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Home My WebLink AboutMULBERRY CONNECTION - FDP200030 - - DRAINAGE REPORT (2)kimley-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:January 22, 2020 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. 6 Sub-basin A Sub-basin A is 5.32 acres and consists of on-site parking, sidewalk, one of the buildings, and landscape areas that surface flow to proposed Rain Garden A. Water quality treatment for runoff from this sub-basin will occur within Rain Garden A. Flows above the WQCV will flow to the detention pond via two overflow spillways between Rain Garden A and the detention pond. Sub-basin B Sub-basin B is 3.16 acres and consists of on-site parking, sidewalk, loading dock, and landscaped areas. Runoff within this sub-basin surface flows to a proposed 8-foot valley gutter routing water south to a proposed curb cut. The curb cut conveys flows to a lowpoint in the landscape area with a Type D grate inlet. The inlet will have a low-flow outlet pipe connected to an underground water quality facility, which will convey the WQCV. Flows over the WQCV will pond within the inlet structure to the “overflow outlet pipe”, which will convey flows to the detention pond. Sub-basin B1 Sub-basin B1 is 0.31 acres and consists of sidewalk, access drive, and landscaped area. Runoff within this sub-basin surface flows to a proposed conveyance swale. The conveyance swale conveys flows from sub- basin C and sub-basin B1 to two proposed flared end sections. The proposed longitudinal slope of the swale is 0.5%, therefore, a soft pan and an underdrain is proposed. Sub-basin OSB Sub-basin OSB is 0.38 acres and consists of landscaped area, and the north half of Redman Drive. Runoff within this sub-basin surface flows to the proposed conveyance swale. Sub-basin C Sub-basin C is 3.68 acres and consists of on-site parking, sidewalk, one of the buildings, and landscaped areas. Runoff within this sub-basin will surface flow east into Rain Garden C, which will provide water quality treatment. Storm events greater than the WQCV will be routed through Rain Garden C, into the proposed conveyance swale. Sub-basin O1 Sub-basin O1 is 0.44 acres and consists of on-site landscaped area and a portion of the west access drive. Runoff within this sub-basin will surface flow offsite and water quality treatment nor detention will be provided for this sub-basin. Sub-basin O2 Sub-basin O2 is 0.40 acres and consists of on-site landscaped areas. Runoff within this sub-basin will surface flow offsite to the proposed swale on the eastern property boundary. Water quality treatment nor detention will be provided for this sub-basin. Sub-basin OS1 Sub-basin OS1 is 0.21 acres and consists of off-site landscaped area and I-25 Frontage Road. Runoff within this sub-basin will flow to the proposed curb and gutter, following historic drainage patterns. An Preliminary Drainage Report Mulberry Connection – Fort Collins, Colorado 7 existing culvert is at the corner of I-25 Frontage Road and Redman Drive, conveys flows south to an existing roadside swale. A curb cut will convey flows to this culvert. Water quality treatment nor detention will be provided for this sub-basin on the proposed site. Additionally, this basin is not included as a part of the total Site improvement area, as proposed off-site improvements will improve existing conditions and maintain historic drainage patterns. Sub-basin OS2 Sub-basin OS is 0.55 acres and consists of a portion of Redman Drive and offsite landscape area. Runoff within this sub-basin will surface flow to proposed curb and gutter adjacent to the Site and the existing drainage ditch on the north side of Redman Drive. Water quality treatment nor detention will be provided for this sub-basin. Additionally, this basin is not included as a part of the total Site improvement area, as proposed off-site improvements will improve existing conditions and maintain historic drainage patterns. Table 1. Summary of proposed drainage sub-basins. Basin ID Total Tributary Area (acres) Impervious Area (acres)1 Imperviousness 2-Year Peak Flow (CFS) 100-Year Peak Flow (CFS) A 5.51 3.67 63%8.81 42.73 B 3.16 2.77 88%5.73 31.41 B1 0.31 0.05 18%0.19 0.86 OSB 0.38 0.21 57%0.57 2.51 C 3.68 2.58 67%6.29 32.61 O1 0.46 0.03 7%0.25 1.11 O2 0.40 0.18 46%0.46 2.51 OS1 0.21 0.21 100%0.58 2.14 OS2 0.55 0.34 62%0.72 3.67 Total On-Site2 13.52 9.28 66%21.73 111.23 Total On-Site to Detention3 13.04 9.28 69%21.59 110.12 Total Off-Site4 1.62 0.76 47%2.01 9.43 1. Includes pavement and roof areas. 2. Does not include OSB, includes O1 and O2. 3. Includes A, B, B1, C, and OSB. 4. Includes O1, O2, OS1, and OS2. 8 Rain Garden A Rain Garden A is proposed to treat runoff from, sub-basin A. Runoff will surface flow to a sediment forebay proposed along the length of the edge of asphalt, along the west side of the parking area. Rain Garden A is designed with 4:1 side slopes from the sediment forebay to a 12-foot width bottom and 4:1 side slopes to the berm between the rain garden and detention pond. An underdrain is proposed along the length of the rain garden. The underdrain will discharge directly into the detention pond. Two spillways will convey flows greater than the WQCV to the detention pond. The spillway crests will be at an elevation 1-foot above the bottom of the rain garden. Final design details of the rain garden will comply with City of Fort Collins standard details and be provided with the Final Drainage Report. Underground Facility An ADS SC-310 underground chamber system is proposed to treat the WQCV from sub-basin B. Runoff from sub-basin B will be collected by a Type D inlet with two connecting outfall pipes. The “low-flow” outlet pipe will convey the WQCV to the underground facility. Flows greater than the WQCV will pond within the inlet structure and enter the 36” outlet pipe, conveying flows to the detention pond. The underground facility will be designed per ADS recommendations, including an underdrain. The underdrain will outfall to the dry well proposed within the detention basin. Rain Garden C Rain Garden C is proposed to treat the WQCV from sub-basin C. Runoff will surface flow to a sediment forebay proposed along the length of the edge of asphalt, along the east side of the parking area. Rain Garden C is designed with 4:1 side slopes from the sediment forebay to an approximately 35-foot width bottom and 4:1 side slopes to the berm between the rain garden and I-25 Frontage Road. An underdrain is proposed along the length of the rain garden. The underdrain will discharge to a dry well located within the detention pond. An overflow grate will convey flows greater than the WQCV to the proposed conveyance swale and ultimately the detention pond. Final design details of the rain garden will comply with City of Fort Collins standard details and be provided with the Final Drainage Report. Detention Pond A detention pond is proposed to detain site flows from the 100-year storm event to the 2-year historic release rate. Due to Site constraints, the inflow pipe to the pond is proposed with an “inflow basin”. The bottom elevation of the inflow basin will be approximately 4-feet below the bottom of the pond. Table 2 summarizes the pond bottom elevation, “inflow basin” elevation, and groundwater elevation in the approximate location of the pond (boring B-17) as shown in the Draft Geotechnical Evaluation prepared by Ninyo & Moore, dated July 2, 2019. Preliminary Drainage Report Mulberry Connection – Fort Collins, Colorado 9 Table 2. Comparison of pond elevations and groundwater elevation. Finished Floor Elevation 4955.00 Pond Berm Elevation 4954.00 100-Year Water Surface Elevation 4952.31 Pond Bottom Elevation 4949.61 “Inflow Basin” Bottom Elevation (rim of dry well)4945.61 Underdrain Invert Elevation into Dry Well 4945.23 Approximate Groundwater Elevation 4943.001 1. Existing ground elevation at approximate location of boring B-17 is 4951.00. Groundwater was encountered approximately 8-feet below ground surface. The elevation of the bottom of the pond is set to allow positive drainage from the pond outlet to the existing drainage ditch adjacent to the Site. The inflow basin was preliminary sized following Figure 9-37 from UDFCD Chapter 9 for low tailwater riprap basins. A dry well proposed within the bottom of the inflow basin will discharge ponded flows within the inflow basin. Flows greater than the volume of the inflow basin will overtop the basin and enter the detention pond, discharging via the outlet structure. Final design of the detention pond will include detailed design of all features required in the Criteria Manual, including a maintenance path, trickle channels, forebays, outlet structure, and overflow spillway. Additionally, design of the dry well will be provided. Final design details will be provided with the Final Drainage Report. VARIANCE REQUESTS No variances are requested at this time. EROSION CONTROL The Erosion Control Criteria and all Erosion Control Materials will be provided in the 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. PROJECT NAME:POUDRE VALLEY DEVELOPMENT DATE: 11/19/2019 PROJECT NUMBER:096635000 CALCULATED BY:HMO CHECKED BY:DLS TOTAL IMPERVIOUS AREA IMPERVIOUS AREA TREATED (AC)(AC)Q2 Q100 EX1 EX1 13.67 0.00 0.00 4.28 18.70 EX2 EX2 0.26 0.00 0.00 0.53 2.55 EX3 EX3 0.21 0.00 0.00 0.08 0.35 14.13 0.00 0.00 4.89 21.60 A A 5.51 3.67 3.48 8.81 42.73 B B 3.16 2.77 2.77 5.73 31.41 B1 B1 0.31 0.05 0.00 0.19 0.86 OSB*B1 0.38 0.21 0.00 0.57 2.51 C C 3.68 2.59 2.59 6.29 32.61 O1 O1 0.46 0.03 0.00 0.25 1.11 O2 O2 0.40 0.18 0.00 0.46 2.51 13.90 9.28 --22.31 113.75 12.85 9.29 8.84 ---- 13.04 9.29 --21.60 110.12 OS1 OS1 0.21 0.21 0.00 0.58 2.14 OS2 OS2 0.55 0.34 0.00 0.72 3.67 0.76 0.55 0.00 1.30 5.80 * NOTE: OSB IS OFFSITE BUT WILL FLOW TO THE PROPOSED WATER QUALITY TREATMENT AND DETENTION POND. ON-SITE BASINS OFF-SITE BASINS TOTAL TOTAL TOTAL TOTAL ON-SITE TO DETENTION TOTAL ON-SITE TO WQ TREATMENT EXISTING BASINS DESIGN POINT RATIONAL CALCULATIONS SUMMARY TRIBUTARY BASINS TRIBUTARY AREA (AC) PEAK FLOWS (CFS) PROJECT NAME:Mulberry Connection PROJECT NUMBER:096501004 CALCULATED BY:HMO CHECKED BY:DLS 2-yr Historic Flow (EX1)4.28 CFS DATE:1/6/2020 O1 100-yr Flow 1.11 CFS O2 100-yr Flow 2.51 CFS Allowed Release Rate 0.66 CFS 12.85 0.66 0.87 Duration, Td Rainfall Intensity, I Inflow Volume, Vi Outflow Volume, Vo Storage Volume, Vs Storage Volume, Vs (min)(in/hr)(CF)(CF)(CF)(AC) 5 9.95 33371 198 33173 0.76 10 7.72 51783 396 51387 1.18 15 6.52 65601 594 65007 1.49 20 5.60 75126 792 74334 1.71 25 4.98 83511 990 82521 1.89 30 4.52 90956 1188 89768 2.06 35 4.08 95786 1386 94400 2.17 40 3.74 100347 1584 98763 2.27 45 3.46 104439 1782 102657 2.36 50 3.23 108329 1980 106349 2.44 55 3.03 111784 2178 109606 2.52 60 2.86 115104 2376 112728 2.59 65 2.71 118156 2574 115582 2.65 70 2.59 121611 2772 118839 2.73 75 2.48 124763 2970 121793 2.80 80 2.38 127715 3168 124547 2.86 85 2.29 130565 3366 127199 2.92 90 2.21 133416 3564 129852 2.98 95 2.13 135730 3762 131968 3.03 100 2.06 138179 3960 134219 3.08 105 2.00 140862 4158 136704 3.14 110 1.94 143142 4356 138786 3.19 115 1.88 145020 4554 140466 3.22 120 1.84 148106 4752 143354 3.29 ALLOWED RELEASE RATE CALCULATION Release Rate (R)' 100-Year Runoff Coefficient (C)' CALCULATIONS cfs (Historic 2-yr flow) Composite of entire proposed area MODIFIED FAA DETENTION SIZING BASIN A 100-YEAR Area (A) DETENTION VOLUME BY MODIFIED FAA METHOD - 100 YEAR acres (Basins A, B, B1, OSB, & C) PROJECT NAME: PROJECT NUMBER:096755001 CALCULATED BY:HMO CHECKED BY:DLS DATE:11/19/2019 Volume = (D/3)(A + B + (AB)^(1/2))D=depth between two contours, ft A=area of bottom contour, SF; B=area of top contour, SF ELEV.AREA VOLUME ACCUM. VOL.ACCUM. VOL. (FT)(SQ FT.) (CU FT)(CU FT)(AC-FT) 4949 37,114 0 0 0.00 4950 40,624 38,856 38,856 0.89 4951 44,231 42,415 81,271 1.87 4952 47,942 46,074 127,345 2.92 100-YR (3.29 AC-FT) = 4952.31 4953 51,775 49,846 177,191 4.07 4954 55,753 53,752 230,942 5.30 STAGE-STORAGE ANALYSIS BASIN A DETENTION BASIN