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Home My WebLink AboutDrainage Reports - 05/29/2018June 28 , 2016 Basil Hamdan, P .E., CFM Stormwater Quality Engineer Fort Collins Utilities , Stormwater 700 Wood Street Fort Collins , CO 80521 City of Fort Coffins Approved Plans Approved by: w/11.. -NOT 12E." •~"'-' U Date:,;/z.tlj,/2.0t~ ~'i <;Toe.M.w,~ RE: City of Fort Collins Gardens on Spring Creek -Drainage & Erosion Control Memo NA Job No. 2257c Dear Mr. Hamdan: NA. Incorporated 25 Old Town Square Suite 200 Fort Cdlins, CO 00524 970 .225 .9099 info@jvajva.com www.Jvajva.com This letter has been prepared to summarize the drainage concept , solutions and changes in impervious area associated with the site improvements for the Gardens on Spring Creek project located at the Community Horticulture Center, 2145 Centre Avenue , Fort Collins , Colorado. This letter represents an addendum to the Center for Advanced Technologies 22 nd Filing ("Community Horticulture Center") Final Project Development Drainage and Erosion Control Report , dated January 31, 2003, prepared by EDAW , Inc . (EDAW). Overview The existing 18.3 acre site consists of the Community Horticulture Center (CHC) and outdoor classrooms, visitor parking lot , Children 's Garden , Garden ofEatin', Experiential Garden, and the Rock Garden , all constructed as part of the first phase of the Master Plan for the development of the City-owned property . In general , as part of the final buildout of the project (phase 2) on the remaining 5+ acres of undeveloped land , proposed improvements include the addition of the Great Lawn and bandstand/stage , the Prairie Garden , Foothills Garden , Undaunted Garden , extensive trail system expansion , and enlargement of the stormwater detention pond /Wetlands Demonstration Site to the east. The project site is located in the FEMA 100-year regulatory flood way and City of Fort Collins Spring Creek Drainage Basin (SWMM Subbasin 130), and no stormwater detention is required (refer to the project Floodplain Memo prepared by JV A, Inc. dated September 8, 2015 for compliance with Chapter IO of City Municipal Code: Flood Prevention and Protection). The referenced Final Drainage and Erosion Control Report for the project accounts for the anticipated phase 2 buildout of the gardens and corresponding increase in runoff into Spring Creek. This letter will serve to supplement the original report and demonstrate that the final development of the Gardens on Spring Creek complies with the approved stormwater drainage plan designed by EDAW. Existing Drainage Design As discussed in the approved drainage report, the predeveloped site historically drained overland directly into Spring Creek. The original storm design by EBA W mimics the historic drainage pattern; runoff from the site flows via curb & gutter (in the existing parking lot), grass swales , culverts , and through a perforated underdrain system within the water quality porous landscape detention areas , ultimately discharging into Spring Creek. The southern portion of the site is interrupted by the Sherwood Ditch Lateral , and the original suspended CMP storm culverts were replaced with RCP culverts underneath the ditch to convey runoff into Spring Creek as part of the phase l project. Other than the area immediately adjacent to the Sherwood Lateral , no runoff will enter the ditch except in the I 00-year event. BOULDER FORT COLL I NS WINTER PARK GLENWOOD SPR I NGS DENVER Proposed Drainage Design CoFC Gardens on Spring Creek Drainage Memo June 28 , 2016 2 of2 The drainage characteristics of the phase 2 bui ldout of the Gardens on Spring Creek comply with the original stormwater design intent as described in the approved drainage report and no major variations are proposed. Runoff will continue to be conveyed to Spring Creek via a combination of overland flow, vegetated swales , and perforated piping. The existing constructed wetlands basin (previously referred to as the Wetlands Demonstration Site) will be expanded to approximately 15 ,000 square feet of area and 0.491 acre-feet of volume. This enlarged basin will encompass and replace the existing southeastern porous landscape detention (PLD) area (identified as water quality basin Ala in the original Report) while retaining the existing concrete sill structure and downstream PLD area to the north , eventually outfalling into Spring Creek. The wetlands basin will provide 11,843 cubic feet of storage volume above the permanent water surface created by the concrete sill at an elevation of approximately 4990.94 feet (NA VD88 datum), an increase of 8,149 cubic feet over the existing condition. The small PLD area to the north referred to in the original plans as basin A I b will also remain . See the attached grading plans , sheets C 1.0 -C 1.2. EBA W's Report anticipated the Gardens full buildout by assuming an overall runoff coefficient value of 0.35 and an approximated 10% imperviousness. Based on as-built drawings of the Horticulture Center, the existing/post-phase I overall site imperviousness was calculated to be 12.5%, with a coefficient of 0.40. The Gardens final proposed layout was calculated to be 17 .8% and 0.44 respectively, resulting in a minor increase over the existing and an insignificant deviation from the original assumptions . The associated increase in runoff is more than offset by the increased volume in the constructed wetlands basin. See attached calculations. Erosion and Sediment Control During construction , temporary erosion and sediment control practices will be used to limit soil erosion and sediment discharge off the site. An erosion and sediment control plan with stormwater management plan and details will be provided in the utility plans for construction. Refer to the Erosion Control Report prepared by NA dated June 28, 2016. Please feel free to contact myself or Karen Brigman to discuss any questions you may have. Sincerely, JV A , rNCORPORA TED By: By: -----------.. __ _ 'v 'v 'v 'v 'v 'v o.:::.. __ -:?-_-_-;..-""'."i -. --:-.--• ~-~-------::-;:.._~.::--·.--... -:-.... -_-:-_---------'•-.. ·:_·.: ,. ~~~ ... ::-."_ i 1! NVld lO!ilNO::J NOISOlJ3 ON'v' '3£lVNl~O '£lNIO~£l ! 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JVA In corporated Job Name : Gardens on Spring Creek 1% C2 cs C10 c,oo 25 Old Town Square Job Number: 2257c Streets Paved 100% 0.95 0 .00 0 .95 1.00 Fort Collins , CO 80524 Oate : 9/15/15 Concrete Drives/Walks 90% 0.95 0 .00 0.95 1 .00 Ph : 970 .225 .9099 By: KRB Roof 90% 0.95 0 .00 0.95 1 .00 Fax: 970 .225 .6923 Gravel 40% a.so 0 .00 a.so 0 .63 Landscaping (B soil) 0% 0.15 0 .00 0.15 0 .19 Gardens on Spring Creek Landscaping (CID soil) 0% 0.25 0 .00 0.25 0 .31 Historic Runoff Coefficient Calculations Playground 10% 0.11 0.00 0.11 0 .14 Location: Fort Collins Artificial Turf 25% 0.20 0 .00 0.20 0 .25 Minor Design Storm: ~ Major Design Storm : Soil Type : D Basin Desiqn Data I(%}= 100% 90% 90% 40% 10% 25% 0% 0% 1(%) Runoff Coeffs Basi n Design A paved ~rives/c A ,oof Asravel A plygnd A art.turt A 1scape (B A 1scape A rolal Arolal Imp C2 cs C10 C100 Name Poi nt streets (sf} (sf} (sf} (sf} (sf} soil) (C/Dsoil) (sf} (ac} (%} (sf} one (sf} (sf} H 1 49 ,942 39 ,309 10,331 12,087 5 ,900 678,427 795,996 18.27 12 .5% 0 .34 0.00 0 .34 0.40 2257c -Rational Cal cul ations.xlsx Historic C Page 1 of 1 JVA Incorporated Job Name: Ga rdens on Spring Creek 1% C2 cs C10 C100 25 Old Town Square Job Number: 2257c Streets Paved 100% 0 .95 0 .00 0.95 1.00 Fort Coll ins, CO 80524 Date : 9/15/15 Concrete Oriv esiWalks 90% 0 .95 0 .00 0.95 1.00 Ph : 970.225.9099 By : KRB Roof 90% 0.95 0.00 0.95 1.00 Fax: 970 .225 .6923 Gravel 40% 0.50 0.00 0 .50 0 .63 Landscaping (B solQ 0% 0.15 0.00 0 .15 0 .19 Gardens on Spring Creek Landscaping (CID soiO 0% 0.25 0 .00 0 .25 0 .31 Developed Runoff Coefficient Calculations Playground 10% 0 .11 0 .00 0.11 0 .14 Location : Fort Collins Artificial Turf 25% 0 .20 0.00 0 .20 0.25 Minor Design Storm : §E Majo r Design Storm: Soi l Type : D Basin Desion Data I(%)= 100% 90% 90% 40% 10% 25% 0% 0% 1(%) Runoff Coeffs Basin Design A paved A drives/c Aroof A gravel A plygnd A art.turf A 1scape (8 A 1scape A Total ATotal Imp C2 cs C10 C100 Name Point streets (sf) (sf) (sf) (sf) (sf) soil) (CID soil) (sf) (ac) (%) (sf) one (sf) (sf) D 1 49 ,942 79 ,073 10,817 27 ,864 5 ,900 622.400 795 ,996 18.27 17 .8% 0.38 0.00 0 .38 0.44 2257c -Rational Calcu lations .xlsx Developed C Page 1 of 1 Gardens on Spring Creek Critical Pond Elevations Design Eng ineer: Design Firm: Project Number: Date : DESIGN CRITERIA K. Brigma n JVA , Inc. 2257c September 9, 2015 Urban Storm Dra inage Criteri a Manua l, Urban Dra inage and Flood Control District, June 2001 Stage Storage Volume (pond volume calculated using the prismoidal formula ): V = (A, + A2 + p:;i;)Depth 3 CONTOUR (FT) AREA (Fr) AREA (ACRE) VOLUME DEPTH (FT) CUMULATIVE VOLUME CUMULATIVE VOLUME 4987.50 4988.0 4989.0 4990.0 4990.94 4991 .0 4992 .0 4992 .5 4993 .00 4992 .00 ;:- !:. 4991 .00 z 0 4990.00 ~ > 4989.00 w ..J w 4 988.00 4987 .00 0.00 790.00 1,480.00 4 ,200.00 7 ,860.00 8,075.00 15,0 10.00 / / (ACRE-FT) 0.000 0.000 0.00 0.018 0.003 0.50 0.034 0.026 1.50 0.096 0.063 2.50 0.180 0.128 3.44 0.185 0.011 3.50 0.345 0.261 4.50 DETENTION POND VOLUME VERSUS ELEVATION -------- 0 .0 0.1 0.2 0.3 0.4 DETENTION POND VOLUME (AC-FT) 2257c Pond Cales.xis -Pond Stage Storage #1 (ACRE-FT) (CUBIC FT) 0.000 0.00 0.003 131 .67 0.029 1248 .77 0.091 3973 .16 0.219 9552 .25 0.230 10030.29 0.491 21395.07 0 .5 0 .6 'Permanent WSE JVA, Inc.