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Home My WebLink AboutMULBERRY CONNECTION - FDP200030 - DOCUMENT MARKUPS - ROUND 4 - DRAINAGE REPORTkimley-horn.com 4582 South Ulster Street, Suite 1500, Denver, CO 80237 303 228 2300 DRAINAGE REPORT COVER LETTER To: City of Fort Collins Matt Simpson, P.E., CFM Development Review Engineer From:Dan Skeehan, P.E. Kimley-Horn and Associates, Inc Date:March 3, 2021 Subject:Mulberry Connection, PDP190015 Drainage Report Cover Letter Kimley-Horn and Associates, Inc. (Kimley-Horn) is submitting this Drainage Report Cover Letter for the above referenced project. The purpose of this letter is to serve as an outline for the proposed development and associated drainage. PROJECT DESCRIPTION The proposed Mulberry Connection Project is located within the Northeast Quarter of Section 9, Township 7 North, Range 68 West of the Sixth Principal Meridian, City of Fort Collins, County of Larimer, Colorado. A vicinity map is provided below. Final Drainage Report Mulberry Connection – Fort Collins, Colorado 14 conveys these flows to the inflow infiltration basin. The detention pond also includes a maintenance path the length of the pond. A modified CDOT Type D inlet is proposed to serve as the outlet structure for the detention pond. The trickle channel is proposed to flow up to the base of the inlet. A 0.5-foot wide by 0.9-foot high square orifice is proposed at the trickle channel flowline/bottom of structure on the outside face of the structure. A trash rack is proposed over the orifice plate. The orifice is sized to release 3.40 CFS, which is the required 100- year release rate. The top of the inlet is set at an elevation of 4951.00, allowing any ponded water into the inlet. A second orifice plate over the outlet pipe with the same orifice dimensions is required to ensure the 100-year release rate is met. A 15-inch RCP storm pipe will convey flows from the outlet structure to the existing roadside swale. The crest of the emergency overflow spillway is set at the 100-year water surface elevation and 1.5-foot below the top of berm elevation. Spillway calculations provided in Appendix D indicate that the undetained 100-year flow rate flowing over the spillway will pond 0.5 feet above the crest. Maintenance of the detention pond is provided via a 12-foot gravel maintenance path accessible north of the pond via mountable curb in the parking lot. A 10-foot grass maintenance path along the bottom of the pond to allows access to the inflow infiltration basin and outlet structure. A drainage easement has been provided around the detention pond. Table 7. Detention Pond Summary Table Approx. Bottom of Pond Elevation 4949.00 100-Year Orifice Open Area 0.45 SF Top of Outlet Structure 4951.00 100-Year Water Surface Elevation 4952.00 Crest of Spillway Elevation 4952.00 Top of Berm Elevation 4953.50 Storm Sewer Conveyance Storm sewer pipe is proposed to connect sub-basins A, B, C, and D to the detention pond. The main stormline (rain garden C to detention pond) has been designed so that there is minimal ponding in the 100- year storm event. The tailwater condition into the pond was set at an elevation of 4949.66, which is the maximum ponding elevation with the inflow infiltration basin. This tailwater elevation causes minimal surcharging to occur at the Rain Garden C inlet and Inlet D4, just downstream of Rain Garden C in the 100- year storm event. Ponded water within Rain Garden C will pond to an elevation of 4953.70 (2.4 feet below FFE) before overtopping the south sidewalk and flowing into Redman Drive and the I-25 Frontage Road. A ponding depth above the inlet rim of 0.60 feet will occur. Ponded water at Inlet D4 will pond to an elevation of 4955.42 (0.68 feet below FFE) before spilling over the berm at the southern property line and flow into Redman Drive. A ponding depth of 2.42 feet above the rim of the inlet will occur before the berm if overtopped. Due to the required elevations within the loading dock area, ponding will occur within this area during the 100-year storm event. In addition to the HGL surcharging at all these locations, when the detention pond is at the 100-year water surface elevation (4952.00), the water surface will equalize into this area. The Final Drainage Report Mulberry Connection – Fort Collins, Colorado 17 flows, therefore, the runoff will overtop the crown and utilize both sides of the street. A ponding depth of 8.1-inches at the curb flowline will result in a spread of 33.8 feet. Street capacity calculations are included in Appendix D. Flows that overtop the crown will flow south into the existing swale and culverts along the south side of the road. In the 100-year storm event, flows from the north side of the road will combine with flows from the south side of the road. This combined runoff will flow into the existing swales, which have depths between 3 and 5 feet. Runoff will continue to flow south through the existing 12” CMP culverts beneath the southern driveways. These culverts do not have capacity to convey all the proposed 100-year flows, therefore, water will pond in the swales and then overtop into the roadway, continuing to flow west, bypassing the culverts. All flows in Redman Drive will flow west into Cooper Slough. VARIANCE REQUESTS A variance is requested to allow the inflow infiltration basin within the detention pond. A separate variance request has been submitted. EROSION CONTROL A separate Erosion Control Report will be provided in addition to this Final Drainage Report. CONCLUSIONS COMPLIANCE WITH STANDARDS The Mulberry Connection project is in compliance with City of Fort Collins criteria for storm drainage design. The “Fort Collins Stormwater Criteria Manual, December 2018 Edition,” and the Urban Drainage Flood Control District “Urban Storm Drainage Criteria Manual Volumes 1, 2, and 3 have been utilized for reference. SUMMARY OF CONCEPT The proposed drainage concept is to surface flow runoff from impervious areas to pervious area to the extent practicable. Two rain gardens and one underground facility are proposed to treat the WQCV from as much of the Site as possible. A detention pond is proposed to detain the on-site 100-year runoff volume to the 2-year historic on-site flow. Downstream impacts due to the development of this Site are not anticipated. REFERENCES Fort Collins Stormwater Criteria Manual, December 2018 Edition, City of Fort Collins. Urban Storm Drainage Criteria Manual, Volume 1-3, Mile High Flood District, Denver, CO; January 2016, with latest revisions. Stormwater Facility Name: Facility Location & Jurisdiction: User Input: Watershed Characteristics User Defined User Defined User Defined User Defined Watershed Slope =0.010 ft/ft Stage [ft]Area [ft^2]Stage [ft]Discharge [cfs] Watershed Length =1000 ft 0.00 503 0.00 0.02 Watershed Area =12.31 acres 1.00 862 1.00 0.02 Watershed Imperviousness =70.0%percent 2.00 1,322 2.00 0.02 Percentage Hydrologic Soil Group A =0.0%percent 3.00 1,894 3.00 0.02 Percentage Hydrologic Soil Group B =100.0%percent 4.00 2,619 4.00 0.02 Percentage Hydrologic Soil Groups C/D =0.0%percent 5.00 31,274 5.00 1.58 6.00 47,292 6.00 2.65 User Input 17 7.00 51,508 7.00 3.40 8.00 55,516 8.00 104.86 WQCV Treatment Method =hours After completing and printing this worksheet to a pdf, go to: https://maperture.digitaldataservices.com/gvh/?viewer=cswdif create a new stormwater facility, and attach the pdf of this worksheet to that record. Routed Hydrograph Results Design Storm Return Period =WQCV 2 Year 5 Year 10 Year 50 Year 100 Year One-Hour Rainfall Depth =0.53 0.86 1.14 1.44 2.40 2.93 in Calculated Runoff Volume =0.572 0.797 1.094 2.062 2.640 acre-ft OPTIONAL Override Runoff Volume =acre-ft Inflow Hydrograph Volume =0.571 0.797 1.093 2.061 2.640 acre-ft Time to Drain 97% of Inflow Volume =76.1 73.4 69.7 56.4 48.0 hours Time to Drain 99% of Inflow Volume =83.0 83.1 83.0 81.3 80.0 hours Maximum Ponding Depth =4.88 5.13 5.43 6.25 6.71 ft Maximum Ponded Area =0.63 0.76 0.87 1.11 1.15 acres Maximum Volume Stored =0.432 0.611 0.857 1.688 2.206 acre-ft Location for 1-hr Rainfall Depths (use dropdown): Workbook Protected Worksheet Protected Stormwater Detention and Infiltration Design Data Sheet Mulberry Connection Detention Pond Fort Collins, CO SDI_Design_Data_v1.08_Detention Pond.xlsm, Design Data 1/19/2021, 1:31 PM WQCV_Trigger =1 RunOnce=1 CountA=1 Draintime Coeff=1.0 0 1 2 3 #N/A #N/A 0 1 2 3 #N/A #N/A Check Data Set 1 Check Data Set 1 Area Discharge Stormwater Detention and Infiltration Design Data Sheet 0 5 10 15 20 25 30 35 40 45 0.1 1 10FLOW[cfs]TIME [hr] 100YR IN 100YR OUT 50YR IN 50YR OUT 10YR IN 10YR OUT 5YR IN 5YR OUT 2YR IN 2YR OUT WQCV IN WQCV OUT 0 1 2 3 4 5 6 7 8 0.1 1 10 100PONDINGDEPTH[ft]DRAIN TIME [hr] 100YR 50YR 10YR 5YR 2YR WQCV SDI_Design_Data_v1.08_Detention Pond.xlsm, Design Data 1/19/2021, 1:31 PM