, because at sometime it will occur. From a safety standpoint, alternatives #1 and #2 would be fairly Iow risk. Alternative #3 poses more risk. especially for skiers and mountain bikers. Steps would have to be taken to keep people out of the box itself. Permanent impact again may also be somewhat subjective. However, WRC believes that alternative #3 has more impact on mountain operations that alternatives #i and #2. The do nothing alternative will not impact the mountain until an event occurs. To implement alternative #1, some stability analysis would need to be done to make sure that the proposed structures will, physically, stay up on the mountain. The type and availability of materials would also need to be considered. Regarding alternative #2, there's some potential for erosion that could expose the walls. There is also potential for runoffto go around the walls. Mr. Leak stated that the wall system probably would not work very well. A structural analysis would need to be done for alternative #3 to determine what it would take to put the boxes through the city. The impact on buildings and utilities would need to be considered. The potential for millions of dollars in damage and loss of life is great with the do nothing alternative. Mr. Leak stated that the model shows potential for mudflows occurring. WRC has identified an area of the City that is most likely to have a mudflow depths of a foot or greater. An option would be to impose certain regulations on development within this area. The regulations could be for new development and for redevelopment. These may include such things as not permitting reconstruction if a building is destroyed; requiring any new structure to have a concrete foundation that extends high enough to prevent damage when a mudflow occurs, requiring that conveyances be built around the structure, or reqmring a mudflow analysis prior to development. The City may choose to permit development only outside the critical area, where less regulation may be necessary. In response to a question from Mr. Erickson, Mr. Leak indicated that mudflows or development would not need to be regulated within the city itself if alternative #1, #2, or #3 were implemented. Mr. Erickson expressed concern that alternatives #1 and #2 would need to take place in the county, and that this would require an intergovernmental agreement. Mr. Adeh indicated that there would need to be shared responsibility because of the direct impact on the City. The importance of getting the County involved in the dialogue was stressed. The possibility of having a joint session was discussed. Savanah Limited Partnership's role in mitigating any potential problems was discussed. Mr. Sarpa agreed that all of the alternatives appear to recommend ways of moving the mud through the problem area to a safe location. The problem area is the site of Savanah's proposed development. The cost of the alternatives and the County's reaction to that cost was discussed. The importance of getting County input and initiating an intergovernmental agreement was reiterated. Mr. Leak provided of overview how he envisions WRC proceeding in the future. The first step is selection and conceptual desi .gn of some sort of storm sewer systein through the city itself, because the storm sewer deals with water flow. The costs associated with water flow and flooding issues are small compared to the substantial costs of mitigating the mudflow. Secondly, the question of whether or not it's feasible to prevent the mudflow from occurring or feasible to move it on through the city will need to be addressed. Thoughts from the technical committee will be needed soon. The third item is development of drainage mudflow criteria. If the City chooses not to implement any of the three alternatives, the criteria would address the kinds of regulations that would protect the lives and interests of the City's residents and businesses to help minimize and mitigate potential future damages. Mr. Leak reiterated the need for input regarding technical feasibility and political feasibility, when taking into account the costs. The risk to Savanah's proposed development was discussed. Mr. Sarpa stated their awareness of the risks involved and indicated that they can work with whatever alternative(s) the City chooses. The risk to the City is high whether or not Savanah develops the property in the problem area. In the event of a debris flow/mudflow, Savanah's development may have a ripple effect on the adjacent properties. Mr. Mooney asked if these issues were being discussed statewide. Mr. Leak indicated that the Colorado Department of Transportation has concerns regarding road blockage. In addition, next year the State of Colorado is looking at developing statewide drainage criteria that may include issues related to mudflows and debris flows. The criteria would not be imposed on communities; however, it could be adopted. This issue is related to flood plains. FEMA allows consumction within flood plains as long as you mitigate for the impacts that you have upon the flood plain and you protect the buildings from additional damage. In the case of a mudflow, you protect the building from damaging others by whatever facilities are put in, and you design the facilities so that they can withstand and not be damaged during the event. Mr. Markalunas expressed concern about the lack of active data. He reviewed an incident during August where he recorded more than twice the precipitation of the local weather station; however, he had now way of corroborating it. He expressed concern that assumptions were being made without technical backup data. Historically, these events have occurred in the past. He referred to the Little Nell slope, which is an ancient debris flow. The problem is that we are making assumptions, but what kind of data are we going to be able to collect in the future to give a correct analysis of what we're dealing with. We don't really have the data to base the decisions on." Leak: "Within the general mountain community in the State of Colorado there is limited data available to make these types of decisions. We utilize the best available information that we have and hope that in the future additional monitoring will be done and over time you'll get additional data. Now as an example, we just fluished analyzing precipitation data for the city of Fort Collins. Even though we had over 100 years of data, we still don't feel as though we can accurately predict a 100 year event without having 1,000 years of data or somehow extend the record out. Markalunas: "They have a satellite dish now on the Crystal River for forecasting. Can we help this situation by initiating quick data that goes into the weather collection system by beaming it up to the satellite so that we can have an extended duration to evacuate the area? Leak: "It would be very helpful to get additional rain gauges up on the mountain with corroborating runoffganges at the bottom. That would be very helpful. Adeh: "The plans were that hopefully within the next year's budget we would be able to put two or three self-recording rain gauge stations at critical points at the foot of the mountain, particularly where the gulches are to get some idea about rainfall. This last summer's event was basically about 1 ½" per hour flood event that roughly fits into a I0 - 15 year flood event. Everyone thought it was a 100-year event, but it wasn't. Unknown: How many years has it been since we've had a 100-year event? Markalunas: "All I know is that I had over 2" of precipitation in under 15 minutes on August 26. The problem was there was no way to corroborate it, other than the weather station up at the water plant, and that was half of what I recorded." Leak: "That,s a big problem having enough gauges where you can get the aerial extent as well as the intensity of the storm." Mooney: "Are you going to get to the conclusion that there is a feasible mitigation here? Are we basically just because of costs, the size and magnitude of the project, just going to be able to put bandaids on this? Are we basically looking at undertaking an incredible construction project?" Adeh: "Before we answer, Alan can give you a lot of information, they are the experts in the state of Colorado and they have done a lot of drainage criteria evaluation. The bottom line is, we have to ask ourselves how much risk are we willing to accept. That's the biggest question every expert will tell you. Beyond that, there's criteria to regulate. Until we come up with the criteria and how much risk we are willing to accept, it's really hard to define any methodology." Unknown: "I think we're risking a lot." Adeh: "So how much is our community, our City fathers, willing to risk? Beyond that, it's mitigation." Erickson: "Basically we have two problems, the first is movement of water offthe mountain through town to the river, the second is mudflow or debris flow. The bottom line overall, is that we have an inadequate drainage system." Adeh: "The best way to answer that is if you design a system to handle a one-year flood, then we are then accepting a different level of risk, a 5-year level of risk, a 10-year level of risk." Erickson: "And that's basically a political decision based on cost, etc. Basically, what level of risk are we going to take - 5, 10, 15, 1007 The same thing with debris flow, right? Even with a 15-year flood, we have a great deal of potential property damage. But, I don't see a greai deal of loss of life resulting from that - em I wrong?" Leak: "It depends when it occurs. If it occurs at 7 p.m. - 8 p.m. when you have a lot of people in restaurants and businesses, that's a potential risk, Or if it occurs at 3 a.m. Is there any warning that it's going to occur? What I want m mention out of this is the state of the art of rain flow analysis right now isn't exact enough for us to say that its going to take a 50 year event to have a potential for debris flow to occur. We basically have modeled what would occur if you have debris flow with that 100-year rainfall event. We don't know if its going to take a 100-year event to prempitate that type of event, or ifa 50-year event will allow that kind of debris flow. If you get a 50-year event, your depth will be less than what we presented. But one thing I want to make real clear is there's no definitive number that anybody can give you right now where its going to take this level of event to create debris flow. Sarpa: "What have other communities done when they are looking at what level of risk? Obviously it's a big financial investment. How have other communities responded?" Leak: "Typically, communities have responded in two ways. One, in an existing area where you are looking at a retrofit type of situation, most communities have opted for a 5- year storm sewer system, allowing a 100-year event to occur however its going to occur - they assume that type of-risk. For new development, usually they require analysis and protection of that development property of up to a 100-year event. That mainly follows FEMA regulations for flood plain and flood plain management." Adeh: "And for that matter, they also have more criteria. If you design a collection system that will handle a 5-year event, then they define another criteria for how to deal with a major flood. In that case, even in our uniform building code, there is clear language that says the lowest occupied building must be protected to a certain depth to allow front yards and street right-of-ways to act as a major flood conveyance channel. That means that we have the buildings high enough, protected enough that we can prevent the loss of life. So, there are mitigation criteria for major storms, but in this case we have lots of homes below the streets and our risk is very high." Leak: "The next area that FEMA has regulated is alluvial fans, where it is a little less certain. When the water comes down that alluvial fan, where is it going to turn? They've allowed development on those alluvial fans based on a probability of risk of that event occurring or impacting that facility, and then requinng that facility to construct improvements to protect that facility. This is where I see this mudflow/debris flows event issue that FEMA really hasn't regulated and doesn't have a lot of experience with. I want to say, you're not on the cutting edge of teclmology but you're addressing a problem that many others have chosen to ignore. Erickson: ."What are they doing in California because that seems to be one of the number one areas for debris flow? Leak: "I know that the FLOW 2D model is being used in several areas in California. They've also used it m model the Turkey River. They're not looking at mud and debris flow, but it's starting to get more application as people see that this is another risk that needs to be assessed." Erickson: '~qais is really too new for anyone to be able to say, based on our modeling that we've done ovei' the last I0 years that these are the actions we've taken to prevent an episode in the furore. So we don't have much of that." Leak: "There's not a lot of that, no. Jim has had more experience in having modeled these areas. What did you do? Did you do anything or did you just say we've got a problem here. Jim is the person who designed the FLOW 2D model." Adeh: "One thing you may like to hear is that this model was developed based on the geological soil conditions on Aspen Mountain & Glenwood Mountain. So at least, they are giving you some real life, near reality data." Hunt: "It's a common and disturbing thing I've seen how France deals with avalanches. How they break up those avalanches and stop them before they get into the areas, they had space to do it. Unfortunately, we've developed in that space where we Could have done that sort of thing. In other words, we put up the barriers that would keep slowing it down 80% of the time and finally it ends at the city limits instead of over the city. That's why I was sort of tongue in cheek, well hey if we put the development at the top of Mill. that's one little breaking area. But is there any combination. Basically, I understand that the cutoff walls sort of do the trick. But it's almost unpalatable to form a dam at the edge of the city to retain it as its coming down." Leak: "We can put in a dam big enough to stop almost anything. But what I'm saying here is can we build a debris basins up stream somewhere. Because of the steepness, if we build a dam, you would probably be looking a soil cement type of structures and it wouldn't be very aesthetic. Adeh: "The Japanese experienced this in one of their ski villages, and it didn't hold. If you put it too far down, when the momentum builds, nothing is going to stop it. The trick is to catch if up front at the point where it forms. That's why one of the better alternatives Alan was proposing was to go up to the source and handle it right there." Erickson: "I have a political question, what kind of liability/responsibility does the ski company have in tlfis?" Adeh: "That's a good question. As a property owner, its my professional opinion that they should not cause any damage to adjacent and downstream properties. That's the limit I can tell you. The property owner is responsible for not causing damage to downstream and adjacent properties. Erickson: "Not to create the problem." Adeh: And if it's historically happening condition, then they're not a risk, not at fault because they:ye been skiing there before having done any development. Erickson/Mooney: "Except that they have done development, just not residential development. They've cut the vegetation - they've removed those factors that prevent the slopes from sliding." Adeh: If the natural, historic condition is altered then I would say the property owner is responsible for it." Erickson: "That happened in 1983. And more snow making equipment. That's all development. The density ofmanmade snow is a lot heavier, and lot thicker, the water content is a lot higher than natural snow in this area. So if you get I" of man made snow, someone told me it had the equivalent of 6" of natural snow." Leak: "That's because it has to do with the physics and chemistry of snow melt itself. And the fact that you have to have a certain temperature for a certain duration to melt that snow. Without getting into real specifics, it really has to do with the sun and the amount of exposure. The water content is a portion but it's not the major component. It's really temperature and humidity that dictate how quickly that snow is going to melt." Unknown: "Even if you can document that they are putting millions of gallons of water on the mountain." Leak: "And that's what I was saying, it does affect peak flow, it likely affects how long that ground is saturated, so it extends out the saturation season probably takes longer for it to drain out." Unknown: "Does it increase the risk ora rain on snow condition?" Leak: "certainly." "Or mudslides, or landslides?" Mooney: "While we're all casual about the urgency of this, what is the timing. I know it's unpredictable for a 100 year event, the weather's changing everywhere, even in Europe last year they had catastrophic mudslides. Are you going to take this to City Council and ask them to choose a plan? Is there a progression of events here we can follow?" Adeh: "We're hoping re have one more meeting next month to at least prepare a draft o£the master plan. And then go to City Council in their work session and explain to them what we have done so far. This is hex a scientific event to prove anything, but based on what we have learned, practiced we can tell them these are the potentials. The proposed criteria we are bringing along with the master plan, then it becomes just like a uniform building code; this will become another enforcement tool." Mooney: "Is there something that the insurance compames are going to step in with their data on how they insure properties in Aspen now? Are the rates going to increase? Are they going to have to do their studies?" Adeh: "I would say any measures toward reducing the impact will help keep the insurance premiums in check. Commun/ties which have come up with more definitive master plan criteria, would have much Iow. er premiums. Hunt: "First of all I like the idea of an early warmng system. If we had somewhere between 2, 5, 10 rain gauges disbursed throughout the area coming to a central spot as a warnmg area, if all of the sudden we had 2" dump in a half hour in Spar Gulch, we know we've got problems and we had better start warning people in the community. That's one tiring. I would sort of like to see that kind of a system in soon. Number two is my gut feeling that probably what would work best with our cimumstances would be a combination of drainage plans. Put in cut offwails ha a few cr/tical areas. I don't think any one is the solution. Maybe a combination of things because I don't like the idea ora sealed cutoffwall backing up moisture and having ail of that waiting lubrication just backed up hill waiting to move. A cutoffwall is tine, but we've got to figure out a way of draining that hydaulic pressure. But I would certainly, from the community point of view, like the idea of the warning system, and that would be necessary if we had the whole cutoff drainage plan in place." Leak: "Basically emergency management planning. Now we have the knowledge, we know we're halfway there. We know this could occur. Here's the real critical areas, lets put something in place that will assist us in early warning and then be able to deal with that event when it occurs." So are we going to be on any kind of timeline for applications for development that we have in our pipeline. Are we going to have any k/nd of direction for the commun/ty development department so that action can be either temporarily postponed, or what are we going to do?" Adeh: "Under the do nothing proposal, what we are doing with the Top of Mill lot #3 development, we're going to do a localized model that if an event comes in, how much nsc would the blockage ora building cause? That is a potential to identify. The developer's engineers would be able to address that, how they are going to mitigate. That's what we are moving toward in/tially under the do nothing option." "So even under the do nothing, we are going to require or request applicants to mitigate for the 100 year event or their applications don't go forward." Adeh: "That's up to you as P & Z to make that decision. We are tpjing to identify the potential, bring that information to you and to the developer, then its between you and the developer can come up with the decisions. If you leave it up to us, we will tell them what the good suggestions criteria would be. But if you tell them to mitigate." Sarpa: "To answer your question from our standpoint, we've been working with Nick and WRC step for step and it's our intention before final approval that this needs to be decided. We've looked at it during the conceptual phase, and when we come back for final approval. The information should be available by January, their work should be done then the decision will have to be made. The idea for us would be sometime in January, we'll come back for final rewew and that's when these decisions will have to be made. We will have our own ideas on how to mitigate it, whether its ditches or wails, then you guys will decide yes or no." So each developer or each person who comes in and wants to build a home doesn't have to go through 2 ½, 3 years, 7 years of hydrological studies in order to determine whether they can build a house or not. Sarpa: "And of course, what we want to avoid is putting something on that piece when its determined sometime thereafter that there's no way that anything above it would have been done that would have necessitated what we did on our piece. They may not be possible. That would be ideal as far as timing. So for instance, if we were going to mitigate at the 100 year level, and it was determined afterward because of cost that there was no way what was above our stte was going to be mitigated at the 100 year level, it was only going to get 50 or 25 year level, then of course we would want to do something on our site that was compatible with that." Unknown: "You still have responsibilities to the people downstream. Our lawsuit is with the people above you." Ohlson: "I just want to pm the entire study into the context of 1) why it was undertaken in the first place. That was to develop the big picture asto how we wanted to fix the current deficiencies that are there today. That responsibility rests somewhere with government, most likely for the greatest pementage of it. And what new development would add to in the sense tax the current environmental conditions that we have and to try to analyze and deduce what would happen under certain development scenarios as well. That's another portion of the project or maybe it's a question." Leak: "That's developing criteria to say how do we allow a development to occur? What's going to be required of the applicant to show, what information is he going to have to develop and submit for review." Ohlson: "The responsibility of new development is to mitigate their portion of the impacts on this new drainage master plan, what we determine is their share of the impacts and to ther/assist with whatever implementation strategy so that we can come up with whether its walls, or p~pes, or conduits or whatever it tums om to be. That share be fairly placed on the next issue again. This time its an environmental issue. Have them pay their fair share to implement this plan. So, the no situation to not do anything is really the current situation that we're operating under fight now, we don't have a plan except perhaps putting in new streets, gutters that's how we carry the current water fight now, new drainage pipes that we might require the applicant to put in the ground. Other than that we have this big picture that we're trying to get to." Leak: "That's right and I really should say that the do nothing alternative is really do no mudflow improvements. There is the alternative of putting regulations in place requiring storm sewer systems and requiring that the applicant pay their proportionate share of the system. However you analyze what their impact on the system would be." Mooney: "Do you think that there's a way to forecast or predict the probability of the timeframe for that 100 year event. Rain on top of snow happens in spring time and the rate of melt happens in the spring time, run offhappens in the spring time. Is there any way to say that there's a critical timeframe when this 100 year event? What is the liability? Let's say on a normal use day. there's 5,000 people in town, and if we know what the probability of the timeframe of this 100 year event is and if there's 10,000 people in town are we liable for not informing them that basically in the spring time if there's a 100 year event, people shouldn't be standing around." Leak "That's a very difficult question to answer from the standpoint that 1) we cannot predict when and if this event is going to occur. We know that historically events typically will occur between April and October. We usually don't get a big rainfall event in November or December. From that standpoint, we have some good data that there is a general season, but it can occur anytime during the season itself. From a risk standpoint from what I've been involved with water law, once a problem becomes known then you're moving toward some sort of solution and then something occurs it occurs, everybody's game for a lawsuit but at least you've put your ducks in a row and you say we're trying to take care of this the problem, the courts don't require you to fully solve the problem because there's not enough money in this world to solve every problem. So, as long as you have a plan to put in place, and you're implementing some sort of plan usually the court says, this is my engineering legal opinion. that's what the role of government is. To try to assist and protect the people, identify the hazards and to provide whatever assistance to keep them from occurring or mitigate for their occurrences. Adeh: "So criteria becomes very critical, a crucial pan of any new development review and approval. Because they say that we don't like the criteria is one concern. But we did do an analysis, we did de/me a criteria, we did define a set of rules by which we approve development, and that com~s with many things including how much water are you going to allow to spread in the streets when it rains? Do you allow it over the top of the sidewalks? Do you allow it to go into the first levels and wet the carpets? Where do you stop? That's the level of the criteria you have to define. And that determines how elaborate the system should be. Leak: "You'll be a lot better offin the court system if you've got something in place than in you have nothing in place." Unknown: "I understand why mudflow's a hazard. But, you don't have to have a 100 year storm to cause a mudflow. Heavy rain ~s pretty much an annual event here every three or four years. How do you predict a 100 year event with rain. mud and what the damage will be. Leak: "That's the problem. We know what conditions make it right for a potential debris flow, and we can identify the likelihood that a debris flow will occur in this area. but as far a mag~aitude and frequency we cannot guarantee a 100 year storm. Curtis: "Since what has happened all around, the probability increases. The conditions are ripe on Aspen Mountain for it, everything's the same as it is in these other situations." Adeh: "A lot of it is observation, experience. The four years if been observing I've seen Highway 82 covered by two feet of mud that was only about 1 ½ years ago and we saw Ute Ave. covered by 3" of mud." Erickson: "I read that Phase I report, and I had a difficult time because I'm not an expert. You've really illuminated everything and made it a lot clearer as far as concerns, and what our problems are and what potential solutions can be." Leak: "I hate to be the bearer of bad news." Erickson: "The bad news has been there, but now we're beginning to fred out about it. Now we can do something about it." Adeh: "Do you have any suggestions as to what we do? Are we doing too much?" Erickson: "Try to get the county involved as soon as possible in this thing because it's not going to be our solution." Mooney: "I think it's a statewide issue." Erickson: "Maybe we can get federal government disaster funds." Adeh: "Thank you for coming, I appreciate your efforts. We will try to come before you one more time. We will show you the whole master plan and the criteria as a draft. And we are hoping to have a session with the city council early next year. AGENDA PHASE H PROJECT UPDATE ASPEN MOUNTAIN DRAINAGE BASIlq MASTER PLAN NOVEMBER 22, 1999 Introduction II BriefOver~iew of Phase 1 Existing Drainage Facilities Mudflow Analysis Snowmelt Analysi: III Review Scope of Work for Phase II IV Phase II Tasks Completed Revise Storm Sewer Hydraulic Analysis Revise FLO-2D Analysis FLO-2D Analysis of Effect of New Development V Remaining Phase II Tasks Drainage Criteria Development Mudflow Criteria Developmem Preliminary Design Master Plan Report VI Discussion CONSULTING ENGINEERS 950 SOUTH CHERRY STREET · SUITE 404 · DENVER COLORADO 80246 ,, (303) 757-8513 · FAX (303) 758-3208 · wrceng@eazy, net