21 Stormwater Mgmt Report_082025

2.2 Site Soils Site soil information has been obtained from the USDA Natural Resources Conservation Service (NRCS) Web Soil Survey database. The soil types present within the studied watershed are shown in Table 2.2. The soils consist of hydrologic soil group (HSG) Type A, therefore the CN values used in calculations are those associated with Type A soils. TABLE 2.3 - Site Soil Properties 2.3 Soil Symbol Soil Name Characteristics Hydrologic Soil Group Rating DunC Dunellen Sandy Loam 8-15% slopes A DuuB Dunellen – Urban land 0-8% slopes A Curve Numbers For determining the runoff flows, the cover types in combination with the HSG rating must be determined for each drainage area. These are used to determine the runoff curve number (CN) values that are used in the hydraulic calculations. The CN values used in this project are listed in Table 2.3. TABLE 2.4 - CN Values 2.4 Land Cover Description HSG A Open space, Good Condition (grass cover >75%) 39 Impervious Surfaces 98 Time of Concentration The time of concentration (Tc) is calculated in accordance with Chapter 15 of the NRCS National Engineering Handbook and has been calculated separately for the pervious and impervious areas within each drainage area. Where there is a long stretch of sheet flow in the proposed condition, the McCuen-Spiess formula is used to determine the appropriate length, not to exceed 100 feet. McCuen-Spiess sheet flow length calculations are provided in the beginning of Appendix A, and full Tc calculations for each drainage area can be found in the HydroCAD summaries in Appendix A. Pre-Development Site Conditions The studied watershed area is a total of 1.36 acres in size and consists of one unique drainage areas: Existing Drainage Area EDA 1A, which is described in more detail below. In the predevelopment condition, the site is an existing florist shop within a shopping center with associated ▪ Stormwater Management Report for Proposed Retail Store▪ 3 ▪ August 2025 ▪