7800 EVERGREEN WAY HYUNDAI Drainage Report 2025-04-28

Conservatively, the R-tank full-infiltration system has been equipped with an emergency overflow for flows exceeding the targets of the DOE Manual flow control performance standard. The R-tank overflow connects to the existing municipal system downstream in Evergreen Way. Refer to Appendix B for flow control calculations of both systems using MGSFlood. 5. Conveyance The proposed conveyance system for the entire site is designed to convey the 100-year, 24-hour rainfall event without overtopping. The drainage capacity analysis is based on the following assumptions: 1. Peak flow rates are calculated from hydrology model MGSFlood. See Figure 5-1 for screenshot of peak flow results and see Appendix C for full MGSFlood conveyance output.As a conservative, worst-case scenario, the existing undetained, fully impervious roof area is used for calculating peak flow rates. 2. Pipe capacity is calculated using the Manning Equation in StormShed 3G for full flow capacity analysis. As a conservative, worst-case scenario, the smallest, minimum-slope pipe is examined. See Figures 5-1 and 5-2 for comparison showing the capacity of the smallest, minimum-slope pipe exceeds the peak flow generated from the existing undetained, fully impervious roof area. *** Point of Compliance Flow Frequency Data *** Recurrence Interval Computed Using Gringorten Plotting Position Predevelopment Runoff Postdevelopment Runoff Tr(Years) Discharge (cfs) Tr(Years) Discharge (cfs) 2-Year 4.034E-03 2-Year 7.480E-02 5-Year 6.399E-03 5-Year 9.577E-02 10-Year 8.270E-03 10-Year 0.111 25-Year 1.075E-02 25-Year 0.142 50-Year 1.186E-02 50-Year 0.178 100-Year 1.353E-02 100-Year 0.213 200-Year 1.866E-02 200-Year 0.225 500-Year 2.556E-02 500-Year 0.241 ** Record too Short to Compute Peak Discharge for These Recurrence Intervals Figure 5-1: MGSFlood Peak Flow Results LMJ Hyundai—Showroom Remodel 9