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Home My WebLink AboutWELLS FARGO PARKING LOT AND ATMS - MJA190003 - DOCUMENT MARKUPS - ROUND 2 - DRAINAGE REPORTDRAINAGE REPORT FOR FINAL DRAINAGE REPORT PARKING AT 112 WEST MAGNOLIA STREET LOTS 1-2, BLOCK 113, FTC 112 WEST MAGNOLIA STREET FORT COLLINS, CO 80524 Prepared For: Wells Fargo Bank RO Box 2609 Carlsbad, CA 92018 Prepared By: Coffey Engineering & Surveying, LLC 4045 St. Cloud Drive, Suite 180 Loveland, CO 80538 970-622-2095 Project No. 2283.00 November 18, 2019 Parking at 112 West Magnolia Street FINAL DRAINAGE REPORT ENGINEER’S CERTIFICATION I hereby certify that this Final Drainage Report for the design of stormwater management facilities for the Parking at 112 West Magnolia Street Project was prepared by me, or under my direct supervision, in accordance with the provisions of the City of Fort Collins Storm Drainage Design Criteria and Construction Standards for the owners thereof. Troy W. Campbell, P.E. Registered Professional Engineer State of Colorado No. 41159 PG. 3 III. DRAINAGE CRITERIA A. Regulations Drainage design criteria specified in the Fort Collins Stormwater Criteria Manual and the Urban Storm Drainage Criteria Manual, Volume 3 by the Urban Drainage and Flood Control District (UDFCD) have been referenced in the preparation of this study. B. Implementation of the “Four Step Process” Our stormwater management strategy utilizes the “Four Step Process” to minimize adverse impacts of urbanization on receiving waters, as follows: Step 1 – Employ Runoff Reduction Practices. The first consideration taken in trying to reduce the stormwater impacts of this development is the site selection itself. The land use for the site remains unchanged. The existing site is a gravel parking lot, but to provide a higher level of service to their employees, Wells Fargo is paving the parking lot. This will reduce the amount of dirt that is tracked on to the paved streets in the area by vehicles and during precipitation events. Step 2 – Implement BMPs That Provide a Water Quality Capture Volume (WQCV) with Slow Release. The primary water quality control will occur in the permeable paver sub-base, located under the parking spaces of the lot. Refer to the Major Amendment plan set for permeable paver details and cross-sections. The permeable pavers will increase water quality and promote infiltration. Water quality treatment for 50% of the site is provided for in the Udall Natural Area water treatment facility. Step 3 – Stabilize Drainageways. The Poudre River is the governing drainageway for the proposed site. By improving the water quality and increasing infiltration, the likelihood of bed and bank erosion from this site is reduced. PG. 4 Step 4 – Implement Site Specific and Other Source Control BMPs. Permeable Pavement Systems: Flow from the parking lot will directly flow into the void area of the pavers sub-base, avoiding any extra pollutant-flow contact time and providing water quality. Landscaping: Personnel from Coffey Engineering and CS Design worked with the City of Fort Collins Forestry Department to preserve a large tree on the southwest side of the parking lot. Interception storage, the water caught by plants at the onset of a rainstorm, is significant when a large tree shades a small lot. Some amount of the intercepted water evaporates, never reaching the ground. Additionally, water retained by the leaves and branches moves more slowly through the hydrograph. In the case of this tree, the canopy hangs directly over proposed impervious asphalt area. C. Development Criteria Reference and Constraints The criteria used as the basis for analysis and design of stormwater management improvements for this site are found in the references cited. To the knowledge of the author, there are no other capital drainage improvements planned for the site that would constrain or otherwise influence the design of the stormwater improvements for this site. D. Hydrological Criteria Stormwater runoff is analyzed for storms with 2-year and 100-year return frequencies. The Rational Method was chosen for this small site. The Rational Method provides that: Q = CIA, where: Q = Design flow in cubic feet per second (cfs) C = Coefficient of runoff for the area under consideration I = Rainfall intensity for the design storm duration (in/hr) A = Area of the drainage sub-basin under consideration (ac) Peak flows were calculated using the Rational Method for the 2-year and 100-year storm events. This software uses the local 1-hour rainfall depth and Fort Collins rainfall intensities, calculated as a function of the time of concentration. These values were obtained by the City of Fort Collins Rainfall Intensity-Duration-Frequency (IDF) curve/table; Figure 3.4-1 and Table 3.4-1 found in the Fort Collins Stormwater Criteria Manual, and can be found in Appendix A. Additionally, per the City of Fort Collins, the “C’ coefficients have been multiplied by the appropriate storm factors. PG. 6 After treatment within the permeable pavers, stormwater will infiltrate through the bottom of the paver sections and dry well. If the paver system overflows due to clogging, catch basins at the low point of the pavers will direct runoff into the underdrain system. When improvements are being added to an existing developed site, onsite detention is only required if there is an increase in impervious area greater than 5,000 square feet in the Old Town Basin. The effective impervious area for the existing site is 7,602 square feet and the effective impervious area for the proposed site is 12,205 square feet, a difference of 4,603 square feet. Since the effective impervious area added to the site is under 5,000 square feet, detention is not required. Sub-basin P1 produces a 100-year flow of 3.48 cfs. The Modified FAA spreadsheet was created to incorporate rainfall data from the Fort Collins Stormwater Criteria Manual, Table 3.4-1. This spreadsheet was used to calculate the inflows and outflows from the site. 905 ft 3 of water quality is required for this site. The sub-base under the pavers is designed per Fort Collins Detail D-54, Interlocking Pavers Cross Section. The water quality volume is held in the 12” #2 rock layer. 30% of the volume of the #2 rock layer is void space. The detention volume for each paver section is: PAVER SECTION PAVER AREA #2 SUB-BASE DEPTH DETENTION VOLUME 1 731 SQ FT 12" 219 CU FT 2 272 SQ FT 12" 82 CU FT 3 582 SQ FT 12" 175 CU FT 4 1,075 SQ FT 12" 322 CU FT 5 731 SQ FT 12" 219 CU FT TOTAL: 1,017 CU FT DETENTION VOLUME FOUND USING AREA X DEPTH X 30% This compares favorably with the required water quality volume of 905 ft 3 . V. CONCLUSIONS A. Compliance with Standards The drainage design for the Parking at 112 West Magnolia Street follows the requirements of the Fort Collins Stormwater Criteria Manual as well as the City’s floodplain regulations. The criteria and recommendations of the Urban Storm Drainage Criteria Manual are also reflected in the design of the drainage systems. Impeviousness WELLS FARGO PARKING EXPANSION Asphalt Area (acres) Concrete Area (acres) Gravel (acres) Pavers (acres) Lawns (acres) Total Area Percent Effective Impervious Area (I = 100) (I = 100) (I = 40) (I = 50) (I = 0) (acres) Imperviousness (S.F) E1 E1 0.00 0.00 0.44 0.00 0.00 0.44 40% 7,602.40 Alley Alley 0.04 0.00 0.00 0.00 0.01 0.05 88% 1840.0 P1 P1 0.20 0.04 0.00 0.08 0.12 0.44 64% 12,205.69 Alley Alley 0.00 0.05 0.00 0.00 0.00 0.05 100% 2,090.90 Notes: Notes: Values from Fort Collins Stormwater Criteria Manual Table 4.1-3 Design Point Basin (s) Existing Developed RJP Wells Fargo Rational Method.xlsx 10/22/2019 Project: Parking at 112 West Magnolia Street Basin ID: P1 Design Information Input: Detemination of Water Quality Volume Using Modified FAA Method: Catchment Drainage Area A= 0.44 acres Catchment Drainage Imperviousness Ia = 64 percent Return Period for Detention Control T= 2 years (2 or 100) Time of Concentration of Watershed Tc= 5 minutes NRCS Soil Group Type= C A, B, C, or D Allowable Unit Release Rate q= 0.11 cfs/acre Runoff Coefficient C= 0.8 Inflow Peak Runoff Qp-in= 3.48 cfs Allowable Peak Outflow Rate Qp-out= 0.05 cfs Modified FAA Minor Storage Volume= 905 cubic feet Rainfall Rainfall Inflow Outflow Storage Duration Intensity Volume Volume Volume minutes inches / hr cubic feet cubic feet cubic feet (input) (output) (output) (output) (output) 0 0.00 0 0 0 5 2.85 301 15 286 10 2.21 467 29 438 15 1.87 592 44 549 20 1.61 680 58 622 25 1.43 755 73 682 30 1.30 824 87 737 35 1.17 865 102 763 40 1.07 904 116 788 45 0.99 941 131 810 50 0.92 972 145 826 55 0.87 1,011 160 851 60 0.82 1,039 174 865 65 0.78 1,071 189 882 70 0.73 1,079 203 876 75 0.70 1,109 218 891 80 0.66 1,115 232 883 85 0.64 1,149 247 902 90 0.61 1,159 261 898 95 0.58 1,164 276 888 100 0.56 1,183 290 892 105 0.54 1,198 305 893 110 0.52 1,208 319 889 115 0.51 1,239 334 905 120 0.49 1,242 348 893 City of Fort Collins Modified FAA Method Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® Civil 3D® by Autodesk, Inc. Tuesday, Oct 22 2019 6 Inch PVC Outlet Pipe Circular Diameter (ft) = 0.50 Invert Elev (ft) = 4994.78 Slope (%) = 0.50 N-Value = 0.012 Calculations Compute by: Known Q Known Q (cfs) = 0.45 Highlighted Depth (ft) = 0.44 Q (cfs) = 0.450 Area (sqft) = 0.18 Velocity (ft/s) = 2.46 Wetted Perim (ft) = 1.22 Crit Depth, Yc (ft) = 0.35 Top Width (ft) = 0.32 EGL (ft) = 0.53 0 1 2 Elev (ft) Section 4994.00 4994.50 4995.00 4995.50 4996.00 Reach (ft)