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HomeMy WebLinkAboutFile Documents.813 W Smuggler St.0253.2018 (30).ARBK DRAINAGE REPORT FOR RANGER STATION SUBDIVISION Lot 3 813 West Smuggler CITY OF ASPEN, COLORADO PARCEL ID: 2735-124-28-003 PREPARED FOR: Ranger Station Subdivision Aspen, CO 81611 PREPARED BY: High Country Engineering, Inc. 1517 Blake Avenue, Suite 101 Glenwood Springs, CO 81601 (970) 945-8676 March 15, 2019 Revised: September 25, 2019 HCE JOB NUMBER: 2181030.03 09/26/2019 Page 2 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc TABLE OF CONTENTS SECTION PAGE I. GENERAL LOCATION AND DESCRIPTION OF SITE 4 II. DRAINAGE STUDIES 6 III. EXISTING SUB BASIN DESCRIPTION 6 IV. DRAINAGE DESIGN CRITERIA 7 V. DRAINAGE FACILITY DESIGN 10 VI. CONCLUSION 14 VIII. REFERENCES 15 EXHIBITS: 1. Vicinity Map (8.5”x11”) 2. Flow Path (8.5”x11”) 3. Existing Drainage Basin Map (24”x36”) 4. Proposed Drainage Basin Map (24”x36”) 5. FEMA Mapping (11”x17”) 6. USDA Web Soil Survey (8.5”x11”) 7. HP-Kumar Subsoil Study & Percolation Test for Lot 3 (8.5”x11”) Appendices Hydrologic Computations  Existing Conditions  Proposed Conditions Hydraulic Computations  Swale & Pipe Calculations  Trench Drain  Drywell-Detention  Weir Aspen Charts and Figures 09/26/2019 Page 3 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Engineers Certification “I hereby affirm that this report and the accompanying plans for the construction of a residence at 813 Smuggler, Aspen, CO was prepared by me (or under my direct supervision) for the owners thereof in accordance with the provisions of the City of Aspen Urban Runoff Management Plan and approved variances are exceptions listed thereto. I understand that it is the policy of the City of Aspen that the City of Aspen does not and will not assume liability for drainage facilities designed by others.” License No. 29975 Roger D. Neal, P.E. Licensed Professional Engineer, State of Colorado 09/26/2019 Reviewed by Engineering 10/14/2019 4:00:13 PM "It should be known that this review shall not relieve the applicant of their responsibility to comply with the requirements of the City of Aspen. The review and approval by the City is offered only to assist the applicant's understanding of the applicable Engineering requirements." The issuance of a permit based on construction documents and other data shall not prevent the City of Aspen from requiring the correction of errors in the construction documents and other data. Page 4 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc I. GENERAL LOCATION AND DESCRIPTION OF SITE A. Location 813 Smuggler, Lot 3 of the Ranger Station Subdivision, is located in the Castle Creek Drainage Basin. The property is approximately 800 feet east of Castle Creek and 2,300 feet south of the Roaring Fork River. A 20’ ditch easement runs within this lot on the easterly side of this property along a slight rise. North 8th Street is located west of the property separated by lots 1 and 2 of the same subdivision and provides the access via an alley to the garage. North 8th continues northerly for approximately 600 feet and intersects Meadows road. West Smuggler bounds the property to the north. The site is located within the City of Aspen, County of Pitkin, and State of Colorado. A Vicinity Map has been included as Exhibit #1. B. Description of Property The proposed Lot 3 also known as 813 W. Smuggler in the Ranger Station Subdivision site is approximately 6,619 square feet (0.152 acres) in size per the amended and restated plat. The north and west property lines run parallel to West Smuggler Street and North 8th Street. The south and east sides of the proposed lot is bound by Lot 4 to the south and Forest Service Headquarters land to the east. An easement on lots 1-4 create the access to lots 1-4 in a newly created alley that is currently under construction. Roadways and vegetation bind the north and south sides. The east and west sides are bound by lots 2 of Ranger Station Subdivision and Forest Service Headquarters land on the easterly side separated by a 20’ easement for a COA irrigation ditch that runs on both properties at a slight northeast diagonal. The existing site consists of vegetation, the irrigation ditch and the alley under construction. There are no affected trees on the site per the survey conducted by HCE and Sopris Engineering. The only noted trees are in the ROW of W. Smugger and those existing trees are not going to be disturbed. The only other remaining trees on the site are within the ditch easement and are to be protected from the proposed construction activities per the City Foresters recommendations during construction. The storm water on the existing site flows from southeast to the northwest. The eastern side of the site is the irrigation ditch, and stormwater flows will flow to the ditch and then northerly off site. Southern stormwater flows will be intercepted by the proposed driveway trench drains and discharged into the vegetated swale of the Lot 3 storm system. Flows from the west are intercepted by Lot 2 and will be contained within that lot. Stormwater on-site will flow through the storm-water system on the lot and will discharge northerly into the W. Smuggler ROW as it has historically. There are four design points that show the location of historic runoff released on site. These include the alley runoff captured in the driveway, two discharge points into the pond, the ditch east of the site, and the location of site overflow. The main historic release for this site is at the northwest corner of the site, and the flows convey offsite as overflow from a pond system during major events. The irrigation ditch also is considered a historic release but only captures storm flows that fall within the easement or grasses next to the home and no impervious surfaces. This site is located on the boundary of the Castle Creek and Aspen Mountain drainage basins; therefore, no significant offsite basins 09/26/2019 Page 5 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc were used in the design. The historic drainage of the site was delineated into two basins; see attached existing basin map for site layout (EXDR-01). C. Lot Soils Description A site-specific geotechnical soil study was completed for foundation design and percolation testing has been provided for Lot 3 (Exhibit #7). The site is well above the river elevation and groundwater was not encountered to approximately the depth of the drywell level. The City of Aspen soils map locates this site in the Type “B” soils area. According to the USDA Web Soil Survey, the property is within section 107 and the report states that it consists of soil Type “B”; see USDA Web Soil Survey (Exhibit #6). D. Description of Project Goals The proposed project will consist of construction of a new home and the associated grading, access utilities and landscaping. The largest portion of the construction will be the construction of the home including excavation of a deep basement and utilities into the foundation. Since this is a small lot, much of the site will be disturbed. The only vegetation on the site to be preserved are the trees near the alley, the vegetation and bushes in the irrigation easement and trees outside the property to the north in W. Smuggler ROW. Site landscaping will include grass swales, as well as grasses, trees and shrubs around the home to the west, north and east to revegetate the disturbed areas of the site. The site is located in the Castle Creek Basin; therefore, the property owner will not be able to pay the fee-in-lieu of detention. The proposed home drainage will be mitigated by providing site detention and water quality within the site through a stormwater system with a grass-lined swale and bio-retention pond. The pond will collect the required WQCV per the city code at the required volume. An underdrain allows the pond to drain to the storm manhole for additional detention volume. Stormwater will flow directly into the manhole through an overflow inlet in the pond. For larger storms that fill the pond and manhole, an overflow weir will allow flows to follow the historic route once both systems inundate. Also, the manhole will have a sump pump and once it reaches capacity, the pump will activate, flow through a discharge pipe and daylight on site. The manhole and pond will collect the detention volume and will discharge to groundwater to W. Smuggler Street for larger storm events through a controlled weir and discharge pipe. The pond provides the controlled release at the northwest corner of the property via a stepped weir. The site is required to capture 59.7 CF of WQCV. The site is also required to capture 73CF of detention, thus sizing the pond and manhole to capture a minimum of 176 CF, well in excess of the calculated requirement, to allow for a factor of safety since there is room to have some redundancy; see attached WQCV in the Appendices. The site consists of four proposed basin, see attached proposed basin map for site layout (Exhibit #4). 09/26/2019 Page 6 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc II. DRAINAGE STUDIES A. Major Basin Description The proposed site is located on FEMA’s Flood Insurance Rate Map (FIRM) No. 08097C0203C which has an effective date of June 4, 1987. The site is located in Zone-X, this zone is described as areas determined to be outside 100-year and 500-year floodplains. The 100 year floodway line follows the Roaring Fork River to the west of the site per the survey documents and FEMA mapping. There is one 20-foot access and utility easement within the site. The existing site is outside the affected areas per the WRC Engineering mapping and report for 100yr and mudflow events. The Mud Flow Zone mapping in the Storm Drainage Master Plan for the City of Aspen, Colorado by WRC Engineering, Inc. in November of 2001 was utilized for this section. Other residential areas surround the property. B. Previous Drainage Studies There are no previous site specific drainage studies for the site. The proposed site is located within the Study Area Boundary for the “Storm Drainage Plan for the City of Aspen” by WRC Engineering, Inc. in November 2001; however, the site does not drain to the storm sewer system for the City of Aspen. C. Receiving System and Effects of Adjacent Drainage Issues There are no major drainage issues with the adjacent properties that affect the site or that the site affects. The existing site north and east of the site discharges sheet flow to the north and west property lines as it did historically. The area southwest of the existing site discharges to an N. 8th roadside swale, then travels north and discharges into the Nth. 8th and W. Smuggler intersection. From there the road appears to carry the drainage northerly and ultimately draining through some large parcels and into the castle creek wetlands. The area southeast of the site discharges northerly along with this site to West Smuggler and down Nth. 8th as before. This system ultimately delivers the stormwater to Castle Creek then the Roaring Fork River. III. EXISTING SUB BASINS DESCRIPTION The existing site’s historic drainage patterns flow from the southeast to the northwest. As stated above, Castle Creek and the Roaring Fork River are the direct receiving facilities. The difference in the historic conditions versus the proposed conditions with historic condition flowrates will determine the amount of storm water allowed to flow offsite. The historic site is depicted as two onsite drainage basins. Refer to sheet EXDR (exhibit #3) for a map of existing basin layouts. Basin EX-1 encompasses the majority of the lot and the currently being constructed access road. Bain EX-2 encompasses the Irrigation ditch and area on the easterly side of the property. 09/26/2019 Page 7 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Historic Flow Path One: Runoff from basin EX-1 sheet flows to the northwest from the southeast corner of the lot. The runoff continues to sheet flow past the property boundary into an existing roadside swale along W. Smuggler St. before crossing Smuggler Street at the intersection of N. 8th and then flows northerly along N. 8th . Once near Meadows Road, the flow continues to travel northwesterly between two residences onto Sneaky lane and into a large, marshy looking wetlands near Castle Creek. After flowing into Castle Creek, the stormwater runoff will join the Roaring Fork River. Please see Exhibit #2. The location of the properties in the Castle Creek drainage basin prevents excessive runoff from other offsite locations. The basin consists of natural/historic grasses and plants. Design point one has been associated with the northwest corner of the basin. Calculations for onsite flows pertain to this location. Historic Flow Path two: Runoff from basin EX-2 sheet flows from southeast to northeast on Lot 3. This basin includes the irrigation ditch on the easterly side of the property. The runoff flows to the irrigation ditch and northeasterly in the ditch to a pipe that flows under W. Smuggler Drive. This basin consists of natural/historic grasses and plants. Design point two has been associated with the northeast corner of the basin. Calculations for onsite flows pertain to this location. Table 1. Existing Basin Characteristics BASIN AREA, ACRES C, 10YR I, 10YR Q10-YEAR, CFS C, 100YR I, 100 YR Q100-YEAR, CFS EX-1 0.117 0.15 3.15 0.06 0.35 5.02 0.21 EX-2 0.035 0.15 2.99 0.06 0.35 4.78 0.20 TOTAL 0.12 TOTAL 0.42 Existing historic release rates from the site release at Design Point 1 is at a rate of 0.06 cfs (10yr) and 0.21cfs (100yr). IV. DRAINAGE DESIGN CRITERIA A. Criteria This drainage study was prepared in conformance with the City of Aspen, Colorado Urban Runoff Management Plan (URMP), dated April of 2010 and the revised sections dated thereafter (December 2014). More than 1,000 square feet of area will be disturbed with the proposed construction; therefore, the site is viewed as a Major Drainage Report per the URMP. More than 25-percent of the site is being disturbed, so water quality for the entire site will be necessary per the URMP. The existing site was analyzed in its historic condition (i.e. no improvements). The offsite basin consisting of landscape area was analyzed as existing (open space) per the URMP. Water Quality Capture Volume (WQCV) will be determined for the site as per the URMP 09/26/2019 Page 8 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc standards. The WQCV is defined as the treatment for up to the 80th percentile runoff event, corresponding to between a 6-month to 1-year event. The WQCV was determined using the equations and Figure 8.13 from Chapter 8 of the URMP. WQCV was calculated using the effective imperviousness. The WQCV equation is: Volume (ft3)=WQCV in watershed inches x 1 ft/12 in x area (acres) x 43,560 ft2/acre. B. Hydrologic Criteria The hydrologic methods for this study are outlined in the URMP from the City of Aspen, Colorado (revised December 2014) and updates utilizing a Microsoft Excel spreadsheet set up for the Rational Method calculations/equations. The obtained rainfall amounts for each basin used the updated Figure 2.1 “IDF Curves for Aspen, Colorado” in the URMP publication from the City of Aspen, Colorado. Using these curves, the rainfall intensity corresponding to the 2-yr, 1-hr storm 10-yr, 1-hr storm, and 100- yr, 1-hr storm event were determined based on the time of concentration for each basin. Figure 3.3 from the URMP was used to determine the runoff coefficients for the 2-year, 10- year and 100-year storm events since the soils were determined to be Type ‘B’ soils. The Rational Method was used for historic and proposed flowrates as outlined in the URMP calculations/equations. The proposed WQ underground structure was sized using the URMP methodology and the required detention for the entire tributary area of the site. The required detention was determined using the methodology described in the URMP for a drywell with a controlled outlet; in this case, a storm manhole was utilized. Runoff is not capable of discharging to a storm sewer system or directly into the river; therefore, detention beyond WQCV is required for the site runoff. Existing and proposed grades do not allow an outlet structure to discharge historic release rates for small events, so runoff cannot discharge out of the bio-retention ponds until the water surface elevation reaches the weir crest. Due to this condition, required detention calculations have been modified for the storage volume used in design. The amount of storage required was determined by calculating full detention until the runoff hydrograph intersects the outflow hydrograph. By providing full detention until the runoff hydrograph reaches the 100-year peak flowrate, the proposed system is capable of detaining 100-year storage volumes while keeping the weir discharge to 100-year historic peak flowrates or less. Hydrograph calculations for detention can be found in the appendices under drywell detention. All charts and figures mentioned from the URMP are located in the last section of the appendices under the “Aspen Charts/Figures” section. C. Hydraulic Criteria Lot 3 will have roof drains and patio drains piped to the pond, a swale will also conveys stormwater runoff from the west portion of the site and driveway trench drains. There is 09/26/2019 Page 9 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc also a pipe that allows overflow from the pond to the storm manhole that has been sized for the 100-year event. See basin descriptions below for explanation. Lot 3 has a trench drain in the driveway that was calculated utilizing the Hydrology channel calculator with AutoCAD’s system. The trench drain has the ability to carry the 100-year event for the small portion that it collects. We calculated the 10 and 100-year depths for the swale, as well as 2 pipes within the system that carry the stormwater flows to the pond and drywell. The swale and storm pipes have been sized per Civil 3D Hydraflow Express utilizing Manning’s equation. The 6” pipe is located at the overflow inlet and discharges into the drywell. The 4” pipe conveys runoff from the deck drains, downspouts and patio inlet that will discharge into the drywell. During a 100yr storm, the design capacities for the 6” and 4” pipes are 50% and 58% of the pipe depth, respectively. See basin descriptions below for explanation. WQCV and required detention volumes will be stored in the bio retention pond and storm manhole, which will percolate through the sand/peat mix and any excess treated water will flow to the storm manhole via an overflow pipe. Once the water in the manhole reaches capacity, the sump pump will be activated and flow through a discharge pipe and daylight on site. The manhole was designed to accommodate 78.8 CF until the point of overflow. The pond and manhole meets the total required detention of 176 CF. The storm manhole and pond will collect storm water from the site via pipes and swales, which convey runoff to the pond and manhole with minimum slopes of 2.0% in swales and 2.0% in pipes per the design documents. The proposed manhole detention will be held until capacity is reached, to which point a sump pump will be activated and daylight on site via a discharge pipe. If the pond water level surpasses the proposed detention, then the system will overflow via a broad crested concrete weir with a length of 1.25 feet and a notch depth of 0.25 feet. This will allow storm-water to leave the site at less than the historic and proposed 10-year and 100-year flowrates. See appendices for weir release calculations. D. Site Constraints The site is mainly clear of any onsite issues, but has disturbance areas within tree driplines. The site is vegetated with native grasses brush and trees. E. Easements, Irrigation Facilities, Waterways A 20-foot wide access and utility easement runs through the site. There is also a 20’ irrigation easement that runs through the easterly portion of the site. There are standard zoning setbacks from property lines. These setbacks do not affect the overall site drainage design. V. DRAINAGE FACILITY DESIGN. A. General Concept 09/26/2019 Page 10 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc The proposed site plan calls for a new single family home, patios and sidewalks, drainage facilities, and associated landscaping. A drywell and pond are proposed to be constructed on the northwest side of the property. The pond and drywell will allow for the capture of the required WQCV and detention per the URMP code. The existing swale along Smuggler Street will carry the site overflow to the corner of Smuggler Street and North 8th Street. Historically, a total rate of 0.21cfs (100YR) left the site as sheet flow to the northwest. In the proposed condition, a total of no more than 0.21cfs (100YR) will leave the site after flowing to the drywell and pond, which has excess storage capacity to reduce the offsite flowrate, rather than sheet flow directly to the neighboring property. The storm water facilities designed within this report focuses on collection and distribution to the pond and storm manhole systems. Due to constraints, this is the most feasible design for the site. The grass lined swales and roof drains will allow for capture of all hardscape water and the transfer of that water to the bio-pond for required WQCV and manhole for detention on the site. The storm manhole will not have a water quality section due to constraints of the site, but all of the site WQCV will be within the bio-retention pond. A site-specific geotechnical soil study was completed for this site, with gradation of the soils and percolation test provided (Exhibit #7). The 9 Principles for storm water quality management were followed during the design process to create the best storm water design and water quality management for this particular site and design concept. Principle No. 1: From day one of all projects that HCE is reviewing we are always pushing storm water, detention and WQ structures on the architects and landscape planners. In this case we have a limited space for these features due to the preservation of existing trees and landscape and the prevention of disturbed areas near other trees near the site. The new home with the associated landscaping has just enough spacing to allow the pond on the north side of the property. Principle No. 2: The proposed redevelopment allows a vast majority of runoff from onsite, impervious areas to be directed to the stormwater quality treatment system. Pervious terrain or ground cover surround the remaining areas. The main structure is proposed to be built in the northern area of the site where stormwater flows will travel around the site in grass lined swales and pipe to provide treatment before entering the bioretention pond and then storm manhole for detention. Over detention also provides additional attenuation of storms to help reduce impacts by reducing flows to surrounding properties. Principle No. 3: The proposed changes to the site will increase impervious area on the property. Principle No. 4: The proposed removal of various pervious areas within the PR-1 basin increases runoff rates. The increased contact between stormwater and the adequately sloped soil will support infiltration and prevent ponding, thus reducing runoff volume. Disturbance within saved tree driplines will be avoided as much as possible. The proposed redevelopment will more closely match natural conditions than the existing features onsite and will add more 09/26/2019 Page 11 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc intensive and pervious landscaping than the natural conditions. Principle No. 5: The pond and storm manhole being oversized will reduce small storm runoff from the site and will accommodate for 10yr or 100yr storm events. The pond capacity includes the required WQCV. Principle No. 6: The stormwater facilities on this site take great care to direct water away from the neighboring property and to appropriate facilities, resulting in reduced flow leaving the site. This will improve the impact of stormwater on neighboring properties. Principle No. 7: The treatment train for this site is limited, but mostly invisible to the public in nature. The addition of stormwater back into the ground in a fast method adds to the WQ process for the stormwater and recharges the area’s aquafers in a faster manner that just stormwater runoff itself. Principle No. 8: All of the onsite facilitates are easily accessible for cleaning and inspection. The storm manhole will have a 24” removable lid for easy access and cleaning in the event that the system needs maintenance or repair. Principle No. 9: There are no steep or deep channels being proposed that could cause tripping or fall hazards. All methods of actual treatment and storage are out of the public’s site and allow safe passage. The proposed site is divided into three onsite drainage basins and one offsite basin. Refer to sheet PRDR, Exhibit #4, in the exhibit section for a map of the proposed basins and drainage design. Basin PR-1 consists of the westerly side of the lot and approximately half of the proposed home. This basin also includes a sidewalk on the front of the home and garage driveway. The pond on the north side of the lot is also included in this basin. The drainage from PR-1 is routed along the westerly side of the property around the front of the home into the pond. Driveway drainage is captured with trench drains and discharged into the proposed swale at design point 1. The flows release at design point 2 into the bioretention pond. The site overflow is located in this basin at the pond weir and discharges at design point 5. This only happens once the pond and storm manhole have been filled and then will release at the historic 10 and 100-year rates. Basin PR-2 consists of the easterly side of the lot and approximately half of the proposed home. Additionally, the storm manhole is included in this basin. This basin includes a pipe from an upper deck and patio drains that will drain to the pond and flow to the manhole for additional detention. The drainage from PR-2 is routed along the easterly side of the property around the front of the home and into the pond in basin PR-1. The flows release at design point 3. Basin PR-3 consists of the irrigation ditch on the easterly side of the property. The runoff flows to the irrigation ditch and northeasterly in the ditch to a pipe that flows under W. 09/26/2019 Page 12 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Smuggler Street. This basin consists of natural/historic grasses and plants. . The flows release at design point 4 into the ditch into a culvert and across W. Smuggler St. Basin OS-1 consists of the alley that is currently being completed with the permit for the alley improvements. This small basin sheet flows to the previously mentioned trench drains in basin PR-1 at design point 1. The design for proposed conditions on the site satisfies WQCV and the stormwater detention requirements. Table 2, is a summary of the proposed basins. Table 2. Proposed Basin Characteristics BASIN AREA, ACRES C, 10YR I, 10YR Q10-YEAR, CFS C, 100YR I, 100 YR Q100-YEAR, CFS PR-1 0.062 0.55 3.96 0.14 0.64 6.33 0.25 PR-2 0.050 0.54 3.96 0.11 0.63 6.33 0.20 PR-3 0.040 0.15 2.98 0.02 0.35 4.75 0.07 OS-1 0.007 0.92 3.96 0.03 0.96 6.33 0.04 TOTAL 0.30 TOTAL 0.56 B. Low Impact Site Design The proposed site design incorporates as much low impact site design criteria from the URMP as is site feasible. Grass lined swales will convey the proposed flows to the designed pond and then to the storm manhole, for additional detention, located near the north property line. All storm water generated by the home will be conveyed through the bio-pond, thus reducing the transfer of pollutants to the City of Aspen storm sewer system and the Castle Creek and Roaring Fork River release point. C. Specific Details The composite impervious percentage for each basin and corresponding WQCV can be located in Table 3, below. The imperviousness was then used to determine the WQCV in watershed inches on Figure 8.13. Level 1 adjustment was not used for any of the basins to be conservative since this site is very flat and has a minimal amount of release locations available. The system utilized for the WQCV is capable of capturing the volume. Table 3. Proposed WQCV Table BASIN AREA (S.F.) IMPERVIOUS AREA (SF) PERCENT IMPERVIOUS WQCV (Watershed inches) WQCV (CF) PR-1 2,716 2,000 73.6% 0.150 33.9 PR-2 2,194 1,566 71.4% 0.145 26.5 TOTAL 4,909 3,565 72.6% 0.146 59.7 D. Operation and Maintenance 09/26/2019 Page 13 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Correct operation of the system shall include drainage flowing north along the west swale and a series of basins and storm pipes along the east; both drain to the pond. Drainage will percolate through the sand/peat mix and treated water will flow to the storm manhole via an overflow pipe. Once the water in the manhole reaches capacity, the sump pump will be activated and flow through a discharge pipe and daylight on site and then to the ROW. Also, once the pond reaches capacity, the system will overflow via a concrete weir to allow storm- water to leave the northwest corner of the site at historic rates. Grass swales, pond, valley pan and storm manhole shall all be kept clear of debris for site inspections. If the system is not functioning the pond will not be draining and the manhole will need to be cleaned. See below for bio retention pond function. The bio retention pond will need to be inspected and cleared of rubbish and debris quarterly to make sure that the filter media has not become clogged and is functioning properly. The detention areas shall also be inspected after large storm events to determine if the filter media is functioning properly. If standing water is observed after small events, then clogging may be occurring in the filter material. The top three inches of the filter media shall be scarified yearly or more frequently to obtain optimum infiltration of surface runoff. If water is pooling, then the filter material may need to be scarified more frequently. If the filter material continues to fail, then the top six inches of the sand material shall be removed and replaced. This may occur every two to five years. The bio retention ponds will need to be mowed to keep weeds down. The grass shall be two to four inches tall in irrigated areas or four to six-inches tall in non irrigated areas per the URMP. The sandy loam and landscape material may need to be removed and replaced when it becomes clogged and can no longer infiltrate the storm water runoff in an acceptable time. If drain times are longer than 12 hours for the WQCV, then the filter media may need to be removed and replaced with fresh filter media. The grass conveyance swales will be kept up with regular mowing with a bagger to keep fine particles out of the upper layer of the system as much as possible. The swales will also need to be inspected after heavy rainstorms to remove any large debris that may have collected and they will be raked quarterly and during the fall months prior to snow fall to remove any built up tree debris that may fall during those months so they are prepared for spring thaw flows. The storm manhole will need to be inspected and cleared of rubbish and debris quarterly to make sure that the filter/riser pipe has not become clogged and is functioning properly. Review of the sump pump should occur at least every 6 months or after large storm events. The owners or owner’s representative will be responsible for the maintenance and upkeep of the drainage facilities. The property owner shall dispose of sediment and any other waste material removed from a reservoir at a suitable disposal site and in compliance with local, state, and federal waste regulations. 09/26/2019 Page 14 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc VI. CONCLUSION A. Compliance with Standards This drainage report has been prepared in accordance with City of Aspen Regulations. The proposed storm water facilities will capture the WQCV and the detention for the site. Water leaving the site will be controlled at the historic release point. B. Drainage Concept The proposed drainage design will be effective in controlling any adverse downstream impacts on landowners or structures by having the storm water leave the site directed toward City stormwater systems instead of uncontrolled drainage ways as historically exist. Water quality issues will not be a concern as the runoff carrying pollutants is being captured and stored in the proposed pond and drywell structure. 09/26/2019 Page 15 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc VII. References City of Aspen, Colorado: Design and Construction Standards, June 2005. City of Aspen, Colorado: Urban Runoff Management Plan. April 2010, Sept 2014 Update. WRC Engineering, Inc. “Storm Drainage Master Plan for the City of Aspen, Colorado”. November 2001. HP Geotech, Inc. “Subsoil Study for Foundation design, Proposed Residence Lot 3, Ranger Station Subdivision, 851 West Smuggler Street, Aspen, CO” dated March 13, 2017, Job Number 17-7-182. UDFCD. www.udfcd.org. 09/26/2019 Page 15 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Exhibit #1 – Vicinity Map 09/26/2019 Page 16 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Exhibit #2 – Flow Path 09/26/2019 Page 17 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Exhibit #3 – Existing Drainage Basins 09/26/2019 09/26/2019 Page 18 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Exhibit #4 – Proposed Drainage Basins 09/26/2019 09/26/2019 Page 19 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Exhibit #5 – Fema 09/26/2019 Page 20 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc 09/26/2019 Page 21 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Exhibit #6 - USDA Web Soils Survey 09/26/2019 Page 22 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc 107—Uracca, moist-Mergel complex, 1 to 6 percent slopes, extremely s Map Unit Setting  National map unit symbol: jq4g  Elevation: 6,800 to 8,400 feet  Mean annual precipitation: 16 to 19 inches  Mean annual air temperature: 40 to 43 degrees F  Frost-free period: 75 to 95 days  Farmland classification: Not prime farmland Map Unit Composition  Uracca, moist, and similar soils: 50 percent  Mergel and similar soils: 40 percent  Estimates are based on observations, descriptions, and transects of the mapunit. Description of Uracca, Moist Setting  Landform: Structural benches, valley sides, alluvial fans  Down-slope shape: Linear  Across-slope shape: Linear  Parent material: Mixed alluvium derived from igneous and metamorphic rock Typical profile  H1 - 0 to 8 inches: cobbly sandy loam  H2 - 8 to 15 inches: very cobbly sandy clay loam  H3 - 15 to 60 inches: extremely cobbly loamy sand Properties and qualities  Slope: 1 to 6 percent  Depth to restrictive feature: More than 80 inches  Natural drainage class: Well drained  Runoff class: Low  Capacity of the most limiting layer to transmit water (Ksat):Moderately high to high (0.20 to 2.00 in/hr)  Depth to water table: More than 80 inches  Frequency of flooding: None  Frequency of ponding: None  Calcium carbonate, maximum in profile: 10 percent  Available water storage in profile: Very low (about 2.6 inches) Interpretive groups  Land capability classification (irrigated): 6s  Land capability classification (nonirrigated): 6s  Hydrologic Soil Group: B  Ecological site: Stony Loam (R048AY237CO)  Other vegetative classification: Stony Loam (null_82)  Hydric soil rating: No 09/26/2019 Page 23 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Description of Mergel Setting  Landform: Structural benches, valley sides, alluvial fans  Down-slope shape: Linear  Across-slope shape: Linear  Parent material: Glacial outwash Typical profile  H1 - 0 to 8 inches: cobbly loam  H2 - 8 to 20 inches: very cobbly sandy loam  H3 - 20 to 60 inches: extremely stony sandy loam Properties and qualities  Slope: 1 to 6 percent  Depth to restrictive feature: More than 80 inches  Natural drainage class: Well drained  Runoff class: Very low  Capacity of the most limiting layer to transmit water (Ksat):Moderately high to high (0.60 to 6.00 in/hr)  Depth to water table: More than 80 inches  Frequency of flooding: None  Frequency of ponding: None  Calcium carbonate, maximum in profile: 10 percent  Available water storage in profile: Low (about 3.3 inches) Interpretive groups  Land capability classification (irrigated): 4s  Land capability classification (nonirrigated): 4s  Hydrologic Soil Group: A  Ecological site: Stony Loam (R048AY237CO)  Other vegetative classification: Stony Loam (null_82)  Hydric soil rating: No 09/26/2019 Page 24 j:/sdskproj/218/1030.03/word/drainage report Lot 3 ranger station.doc Exhibit #7 – HP Kumar Soil Study Percolation Test 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 09/26/2019 HYDROLOGIC COMPUTATIONS 09/26/2019  EXISTING CONDITIONS 09/26/2019 09/26/2019  PROPOSED CONDITIONS 09/26/2019 09/26/2019 HYDRAULIC COMPUTATIONS 09/26/2019 SWALE & PIPE CALCULATIONS 09/26/2019 Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Sunday, Sep 30 2018 Swale at PR-1 100YR Peak Flowrate Triangular Side Slopes (z:1) = 3.00, 3.00 Total Depth (ft) = 0.50 Invert Elev (ft) = 7896.75 Slope (%) = 2.00 N-Value = 0.024 Calculations Compute by: Known Q Known Q (cfs) = 0.22 Highlighted Depth (ft) = 0.21 Q (cfs) = 0.220 Area (sqft) = 0.13 Velocity (ft/s) = 1.66 Wetted Perim (ft) = 1.33 Crit Depth, Yc (ft) = 0.21 Top Width (ft) = 1.26 EGL (ft) = 0.25 0 .5 1 1.5 2 2.5 3 3.5 4 Elev (ft)Depth (ft)Section 7896.00 -0.75 7896.50 -0.25 7897.00 0.25 7897.50 0.75 7898.00 1.25 Reach (ft) 09/26/2019 Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Sunday, Sep 30 2018 Swale at PR-1 10YR Peak Flowrate Triangular Side Slopes (z:1) = 3.00, 3.00 Total Depth (ft) = 0.50 Invert Elev (ft) = 7896.75 Slope (%) = 2.00 N-Value = 0.024 Calculations Compute by: Known Q Known Q (cfs) = 0.12 Highlighted Depth (ft) = 0.16 Q (cfs) = 0.120 Area (sqft) = 0.08 Velocity (ft/s) = 1.56 Wetted Perim (ft) = 1.01 Crit Depth, Yc (ft) = 0.16 Top Width (ft) = 0.96 EGL (ft) = 0.20 0 .5 1 1.5 2 2.5 3 3.5 4 Elev (ft)Depth (ft)Section 7896.00 -0.75 7896.50 -0.25 7897.00 0.25 7897.50 0.75 7898.00 1.25 Reach (ft) 09/26/2019 DETENTION- WIER CALCULATIONS 09/26/2019 Hydrology Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Sunday, Sep 30 2018 Required Storage with 94% of EX-1 100yr Historic Release Rate Hydrograph type = Rational Peak discharge (cfs) = 0.377 Storm frequency (yrs) = 100 Time interval (min) = 1 Drainage area (ac) = 0.108 Runoff coeff. (C) = 0.63 Rainfall Inten (in/hr) = 5.541 Tc by FAA (min) = 7 IDF Curve = AspenIDFcurve.IDF Rec limb factor = 1.00 Hydrograph Volume = 158 (cuft); 0.004 (acft) 0 5 10 15 Q (cfs) 0.00 0.00 0.05 0.05 0.10 0.10 0.15 0.15 0.20 0.20 0.25 0.25 0.30 0.30 0.35 0.35 0.40 0.40 0.45 0.45 0.50 0.50 Q (cfs) Time (min) Runoff Hydrograph 100-yr frequency Runoff Hyd - Qp = 0.38 (cfs)Outflow Hyd *Req. Stor = 73 (cuft) * * Estimated 09/26/2019 FAA Formula Tc Worksheet Tc = 1.8(1.1 - C) x Flow length^0.5 / Watercourse slope^0.333 Hydraflow Express by Intelisolve Rational Required Storage with 94% of EX-1 100yr Historic Release Rate Description Flow length (ft) = 103.00 Watercourse slope (%) = 2.00 Runoff coefficient (C) = 0.63 Time of Conc. (min) = 7 09/26/2019 Weir Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Sunday, Sep 30 2018 Weir @ Proposed Site 100YR Peak Flowrate Rectangular Weir Crest = Sharp Bottom Length (ft) = 1.25 Total Depth (ft) = 0.25 Calculations Weir Coeff. Cw = 3.33 Compute by: Known Q Known Q (cfs) = 0.44 Highlighted Depth (ft) = 0.22 Q (cfs) = 0.440 Area (sqft) = 0.28 Velocity (ft/s) = 1.58 Top Width (ft) = 1.25 0 .5 1 1.5 2 2.5 Depth (ft)Depth (ft)Weir @ Proposed Site 100YR Peak Flowrate -0.50 -0.50 0.00 0.00 0.50 0.50 1.00 1.00 Length (ft)Weir W.S.09/26/2019 Weir Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Sunday, Sep 30 2018 Weir @ Proposed Site 10YR Peak Flowrate Rectangular Weir Crest = Sharp Bottom Length (ft) = 1.25 Total Depth (ft) = 0.25 Calculations Weir Coeff. Cw = 3.33 Compute by: Known Q Known Q (cfs) = 0.24 Highlighted Depth (ft) = 0.15 Q (cfs) = 0.240 Area (sqft) = 0.19 Velocity (ft/s) = 1.29 Top Width (ft) = 1.25 0 .5 1 1.5 2 2.5 Depth (ft)Depth (ft)Weir @ Proposed Site 10YR Peak Flowrate -0.50 -0.50 0.00 0.00 0.50 0.50 1.00 1.00 Length (ft)Weir W.S.09/26/2019 Weir Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Sunday, Sep 30 2018 Weir @ EX-1 Historical 100YR Release Rate Rectangular Weir Crest = Sharp Bottom Length (ft) = 1.25 Total Depth (ft) = 0.25 Calculations Weir Coeff. Cw = 3.33 Compute by: Known Q Known Q (cfs) = 0.21 Highlighted Depth (ft) = 0.14 Q (cfs) = 0.210 Area (sqft) = 0.17 Velocity (ft/s) = 1.23 Top Width (ft) = 1.25 0 .5 1 1.5 2 2.5 Depth (ft)Depth (ft)Weir @ EX-1 Historical 100YR Release Rate -0.50 -0.50 0.00 0.00 0.50 0.50 1.00 1.00 Length (ft)Weir W.S.09/26/2019 Weir Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Sunday, Sep 30 2018 Weir @ EX-1 Historical 10YR Release Rate Rectangular Weir Crest = Sharp Bottom Length (ft) = 1.25 Total Depth (ft) = 0.25 Calculations Weir Coeff. Cw = 3.33 Compute by: Known Q Known Q (cfs) = 0.06 Highlighted Depth (ft) = 0.06 Q (cfs) = 0.060 Area (sqft) = 0.07 Velocity (ft/s) = 0.81 Top Width (ft) = 1.25 0 .5 1 1.5 2 2.5 Depth (ft)Depth (ft)Weir @ EX-1 Historical 10YR Release Rate -0.50 -0.50 0.00 0.00 0.50 0.50 1.00 1.00 Length (ft)Weir W.S.09/26/2019 ASPEN CHARTS AND FIGURES 09/26/2019 City of Aspen Urban Runoff Management Plan Chapter 3 - Runoff 3-2 Rev 2/2010 Figure 3.1 Natural Resource Conservation Service (NRCS) Soil Map for Aspen 09/26/2019 City of Aspen Urban Runoff Management Plan Chapter 3 - Runoff 3-6 Rev 10/2014 Figure 3.2 – Runoff Coefficients for NRCS Hydrologic Soil Group A Figure 3.3 – Runoff Coefficients for NRCS Hydrologic Soil Group B 09/26/2019 City of Aspen Urban Runoff Management Plan Chapter 2 - Rainfall 2-2 Rev 9/2014 into thunderstorms. Autumn in Aspen is usually dry and warm and during September daytime temperatures can reach 70°F, but night temperatures can drop to freezing. Aspen is renowned for its warm winter sun. Winter daytime temperatures typically range from 20 to 40°F in the City and from 10 to 30°F on the mountain. Once the sun goes down, the temperature drops dramatically. Table 2.1 presents monthly statistics for temperature, precipitation, snowfall, and snow depth in the Aspen area. Table 2.1 Monthly Statistics for Temperature and Precipitation in Aspen Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Average Max. Temperature (F) 35 39 45 52 63 72 78 76 69 58 43 35 55.5 Average Min. Temperature (F) 9.1 12 20 26 35 41 47 46 39 30 19 9.7 27.7 Average Total Precipitation (in.) 1.7 2.1 2.7 2.5 2.1 1.4 1.8 1.6 2.1 2 2.6 1.9 24.37 Average Total Snowfall (in.) 25 27 28 20 7.8 1 0 0 1 11 28 25 173.8 Average Snow Depth (in.) 21 28 27 12 1 0 0 0 0 1 6 14 (Source: Station 050372 at Aspen 1 SW, Colorado) 2.3 Rainfall Depth, Duration, Frequency, and Intensity The rainfall intensity-duration-frequency (IDF) curve is a statistical formula to describe the relationship among the local rainfall characteristics and return periods. The IDF curve is used in the Rational Method for peak runoff predictions of basins smaller than 90 acres. Based on the NOAA Atlas Volume 3, the IDF curve for the City of Aspen can be derived according to the locality and elevation. The City of Aspen is located at approximately 39°11′32″N and 106°49′28″W, at an elevation of approximately 8,100 feet. Based on depth and duration data (Appendix B, Table 1), rainfall intensities can be calculated for various frequencies. Rainfall intensity data, which form the basis of the Intensity-Duration-Frequency (IDF) curves in Figure 2.1 are provided in Table 2.2. Table 2.2 Rainfall Intensity-Duration-Frequency in Aspen, Colorado Return   Rainfall Intensity in inch/hr for Various Periods of Duration  Period 5-min 10-min 15-min 30-min 1-hr (P1) 2-hr 3-hr 6-hr 24-hr 2‐yr 2.06 1.51 1.23 0.77 0.47 0.28 0.21 0.13 0.06  5-yr 2.98 2.17 1.77 1.09 0.64 0.36 0.26 0.16 0.07  10-yr 3.72 2.72 2.22 1.35 0.77 0.43 0.30 0.18 0.08  25‐yr 4.75 3.47 2.82 1.71 0.95 0.53 0.36 0.21 0.09  50‐yr 5.53 4.05 3.30 1.98 1.09 0.60 0.41 0.24 0.11  100-yr 6.32 4.63 3.76 2.24 1.23 0.67 0.45 0.26 0.12  Using the data in Table 2.2 (derived from NOAA Atlas 14 Volume 8), the following equation was derived that can be used to determine intensities not shown in the IDF table or curve: 052.1 1 )10( 8.88 dT PI (Equation 2-1) Where, I = rainfall intensity (inch/hr), P1 = 1-hr rainfall depth (inches), and Td = duration or time of concentration (minutes). 09/26/2019 City of Aspen Urban Runoff Management Plan Chapter 2 - Rainfall 2-4 Rev 9/2014 Note: Accuracy is more reliable at 5 minute increments. Figure 2.1 IDF Curves for Aspen, Colorado 0 1 2 3 4 5 6 7 0 5 10 15 20 25 30 35 40 45 50 55 60 In t e n s i t y  (i n c h / h r ) Duration in Minutes Rainfall IDF for Aspen, Colorado 2‐yr 5‐yr 10‐yr 25‐yr 50‐yr 100‐yr 09/26/2019 City of Aspen Urban Runoff Management Plan Chapter 8 – Water Quality 8-33 Rev 8/2009 Figure 8.13 Aspen Water Quality Capture Volume 09/26/2019