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HomeMy WebLinkAboutDrainage Reports - 04/09/2019 (2)I I IMarch 22, 2019 Dan Mogen Fort Collins Utilities — Stormwater 700 Wood Street Fort Collins, CO 80522 Re: The Gardens on Spring Creek Drainage Letter INTCRW T CONSULTING GROUP City of Fort Collins :,ed I 19 Z1°1 Plans Dear Dan, Please accept the following letter describing the storm drainage intent demonstrating the proposed site improvement's ability to comply with the City's stormwater requirements. INTRODUCTION The Gardens on Spring Creek project proposes to construct an addition to the existing on -site facility located at 2145 Centre Avenue, Fort Collins. SEXISTING CONDITIONS ' The project site is located within the Spring Creek Drainage Basin. The existing site consists of a greenhouse/office building, accessory buildings, gardens, recreational areas, and walking paths_ The site generally slopes away from the existing building to the northeast and the southwest. The site was previously studied with the Project Development Drainage and Erosion Control Report Center for Advanced Technologies 224d Filing "Community Horticultural Center", dated January 2003, and prepared by EDAW Inc. for the City of Fort Collins. The report states, "Although no detention is required on the site, BMP's will address stormwater quality. They will consist of porous landscape, a wetlands basin, and a grassed swale that will treat low flow, while allowing high flow events to bypass into Spring Creek." Following these conditions, no stormwater detention is being provided for this project. However, the proposed improvements will be treated for water quality via the existing water quality features on -site as discussed in this letter. Portions of the site are located within a FEMA mapped 100-yr floodplain and floodway as depicted on the attached drainage plan; however, no grading or improvements are proposed to be within the floodway and a Floodplain Use Permit is not required.. rThe historic basins are described below: Basin HB includes the northern portion of the site and is approximately 1.40 acres and 21.8% impervious. The 100-yr peak runoff is 3.69 cfs. The runoff from this basin overland flows to the Sherwood Lateral. 1218 W. ASH, STE. A, WINDSOR, COLORADO 80550 TEL. 970.674.3300 - FAX 970.674.3303 The Gardens on Spring Creek Stone Drainage Letter March 22, 2019 Page 2 of 5 Basin HA3b consists of the southem portion of the site that drains to Design Point 2. Design Point 2 is an existing inlet within a Porous Landscape Water Quality Pond southwest of the main existing building. The basin is approximately 1.33 acres and is 21.0% impervious. The 100-yr peak runoff is 3.23 cfs. All calculations and exhibits are attached for reference. PROPOSED DEVELOPMENT The proposed improvements will consist of constructing a building addition on the east end of the main structure. Minor adjustments to existing hardscapes will be made and some new hardscape will be added to accommodate the addition. The existing Porous Landscape Water Quality Pond is proposed to satisfy the Low Impact Development (LID) and water quality requirements for the new construction. The proposed improvements are included in two drainage basins and consist of: Basin DB varies slightly from Basin HB due to the building addition that alters the line of division between the site basins. It is approximately 1.35 acres and 22.2% impervious, which is a decrease of 0.05 acres and an increase of 0.4% imperviousness compared to Basin HB. The 100-yr peak runoff is 3.66 cfs, which is a decrease of 0.03 cfs from historic. Therefore, no water quality is required for this basin. The majority of the runoff will continue to sheet flow to the Sherwood Lateral, similar to the historic drainage pattern. Since there is no increase in the runoff being directed to the Lateral we do not anticipate the need to obtain approvals from the Ditch Company. Basin DA3b varies slightly from Basin HA3b due to the building addition that alters the line of division between the site basins. The basin is approximately 1.39 acres and is 27.7% impervious, which is an increase in area of 0.06 acres and a 6.7% increase in imperviousness compared to Basin HA3b. The 100- yr peak runoff is 4.09 cfs which is an increase of 0.86 cfs compared to Basin HA3b. The new roof drains will be connected to the existing perimeter roof drain system which is an 8-inch PVC drain that has a minimum slope of 1.06% and a capacity of 1.26 cfs. The combined flows from the existing and proposed roof areas are 0.47 cfs for a 10-year event and 1.21 cfs for a 100-year event; therefore, this drain system is shown to have adequate capacity even up to a 100-year event. Design Point 2 is the existing water quality outlet structure and will control the runoff in the same manner that it has historically. No detention is required for this basin and the existing. Porous Landscape Water Quality Pond on -site is shown to have excess capacity to meet the water quality requirements. All calculations and exhibits are attached for reference. Existing Porous Landscape Water Quality Pond A comparison was made between the required WQCV for the existing conditions and what would be required including the new building addition. The new addition minimally increases the contributing watershed area from 1.33 acres to 1.39 acres (0.5% increase) and increases the calculated WQCV from 0.016 (674 cf) ac-ft to 0.020 ac-ft (871 cf). The existing topographic survey indicates that approximately 890 cu ft of storage is available at the rim elevation of the outlet structure. Therefore, it is proposed that the minimum required water quality capture volume to treat Basin DA3b is met and exceeded with the current water quality pond. All calculations and exhibits are attached for reference. The Gardens on Spring Creek Storm Drainage Letter ' March 22, 2019 Page 3 of 5 1 Standard Operating Procedure (SOP) Currently, multiple stormwater conveyance and water quality treatment facilities exist on -site. As a part of this project, several SOP's were developed and are attached to this letter. These will be put into practice for the existing facilities. LOW IMPACT DEVELOPMENT TECHNIQUES In reference to the code requirements for implementation of Low Impact Development (LID) techniques, the use of the existing Porous Landscape Water Quality Pond is proposed to satisfy these requirements. The pond will promote infiltration while capturing fine sediment that drains from the impervious areas, prior to discharging through the perforated underdrain. Below is a description of the 4 step process for selecting structural BMPs: Urban Drainage and Flood Control District (UDFCD) recommends a Four Step Process for receiving water protection that focuses on reducing runoff volumes, treating the water quality capture volume (WQCV), stabilizing drainageways and implementing long-term source controls. The Four Step Process applies to the management of smaller, frequently occurring events. Step 1: Employ Runoff Reduction Practices To reduce runoff peaks, volumes, and pollutant loads from urbanizing areas, implement Low Impact Development (LID) strategies, including Minimizing Directly Connected Impervious Areas (MDCIA). Runoff from the building roof will be conveyed to the Porous Landscape Water Quality Pond via roof drains connected to an existing 8-inch perimeter drain pipe that outlets to the water quality pond. The pond will promote infiltration, reduce the runoff rate and filter runoff prior to being released into a tributary for Spring Creek. Step 2: Implement BMPs that Provide a Water Quality Capture Volume with Slow Release The Porous Landscape Water Quality Pond is designed to provide water quality capture volume per Urban Drainage's recommendations and calculations. The capture runoff is designed for a 12-hr drain time. Step 3: Stabilize Drainageways Natural Drainageways are subject to bed and bank erosion due to increases in frequency, duration, rate and volume of runoff during and following development. The site will ultimately drain to Spring Creek and the Sherwood Lateral which are both well vegetated and additional bank stabilization is unnecessary with this project. Step 4: Implement Site Specific and Other Source Control BMPs Proactively controlling pollutants at their source by preventing pollution rather than removing contaminants once they have entered the stormwater system or receiving waters is important when protecting storm systems and receiving waters. This can be accomplished through site specific needs ' The Gardens on Spring Creek Storm Drainage Letter ' March 22, 2019 Page 4 of 5 ' such as construction site runoff control, post -construction runoff control and pollution prevention J good housekeeping. It will be the responsibility of the contractor to develop a procedural best management practice for the site. The proposed development is not expected to increase pollutants that require treatment, as there are no drive surfaces or parking areas being proposed with this project. Therefore, no ' permanent source control concerns will be present on -site, relating to the proposed improvements of this project. All calculations are attached for reference. EROSION CONTROL The disturbance area associated with the proposed project is approximately 0.25 acres, which does not warrant a state SWMP to be provided with this project application. Temporary Erosion Control Erosion and sedimentation will be controlled on -site by use of sediment control logs, rock socks, silt fence, inlet protection, seeding, mulch, and turf. The measures are designed to limit the overall sediment yield increase due to construction as required by the City of Fort Collins. During overlot and final grading the soil will be roughened and furrowed perpendicular to the prevailing winds. During the performance of the work required by these specifications or any operations appurtenant thereto, whether on right-of-way provided by the City or elsewhere, the contractor shall furnish all labor, equipment, materials, and means required. The Contractor shall carry out proper efficient measures wherever and as necessary to reduce dust nuisance, and to prevent dust nuisance that has originated from his operations from damaging crops, orchards, cultivated fields, and dwellings, or causing nuisance to persons. The Contractor will be held liable for any damage resulting from dust originating from his operations under these specifications on right-of-way or elsewhere. It is unlawful to track or cause to be tracked mud or other debris onto city streets or rights -of -way. Wherever construction vehicles access routes or intersect paved public roads, provisions must be made to minimize the transport of sediment by runoff or vehicles tracking onto the paved surface. Stabilized construction entrances are required with base material consisting of 6" coarse aggregate. The contractor will be responsible for clearing mud tracked onto city streets on a daily basis. All temporary and permanent erosion and sediment control practices must be maintained and repaired as needed to assure continued performance of their intended function. Silt fence, rock socks, and sediment control logs will require periodic replacement. Maintenance is the responsibility of the contractor. All disturbed areas must be seeded and mulched within 30 days of project start. Vegetation shall not be ' considered established until a ground cover is achieved, which is demonstrated to be mature enough to control soil erosion to the satisfaction of the City Inspector and to survive severe weather condition. Permanent Erosion Control Where permanent drainage features on -site have the potential to cause erosion and sedimentation, ' permanent protection will be provided and is described below: Roof drains will convey roof runoff to the existing Porous Landscape Water Quality Pond which currently has a pipe outlet to a headwall installed with a flap gate to prevent rodent nesting and backwater from the pond during larger rain events. The Gardens on Spring Creek Storm Drainage Letter March 22, 2019 Page 5 of 5 CONCLUSIONS All computations that have been completed within this letter are in compliance with the City of Fort Collins Storm Drainage Design Criteria Manual and the previously approved report for the site. The proposed drainage concepts presented in this report and on the construction plans adequately provide for stormwater quality treatment of proposed impervious areas. Conveyance elements have been designed to pass required flows and minimize future maintenance. If, at the time of construction, groundwater is encountered, a Colorado Department of Health Construction Dewatering Permit will be required. Thank you in advance for your time, and if you have any questions or comments please contact me at (970) 833-1406. Sincerely, Logan A. Johnson, P.E. Colorado Professional Engineer no. 55076 Attachments Reviewed by: Robert Ahnirall, P.E. 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(S.F.) Runoff Coefficient (in.) (C.F.) Basin Drain Tine Area Area IC" WOCV Design Vol. AU 12 128 55.757 0.4 0145 674 Storawrr Vol. Dssiyn Scent- Ara (Design VoU avg. depth) (C.F.) TO (S.F.) Design Vol D(w0g) Design Ares 674 0.50 1,347 1,347 Surface Area Q*-) (AC.) (S.F.) Runoff Coefficient (ln.) (C.F.) Basin Drain Tune Area Area T, W'OCV Design Vol. Ala 12 2.59 112.820 0.6 0.19 1.7ti6 Stop os Vol. Design Surface Area (Design Val avg. depth) (C-FJ (Ft) (S.F.) Design Vol. D(avg) Design Area 1.786 0.50 3,573 Design Basin Storage Voluuns (Hr_) (Ae.) (S.F.) Runoff Coefficient (kQ r-F) Basin Drain Time Area Area "C' WQCV Design Vol. Alb 12 1.81 78.844 0.25 0.11 723 Vol. Design Surface Area (Design Volt avg. depth) (aF.) (FL) (S.F.) Design Vol D(M) Design Arse 723 0.58 1,246 1,246 Surface Ana Design Basin Storage Voluno dHv.) (Ac.) (S.F.) Runoff Coefficient (in.) (C.F.) Basin Drain Ime Area Area ^C" WOCV Design Vol. Al 24 4.38 190.793 025 0.12 1.M SUoraas VOL Design Surfaca Am (Design Volt Mg. depth) (C.F.) To (S.F.) Design Val. D(avg) Design Ana 1.908 023 8,295 Total 10.06 acres urafficial copy was downloaded on Apr-30.2018 from the City of Fort Collins Public Records Website: hntp*itydocs fcgov corn KO O ZC IP U e Ad ci 3 t5esetd�p tl c= lb . J 4 . I � 1 4 �y� pppK s A ; I ED .1i / 719e. sr N AL �aata V I yy Q aa�ata Y z1i i g 5 ¢aaea t LOCATION: PROJECT NO: COMPUTATIONS BY DATE: EX SPILL EX WO Pond - Stage/Storage Gardens on Spring Creek LAJ 3/20/2019 V = 1 /3 d (A + B + sgrt(A*B)) where V = volume between contours, ft3 d = depth between contours, ft A = surface area of contour Detention Pond Stage (ft) Surface Area (ft2) Incremental Storage (cu-ft) Total Storage (cu-ft) 4998.7 1 4999.0 48 5.17 5.2 4999.5 546 125.76 130.9 5000.0 1356 460.41 591.3 5000.2 1794 298.28 1370-095-00 EX POND VOLUME.xls Design Procedure Form: WQCV Project Name: Gardens On Spring Creek Project Number: 1370-095-00 Company: INTERWEST CONSULTING GROUP Designer: U Date: 3/20/2019 1. Basin Storage Volume (Existing Conditions) A) Tributary Area's Imperviousness Ratio (i=la/100) B) Contributing Watershed Area (Area) C) Water Quality Capture Volume (WQCV) (WQCV = 1.0 - (0.91 -i3-1.19'Q+0.78i)) D) Design Volume: Vol = WQCV/12' Area 1-2 2. Basin Storage Volume (Proposed Conditions) A) Tributary Area's Imperviousness Ratio (i=lall00) B) Contributing Watershed Area (Area) C) Water Quality Capture Volume (WQCV) (WQCV =1.0`(0.91-i3-1.19`Q+0.78i)j D) Design Volume: Vol = WQCV/l2 ` Area " 1.2 la = 21 % i = 0.21 A = 1.334 acres WQCV = 0.119749 watershed inches Vol. = 0.016 ac-ft la = 27.7 % i = 0.277 A = 1.385 acres WQCV = 0.144094 watershed inches Vol. = 0.020 ac-ft percent increase= 25 % Bioretention/Bioswa I e Routine Maintenance Table (Summary from Chapter 6 of UDFCD) ' Required Maintenance Objective Action 1 Frequency of Action Occasional mowing of grasses and Lawn mowing weed removal to limit unwanted Routine - Depending on aesthetic and vegetative vegetation. Maintain irrigated turf requirements, planting scheme and cover. grass as 2 to 4 inches tall and non- Weeds should be removed before they care irrigated native turf grasses at 4 to 6 flower. inches. Remove debris and litter from bioretention area and upstream concrete forebay to minimize clogging of the sand media. Remove debris and litter from the pond area Routine -Including just before annual storm Debris and litter and outlet orifice plate to minimize seasons and after snow season (April or removal and clogging. Remove debris and litter May), end of storm season after leaves have snow from curb channel and sidewalk fallen, and following significant rainfall stockpiling chase outlets adjacent to pond if applicable to minimize clogging. events. Avoid stockpiling snow in the bioretention area to minimize clogging from sediment accumulation. Inspect detention area to determine if the sand media is allowing acceptable infiltration. If standing Inspections water persists for more than 24 hours Routine -Biannual inspection of the after storm runoff has ceased, hydraulic performance. clogging should be further investigated and remedied. ' Growing media replacement Non -routine - Performed when clogging is Restore infiltration capacity of due to the migration of sediments deep into bioretention facilities. the pore spaces of the media. The frequency of replacement will depend on site -specific pollutant loading characteristics. Directly Connected Downspouts Many of the downspouts connect directly to the storm drain system. The following SOP generally applies to all direct downspout connections. This SOP can more specifically apply to those which drain directly to the reservoir areas beneath the Modular Block Pavers. At each of these connections, the downspout discharges to a perforated drain basin. The drain basins discharge directly to the MBP reservoir. The drain basins are designed to prevent debris and sediment from entering the MBP reservoir area. Debris and sediment compromise the functionality and effectiveness of the system. Routine Maintenance Table for Directly Connected Downspouts ' Required Maintenance Objective Frequency of Action Action Inspect the downspout and basin to ensure the system functions as it Inspections Routine was designed. Repair or replace damaged downspouts as needed. Sediment', Remove debris and litter from the Routine -just before annual storm seasons Debris and Litter basin. Remove sediment from the (i.e., April/May); at the end of stone season removal sump. after leaves have fallen; and following significant rainfall events. I Perforated Subdrain The perforated subdrain system storm drain outfall at the bottom of the Low Impact Development (LID) system is critical to the overall function of the system subbase. As such, special maintenance has been identified to ensure these perforated drain systems perform as they were designed. Perforated subdrains leading away from the LID system is designed to provide faster release of water when accumulation occurs under the LID system. Outflow should be seen into downstream storm boxes. If not seen it is recommended that the system is inspected using a video camera to verify no clogging has occurred. Perforated subdrains leading toward the LID system are designed to provide an opportunity for ' infiltration. These subdrains may lead to a drywell where additional infiltration capacity is available to reduce runoff per the stated LID goals adopted by the City. Routine Maintenance Table Required Maintenance Objective Frequency of Action Action ' Use a video camera to inspect the condition of the perforated drain ' Inspection pipes. Cleanout pipes as needed. If Every two to five years. integrity is the of the pipe compromised, then repair the ' damaged section(s). - Where accessible, expose inlet Inspection and/or outlet of perforated pipe Minimum Annually and watch for water inflow and/or Outflow. Storm Drain Lines Storm drain lines are subject to sedimentation as well as tree roots clogging the flow path or altering the pipe slope. Maintenance is important to ensure these storm drain systems perform as they were designed. Routine Maintenance Table Required Maintenance Objective Frequency of Action Action Use a video camera to inspect the condition of the storm drain pipes. Cleanout pipes as needed. If the Inspection Every two to five years. integrity of the pipe is compromised, then repair the damaged section(s).