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HomeMy WebLinkAboutFile Documents.1419 Crystal Lake Rd.0009.2017 (5).ARBK1 Drainage Report 1419 CRYSTAL LAKE ROAD ASPEN, CO 81623 February 13th, 2017 Updated May 19th, 2017 Prepared by Richard Goulding, P.E. Roaring Fork Engineering 592 Highway 133 Carbondale, CO 81623 Reviewed by Engineering 06/21/2017 9:04:37 AM "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. 2 Drainage Report 1419 CRYSTAL LAKE ROAD ASPEN, CO 81623 I HEREBY AFFIRM THAT THIS REPORT FOR THE IMPROVEMENTS AT 1419 CRYSTAL LAKE ROAD WAS PREPARED BY ME FOR THE OWNERS THEREOF IN ACCORDANCE WITH THE PROVISIONS OF THE CITY OF ASPEN URBAN RUNOFF MANAGEMENT PLAN AND APPROVED VARIANCES AND 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. RICHARD GOULDING, P.E. RFE Project # 2016-35 3 Table of Contents 1.0 General .................................................................................................................................................... 4 1.1 Existing Site ........................................................................................................................................ 4 1.2 Proposed Conditions ........................................................................................................................... 5 1.3 Previous Drainage Studies .................................................................................................................. 5 1.4 Offsite Drainage & Constraints........................................................................................................... 5 2.0 Drainage Basins and Sub-basins ............................................................................................................. 5 2.1 Drainage Basins .................................................................................................................................. 6 2.2 Peak Discharge Calculations ............................................................................................................... 6 3.0 Low Impact Site Design .......................................................................................................................... 7 3.1 Principles............................................................................................................................................. 7 4.0 Hydrological Criteria .............................................................................................................................. 9 4.1 Storm Recurrence and Rainfall ........................................................................................................... 9 4.2 Peak Runoff and Storage Volumes Methodology ............................................................................... 9 5.0 Hydraulic Criteria ................................................................................................................................... 9 5.1 Inlets .................................................................................................................................................... 9 5.2 Pipes .................................................................................................................................................. 10 6.0 Proposed Facilities ................................................................................................................................ 11 6.1 Proposed structures ........................................................................................................................... 11 6.2 Infiltration ......................................................................................................................................... 12 6.2.1 Drywell ...................................................................................................................................... 12 7.0 Operation and Maintenance .................................................................................................................. 12 7.1 Drywell ............................................................................................................................................. 12 7.2 Pervious Paver Area .......................................................................................................................... 12 8.0 Appendices ............................................................................................................................................ 13 Drawings 11x17 ...................................................................................................................................... 13 4 1.0 General 1.1 Existing Site The residence under evaluation is located at 1419 Crystal Lake Road in Aspen, Colorado. This property is a 1.03 acre parcel and is located within the Smuggler Hunter Drainage Basin. The existing structure includes an 8,000 square foot two-story residence, a walkout basement, and a finished garage. The residence is built directly above the top of the bank on a flat area. The southern property line runs along the center of the Roaring Fork River, with neighboring houses located to the north and west of the site and the Aspen Club Condominiums located to the east. Along the western property line is an asphalt trail on an easement that provides access to the Aspen Club and Spa, which is south of the property across the river. A large berm lined with trees is located along the north property line to provide privacy from Crystal Lake Road. A majority of the site consists of a grass lawn with large aspen and pine trees. There are a number of landscape beds leading up to the structure and the stone patio to the south of the structure. 1419 Crystal Lake Road 5 A Geotechnical Report has been performed by H-P Kumar for this project as of permit submittal. All calculations with percolation information included in this document currently use percolation test results performed on January 17, 2017. 1.2 Proposed Conditions This project is classified as a ‘Major Project’ as per Table 1.1 of the URMP. The proposed development is over 1000 square feet (sf) and disturbs an area of approximately 16, 500 sf., roughly 37% of the site. The intent of this report is to demonstrate compliance with the requirements of the URMP. The Low Impact Design (LID) Principles in the introduction of the manual were used as a guide throughout the design process. The proposed project consists of a significant remodel of the interior of the existing structure, the demolition of the patio and stairway on the south side of the residence, walkway and pond removal on the west side of the residence, and regrading and resurfacing of existing snowmelted concrete driveway. The existing building footprint will not change. The runoff from impervious surfaces will be collected through a system of inlets, roof drains, and trench drains and routed either to a drywell located at the northwest corner of the driveway or to a screened rock detention bed located under the improved patios to the south of the house. Drainage from northern sections of existing roof and the driveway will be routed to the drywell located at the northeast corner of the driveway. Runoff from southern sections of existing roof and all of the southern patios will be routed to a screened rock bed located under the new patio pavers. Refer to sheet C6 of the civil set for proposed grading and drainage of the site. 1.3 Previous Drainage Studies The City completed a Drainage Master Plan in November 2001 using the consultant WRC, however this project is outside the area of that study. Figure 7.1 of the URMP defines the mudflow zones for the City of Aspen. This property is not impacted by mudflow. 1.4 Offsite Drainage & Constraints There is no offsite drainage that impacts the property. The proposed development will not impact any irrigation ditches or waterways. 2.0 Drainage Basins and Sub-basins The site was divided into two drainage basins, which were then subdivided into smaller sub-basins. Basin and Sub-basin delineations are shown on Sheets C4 and C5 of the permit drawings. These sheets list impervious areas, runoff coefficients, peak flows, and the required volume of runoff to be detained. Sub- basins were created to calculate the maximum flow entering each inlet of the proposed storm systems. The sub-basin peak flows were then used to size pipes and inlet capacities. 6 2.1 Drainage Basins Basin 1 is the largest of the basins at 8560.7 square feet (sf) and is 99% impervious. Impervious sections of this basin include the proposed driveway, the northern section of the roof structure, and the patio located west of the structure. The remaining pervious sections of this basin include landscaping areas near the front entry and west of the existing structure. There are five sub-basins in Basin 1. Two downspout tie-ins collect runoff from the roof portions, two trench drains collect runoff from the patio west of the existing structure and the proposed driveway, and Storm Systems A and B routes runoff from these sub- basins to Drywells A and B. Each drywell is 6 ft. in diameter and 12 ft. deep, located on the northwest corner of the proposed driveway. Basin 2 consists of the southern portions of the roof and patios to the south and east of the structure. Basin 2 is 3684.0 sf and 100% impervious. There are seven sub-basins in Basin 2. Four downspout tie- ins collect runoff from the roof portions, one slot drain collects runoff from the east patio, and one center drain collects runoff on the stairway. All of these inlets drain to Screened Rock Bed C. The 3 ft. deep screened rock bed located below the patio will only extend to the 15 ft. top of slope setback. No treatment will occur within the setback. 2.2 Peak Discharge Calculations The peak flows were calculated for each Major Basin for 5 and 100 year storm events. Rainfall intensity was calculated using a Time of Concentration (Td) of 5 minutes. Actual Time of Concentration on the site is significantly less than 5 minutes, but according to the City of Aspen URMP, equations used to calculate rainfall intensity are only valid for a Time of Concentration of greater than 5 minutes so the smallest valid Time of Concentration value was used. The 1 hour Rainfall depth (P1), given in Table 2.2 as 0.64 inches for a 5-year event and 1.23 inches for a 100-year event. Equation 2.1 was referenced when solving for the Rainfall Intensity (I). I = 88.8P1/(10+Td )1.052 (Equation 2.1) Runoff Coefficients (C), a function of the Soil Group (in this case B) and the percentage of impervious area within each sub basin were developed using Figure 3.2. The Runoff Coefficient (C) was then multiplied by the Rainfall Intensity (I) and the acreage of each Major Basin (A) to determine the peak discharge for each Major Basin. Qp = CIA Qp = Peak Discharge (cfs) A = Area (Acres) I = Rainfall intensity (inches per hour) C = Runoff Coefficient These peak flow values were used to calculate the size of the proposed detention and conveyance structures, such as drywells, inlets and pipes. The tables below contain the peak flows for developed and undeveloped conditions for 5 and 100 year storm events. 7 3.0 Low Impact Site Design 3.1 Principles Principle 1: Consider storm water quality needs early in the design process. The Grading and Drainage design was coordinated between the architect, landscape architect and civil engineering teams throughout the design process. Multiple site visits ensured proper understanding of existing conflicts and opportunities to improve existing drainage patterns. Principle 2: Use the entire site when planning for storm water quality treatment. 5 Year Peak Discharge Developed Calculations 1 Hour(P1)0.64 Return Period 5 Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max See(D1)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec) 1 8560.72 8508.72 99.39%0.896 5 3.29 0.58 2 3749.43 3749.43 100.00%0.896 5 3.29 0.25 5 Year Peak Discharge Pre Development Calculations 1 Hour(P1)0.64 Return Period 5 Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max See(D1)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec) 1 8560.72 0.00 0.00%0.080 5 3.29 0.05 2 3749.43 0.00 0.00%0.080 5 3.29 0.02 100 Year Peak Discharge Developed Calculations 1 Hour(P1)1.23 Return Period 100 Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max See(D1)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec) 1 8560.72 8508.72 99.39%0.950 5 6.33 1.18 2 3749.43 3749.43 100.00%0.950 5 6.33 0.52 100 Year Peak Discharge Pre Development Calculations 1 Hour(P1)1.23 Return Period 10 Basin ID Total Area Imp. Area Impervious C Value Time of C Intensity Q Max See(D1)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 (ft3/sec) 1 8560.72 0.00 0.00%0.350 5 6.33 0.44 2 3749.43 0.00 0.00%0.350 5 6.33 0.19 8 Storm water quality was considered in the design of every part of the site that is being affected by the proposed construction. Gutters and downspouts will be added to collect runoff from the entire roof and routed to either to a drywell or a screened rock bed. Principle 3: Avoid unnecessary impervious area. All pervious patios are to be refinished with infiltration beds underneath. Although the size of the driveway is increasing, full detention will be infiltrated through the drywell next to the drive. Principle 4: Reduce runoff rates and volumes to more closely match natural conditions. All runoff from impervious surfaces on the property is collected and routed to either a drywell or a screened rock bed. Because the drywells are sized to fully detain a 100 year storm event, runoff rates from the property will be greatly reduced. Principle 5: Integrate storm water quality management and flood control. The use of the drywells and screened rock bed to collect runoff from all impervious surfaces simultaneously aids in storm water quality and flood control on the site. Principle 6: Develop storm water quality facilities that enhance the site, the community and the environment. The use of the two chambered drywell for water quality reduces the runoff of sediment and contaminants from the site. This reduces the site’s effect on the Roaring Fork River and the community. Principle 7: Use treatment train approach. Because storm water drywells are sized for full detention, the treatment train approach is not applicable in this project. Principle 8: Design sustainable facilities that can be safely maintained. Inlets and piping will be vacuumed or flushed periodically to maintain adequate flow. Proper grading reduces dangerous slopes and proper drainage to reduce ice buildup. Both the drywell and screened rock bed will be easily accessible. Principle 9: Design and maintain facilities with public safety in mind. The proposed storm system adds gutters and drains to roofs. This reduces ice buildup and the damages that follow. There are no drop-offs or steep grades proposed. 9 4.0 Hydrological Criteria 4.1 Storm Recurrence and Rainfall The property is located outside of the commercial core and isn’t served by any storm drains so this property classifies as a “Sub-urban area not served by public storm sewer”. The total site shall meet detention requirements for 5- and 100-year historical storm events. The 1 hour Rainfall depth (P1) is given in Table 2.2 as 0.64 inches for the 5-year event and 1.23 inches for the 100-year event. The Intensity in inches per hour for different storm duration (Td) is calculated using the Equation 2.1 from the Aspen URMP. 4.2 Peak Runoff and Storage Volumes Methodology Using the peak flows for each Major Basin established in section 2.2, storage requirements were calculated for a 100 year storm event. Because there is no storm infrastructure available for detention system overflow, Drywell A, Drywell B, and Screened Rock Bed C were sized to fully detain the runoff from a 100 year storm event. The table below shows storage volume requirements for the proposed detention system. 5.0 Hydraulic Criteria This property is not connected to the COA’s storm water infrastructure. All hydraulics are sized for onsite infrastructure. 5.1 Inlets Basins 1 and 2 were divided into sub-basins according to which inlet they discharged into. The peak flows for the 100 year event in each sub-basin were used as the flowrate to size the proposed inlet drains, trench drains, and slot drains. Equations 4.17 through 4.20 from the URMP were used in these calculations. The equations incorporate a 50% clogging factor and assume a 40% opening in the grates. A water depth of 0.5 in. was assumed and all the inlets were treated as sumps, as they will be set a minimum of 0.5 in. below flow lines. Full Detention Storage Basin Total Area Impervious Area Impervious Full Detention Depth Factor of Safety Required Storage BMP (ft2)(ft2)(%)(in)F.O.S.(ft3) 1 8560.72 8508.72 99.39%1.23 1 872 Drywell A, Drywell B 2 3749.43 3749.43 100.00%1.23 1 384 Screened Rock Bed C Sub Basin and Rectangular Inlet Calculations 1 Hour(P1)1.23 m=40%Ys=.04 (Depress inlet by 0.04') Return Period 100 Cg=50%Co=0.65 Inlet ID Basin ID Total Area Imp. Area Impervious C Value Time of Concentration Intensity Q Max Inlet Type Inlet Width Inlet Length Effective Open Area (EQ. 4-20)Inlet Capacity (EQ 4-19)Has Capacity See(D1)(ft2)(ft2)(%)(From Table) (Td)I=88.8P1/(10+Td)1.052 (ft3/sec)Rectangular Wo (inches)Lo (inches)Ae=(1-Cg)mWoLo Q=CoAe√2gYs (Yes/No) TRENCH DRAIN-A1 1.1 787.08 787.08 100.00%0.950 5 6.33 0.109 4" x 19.7'4 236.4 1.313 1.318 Yes TRENCH DRAIN-B3 1.4 3652.57 3600.57 98.58%0.880 5 6.33 0.467 4" x 39.0'4 468 2.600 2.609 Yes SLOT DRAIN-C3 2.3 272.51 272.51 100.00%0.950 5 6.33 0.038 0.25" x 19.8'0.25 237.6 0.083 0.083 Yes 10 5.2 Pipes Pipes used will be PVC SDR-35 with a Manning’s coefficient (n) of 0.01. The pipes were sized to accommodate peak flows for a 100 year event from all contributing sub-basins. A table delineating which sub-basins contribute to which pipes is shown below. Calculated pipe sizes were tested for hydraulic capacity at 80% of their full flowrate. Calculations of depth of flow for each pipe were calculated. Design charts giving Qdesign / Q full were downloaded from FHWA and the equations in Section 4.8.4 were used as the basis for these calculations. Calculated pipe sizes and depth of flow for onsite pipes are shown below. Sub Basin and Circular Inlet Calculations 1 Hour(P1)1.23 m=40%Ys=.04 (Depress inlet by 0.04') Return Period 100 Cg=50%Co=0.65 Inlet ID Basin ID Total Area Imp. Area Impervious C Value Time of Concentration Intensity Q Max Inlet Type Diameter Effective Open Area(EQ. 4-20)Inlet Capacity (EQ 4-19)Has Capacity See(D1)(ft2)(ft2)(%)From Table (Td)I=88.8P1/(10+Td)1.052 ft3/sec Wo (inches)Ae=(1-Cg)mA Q=CoAe√2gYs (Yes/No) INLET-C4 2.4 198.57 198.57 100.00%0.950 5 6.33 0.027 6" Round 6 0.039 0.046 Yes Storm System Pipes Pipe System Pipe Contibuting Sub-Basins Peak Flows (CFS) A A1 1.1 0.11 A2 1.1 0.11 A3 1.1, 1.2 0.18 B B1 1.3 0.28 B2 1.3 0.28 B3 1.4 0.47 C C1 2.1 0.11 C2 2.1, 2.2 0.22 C3 2.1-2.3 0.25 C4 2.1-2.4 0.28 C5 2.1-2.4 0.28 C9 2.6 0.11 K=0.462 Pipe Combined Design Flow Manning Coefficient Flattest Slope Equation 4-31 Required Diameter Design Diameter Design Diameter-80% Has Capacity@ 80% full (ID)Q (ft3/sec)n (%)S0 d={nQ/K√So}3/8 (inches) (inches) (inches)Yes/No A1 0.11 0.01 3.00%0.20 2.393 4.0 3.200 Yes A2 0.11 0.01 2.00%0.22 2.582 4.0 3.200 Yes A3 0.18 0.01 8.00%0.20 2.411 4.0 3.200 Yes B1 0.28 0.01 2.00%0.31 3.699 6.0 4.800 Yes B2 0.28 0.01 4.00%0.27 3.249 6.0 4.800 Yes B3 0.47 0.01 2.00%0.37 4.461 6.0 4.800 Yes C1 0.11 0.01 6.00%0.18 2.126 4.0 3.200 Yes C2 0.22 0.01 20.00%0.18 2.169 4.0 3.200 Yes C3 0.25 0.01 4.00%0.26 3.115 4.0 3.200 Yes C4 0.28 0.01 5.00%0.26 3.104 6.0 4.800 Yes C5 0.28 0.01 25.00%0.19 2.296 6.0 4.800 Yes C9 0.11 0.01 2.00%0.21 2.553 4.0 3.200 Yes Onsite Piping Capacity 11 6.0 Proposed Facilities 6.1 Proposed structures Drywells and a screened rock bed are being utilized to meet URMP requirements for storm water management. Detention volumes were calculated using equations 5-1 through 5-4 from the City of Aspen URMP. Drywell A and Drywell B are located at the Northwest corner of the driveway and collects all runoff from Basin 1. These drywells have a diameter of 6 ft., a storage depth of 12 ft. each with 18 in. of screened rock surrounding the drywells, giving each drywell a storage capacity of 467 ft3, for a total capacity of 934 ft3. The total capacity for the drywells exceeds the required detention volume of 872 ft3 for Basin 1. Screened Rock Bed C is located under the snowmelted patio area along the south edge of the building and collects runoff from Basin 2. This bed is 3 ft. deep with a footprint of 427 ft2, and a gravel void ratio of 0.3 giving the bed a total storage volume of 386 ft3, which exceeds the required capacity of 384 ft3 for Basin 2. Pipe Combined Design Flow Manning Coefficient Design Diameter X-section Slope Q -Full Q-Design/Q Full d/D Depth (ID)Q (ft3/sec)n (inches) (ft2) (%) (ft3/sec)Q/Qfull (from Chart)d= (d/D)*D A1 0.109 0.01 4.0 0.087 3.00%0.429 0.253 0.38 1.52 A2 0.109 0.01 4.0 0.087 2.00%0.351 0.310 0.43 1.72 A3 0.181 0.01 4.0 0.087 8.00%0.701 0.258 0.38 1.52 B1 0.283 0.01 6.0 0.196 2.00%1.034 0.274 0.38 2.28 B2 0.283 0.01 6.0 0.196 4.00%1.462 0.194 0.34 2.01 B3 0.467 0.01 6.0 0.196 2.00%1.034 0.451 0.53 3.18 C1 0.112 0.01 4.0 0.087 6.00%0.607 0.184 0.33 1.30 C2 0.216 0.01 4.0 0.087 20.00%1.109 0.195 0.34 1.34 C3 0.253 0.01 4.0 0.087 4.00%0.496 0.511 0.57 2.28 C4 0.281 0.01 6.0 0.196 5.00%1.635 0.172 0.31 1.88 C5 0.281 0.01 6.0 0.196 25.00%3.655 0.077 0.22 1.32 C9 0.105 3.01 4.0 0.087 2.00%0.001 90.505 0.90 3.60 Depth Of Flow-section 4.8.4 Storm Sewer Sizing Drywell Storage Drywell Basins Diameter Storage Depth Internal Volume External (18" of Screened Rock) Volume Total Capacity Required Capacity (Name)(#)(ft)(ft)π*H*(D/2)2) (ft3)0.3*π*8*((D/2)+1.5)2 - (D/2)2) (ft3)(ft3)(ft3) Drywell A 1 6 12.5 353 85 438 431.98 Drywell B 1 6 12.5 353 85 438 431.98 12 6.2 Infiltration 6.2.1 Drywells and Screened Rock Bed Part of the Analysis is to ensure that the drainage structures can completely drain within 24 hours. The minimum depth of perforation a drywell must have is 4 ft. Below is a calculation showing that there is enough perforation area for the drywells and screened rock bed to drain within 24 hours using the percolation rate determined on January 17, 2017 of 3 inches per hour for the entire site. Section 8.5.4.2 was referenced for these calculations. 7.0 Operation and Maintenance 7.1 Drywell Drywells must be inspected and maintained quarterly to remove sediment and debris that has washed into them. Minimum inspection and maintenance requirements include the following:  Inspect drywells at least four times a year and after every storm exceeding 0.5 inches.  Dispose of sediment, debris/trash, and any other waste material removed from a drywell at suitable disposal sites and in compliance with local, State, and Federal waste regulations.  Routinely evaluate the drain-down time of the drywell to ensure the maximum time of 24 hours is not being exceeded. If drain-down times are exceeding the maximum, drain the drywell via pumping and clean out the percolation area (the percolation barrel may be jetted to remove sediment accumulated in perforations. If slow drainage persists, the system may need to be replaced. 7.2 Pervious Paver Area As per section 8.5.3.1 of the URMP, the following schedule will be undertaken by the owners of the property to achieve long term performance of the BMP’s. Screened Rock Bed Storage Storage System Basins Area Depth Void Ratio Total Capacity Required Capacity (Name)(#)(ft2)(ft)(ft3)(ft3) Screened Rock Bed C 2 428.64 3 0.3 385.78 384 Drywell Infiltration Name Diameter Perforation Height Perforated Area Total Capacity Infiltration Rate Infiltration Time Volume Infiltrated in 24 Hours (Name)D (ft)H (ft)A (ft2) = 3.14*D*H V (ft3)I (in/hr)T (hr) = V/(A*I/12)Vtotal (ft3) = V*T Drywell A 6 8 150.80 438.25 3 11.63 5094.68 Drywell B 6 8 150.80 438.25 3 11.63 5094.68 Screened Rock Bed Infiltration BMP Max Volume Infiltration Area Infiltration Rate Time To Drain Volume Infiltrated in 24 Hours (name)V (ft3)A (ft2)I (in/hr)(hr)Vtotal (ft3) = V*T Screened Rock Bed C 384.32 204 3 7.54 22.61 13 8.0 Appendices Drawings 11x17