48 001 STORMWATER REPORT 20240823 Sealed

G. UNDERGROUND DETENSION SYSTEM DESIGN: The underground detention system is designed to temporarily store the increased runoff from proposed conditions of Basin 1 and making sure that total post-development peak discharge is equal to or less than the pre-development peak discharge. Detention system will be constructed using Stormtech SC-310 chambers. Below are the design metrics of the detention system: Chamber Model Units Number of Chambers Number of End Caps Voids in the stone (porosity) Base of Stone Elevation Amount of Stone Above Chambers Amount of Stone Below Chambers Area of system - • SC-310 Imperial 24 12 40 240.70 12 18 541 % ft in in sf Outlet Structure: • • • 2x9 INCH.ORIFICE HOLE @ I.E.:243.36 2x9 INCH.ORIFICE HOLE @ I.E.:242.95 5 DIA.ORIFICE HOLE @ I.E.:242.20 Refer to the attached Exhibit 1 – Stormwater Modeling Report and Sheet 7 – Stormwater management plan for design details of the proposed detention system. Table No. 3 Comparison of Peak Discharge (See Exhibit 1 for Stormwater Modeling Report) Peak Discharge (cfs) 2-Year 10-Year 100-Year Existing Condition 0.637 1.156 2.183 Proposed Condition (Discharge from Detention System + Basin 2 & 3) 0.546 1.063 2.081 H. CONCLUSIONS The proposed underground detention system as described herein will meet the design objectives set forth in Section C above. As indicated in Table No. 3 above, the discharge from the site in proposed conditions is less than discharge in existing condition for each of the 2, 10, and 100-year rainfall events. It is my professional opinion that the proposed BMP improvements to the site can be constructed without negatively impacting the offsite drainage.