Drainage Report
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ENGINEERING REPORT STORMWATER MANAGEMENT PLAN 132 FLAX HILL ROAD NORWALK, CONNECTICUT September 21, 2020 Revised April 20,2021 Prepared for: Workforce Partners, LLC PO Box 692 South Norwalk, CT 06856 Prepared By: Ricardo Ceballos, P.E. Professional Engineering Consulting Services 245 Sturges Highway Westport, CT 06880 (203)635-0922 STORMWATER MANAGEMENT PLAN 132 Flax Hill Road Norwalk, CT PROJECT DESCRIPTION The descriptions and computations included within this Stormwater Management Plan and Appendix are provided in support of applications submitted by Workforce Partners, LLC to the City of Norwalk for permitting purposes related to site and building improvements at 132 Flax Hill Road in Norwalk. The site's storm water management system shall be sized to accommodate runoff from a 25-year design storm and the system shall be designed so that post development peak discharge rates, and stormwater runoff volumes, do not exceed predevelopment quantities. Per City requirements, the storm water management plan has been prepared by a Connecticut-licensed professional engineer. Location The project site is located at 132 Flax Hill Road, Norwalk, Connecticut (site); As shown in Figure 1 the site is located in South Norwalk. The 0.217-acre site is a fully developed urban area. Project Location Figure 1 Project Location – 132 Flax Hill Road, Norwalk, CT. Page 2 Existing Conditions The site has 1 existing residential building, a detached garage structure, a driveway, and walkways. The site’s open space are landscaped grass areas. The purpose of the proposed project is to demolish the existing garage structure and add additional parking and driveway areas. As shown on Appendix A, there are no stormwater features, catch basins, yard drains or storm pipes. All runoff infiltrates or discharges overland to the back of the property and a mostly grass covered area in the front discharges to Flax Hill Road where it enters the City’s storm system. Based upon FEMA’s Flood Insurance Rate Map (FIRM) Map number 09001C0531G for Fairfield County, revised date July 8, 2013, the site is inside Zone X or area of minimal flood hazard (See Appendix C). According to the Soil Survey of Fairfield County, the site’s soil type of the site and where the storm water management infiltration system will be installed is entirely Urban land-Chatfield-Rock outcrop complex, 15 to 45 percent slopes – Hydrologic Soil Group D (see Appendix D). The results of a test hole and percolation test at the site show there is a topsoil layer approximately 3 inches deep, and then brown fine/medium sand, and urban fill to a depth of 5 feet. There are no wetlands or surface water bodies on site. There was no groundwater present during the test hole or percolation test. Groundwater in the area is classified with a GB ground water quality class. The water quality classes are based on the “Water Quality Classification Norwalk, CT” map prepared by CT DEEP dated November 2015 (See Appendix E). Proposed Project The proposed stormwater management system for this project is designed to collect and direct runoff to a subsurface infiltration system. The system will provide infiltration, temporary storage of runoff, reduction of post development peak discharge rates, and meet groundwater recharge volume and pollutant reduction requirements. The system has been designed in accordance with the City of Norwalk Zoning Regulations, Drainage Manual, and the guidelines and procedures set forth in the 2004 Connecticut Storm Quality Manual. The proposed development impervious area is a paved driveway and parking area, and the footprint of the existing residential building and walkways (Appendix B). The proposed stormwater management system includes a catch basin, and the stormwater infiltration chambers system. The proposed drainpipe is sized to convey the peak runoff discharge of the 25-year storm. Details of the size and location of the stormwater management system can be found on the site plan for the project. The results of the percolation test indicate a percolation rate of 1” in 10 minutes. Conservatively for design purposes an exfiltration rate of 6” per hour will be used. The system has also been designed to provide volume storage and infiltration of the first inch of runoff removing at least 80% of the average annual total suspended solids (TSS) load, removing at least 80% of floatable debris, providing peak runoff rate attenuation, and volume attenuation to improve the existing site hydrology, as well as to minimize sediment transport. Page 3 PEAK RUNOFF CALCULATIONS Calculations for the existing and proposed conditions were analyzed using the US Department of Agriculture Natural Resources Conservation Service (NRCS, Formerly known as the Soil Conservation Service or SCS) TR-55 methodology which outlines procedures for calculating storm runoff volume, peak rate of discharge, and hydrographs on small watersheds. To complement the design results, the method utilized to predict the surface water runoff rate in this analysis and the impact of the proposed infiltration/detention system is a computer program entitled Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2020. The Hydrographs computer modeling program utilizes the same methods for computing runoff rates that were originally developed by the US Department of Agriculture NRCS, and they are also utilized in the TR-20 computer modeling program and others. Calculations are included in Appendix F. The runoff curve number is a land sensitive coefficient that dictates the relationship between total rainfall depth and direct storm runoff. The site as stated before is a fully developed urban area with paved parking lots, roofs, and driveways. The land cover types present at the project site are listed below and calculations are included in Appendix F: • Open space (Lawns, parks etc.) – Fair Condition; grass cover 50% to 75%: CN 84 • Open space (Lawns, parks etc.) – Good Condition; grass cover > 75%: CN 80 • Impervious Areas – Paved parking lots, roofs, driveways: CN 98 The time of concentration (Tc) is defined as the time for runoff to travel from the hydraulically most distant point in the watershed to a point of interest. Values of time of concentration were determined for both conditions based on land cover and slope of the flow path using methods outlined in TR-55. Considering how small the site is, a time of concentration of 0.1 hours was used, the minimum Tc allowed in TR-55. For this study, all rainfall depth and rainfall intensity were obtained from the National Oceanic and Atmospheric Administration (NOAA) Weather Service Hydrometeorological Design Studies Center Precipitation Frequency Data Server (PDFS). Enclosed for reference is a copy of the downloaded NOAA Atlas 14 table for rainfall depth and rainfall intensity (See Appendix G). The peak flow and total runoff volume were determined for all frequency storms analyzed including 1- year, 2-year, 5-year, 10-year and 25-yr Storm. As shown on the tables below, the reduction of the peak runoff rate and runoff volume is achieved through the installation of the underground infiltration/detention system which provides temporary storage for runoff. The proposed retention system will be surrounded by 6 inches of stone to provide a stable base and to increase the storage volume. The total runoff volumes, and peak flows are as follows: Page 4 Table 1 Site Runoff Volume (cf) Comparison for 132 Flax Hill Road WS-1 Storm Existing Proposed Proposed Frequency Conditions Conditions Conditions CULTEC % Change Runoff Volume (cf) Runoff Volume (cf) Runoff Volume (cf) 1-Year 738 807 0 -100% 2-Year 971 1046 0 -100% 5-Year 1357 1440 160 -88% 10-Year 1688 1776 330 -80% 25-Year 2146 2239 583 -73% Table 2 Site Peak Discharge (cfs) Comparison for WS-1 Storm Frequency Existing Conditions Proposed Conditions % Change Peak Discharge (cfs) Peak Discharge (cfs) 1-Year 0.238 0 -100% 2-Year 0.311 0 -100% 5-Year 0.429 0.173 -60% 10-Year 0.528 0.421 -20% 25-Year 0.663 0.623 -6% 1-inch storm 0.026 0 -100% The underground infiltration/detention system will be located under the parking lot area on the South- East side of the property, and the proposed system storage volume capacity is 415 cubic feet. The system is a Cultec Recharger 330XLHD (4 chambers) and it is designed to also capture the stormwater runoff generated by the first inch of all rainfall events as shown above. The volume of the infiltration system meets the requirements of the groundwater recharge criteria and the pollutant reduction criteria set on the 2004 CTDEEP Stormwater Quality Manual. Included as Appendix H is a copy of the “Maintenance & Inspection Schedule” which has been prepared for the site to promote good housekeeping measures. Design plans accompany this report to provide additional design detail regarding the underground storage system proposed. The proposed improvements will have no adverse impacts on surrounding properties or city infrastructure. Page 5 APPENDIX A APPENDIX B APPENDIX C National Flood Hazard Layer FIRMette Legend 73°25'42"W 41°6'10"N SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT Without Base Flood Elevation (BFE) Zone A, V, A99 With BFE or DepthZone AE, AO, AH, VE, AR SPECIAL FLOOD HAZARD AREAS Regulatory Floodway 0.2% Annual Chance Flood Hazard, Areas of 1% annual chance flood with average depth less than one foot or with drainage areas of less than one square mileZone X Future Conditions 1% Annual Chance Flood HazardZone X Area with Reduced Flood Risk due to OTHER AREAS OF Levee. See Notes.Zone X FLOOD HAZARD Area with Flood Risk due to LeveeZone D NO SCREEN Area of Minimal Flood Hazard Zone X Effective LOMRs OTHER AREAS Area of Undetermined Flood HazardZone D GENERAL Channel, Culvert, or Storm Sewer STRUCTURES Levee, Dike, or Floodwall 20.2 B Cross Sections with 1% Annual Chance 17.5 Water Surface Elevation 8 Coastal Transect Base Flood Elevation Line (BFE) Limit of Study Jurisdiction Boundary Coastal Transect Baseline OTHER Profile Baseline FEATURES Hydrographic Feature Digital Data Available No Digital Data Available MAP PANELS Unmapped Ü The pin displayed on the map is an approximate point selected by the user and does not represent an authoritative property location.