5901 23RD DR W SEAWAY CORP 3C 2025-07-29

I Stormwater Management Report Everett, WA I 100-Year Flood/Overflow Condition The stormwater system for this project has been designed to address all storm events in accordance with design criteria described previously. In the event of a larger storm, the primary system may fail. In that event the following series of overflows would convey the stormwater to the outfall: 1 1. Primary Overflow — If the control structure orifice and wier fail or become plugged, the water would overflow into the control structure through the jailhouse window in the manhole, bypassing the orifice and wier and discharging directly to the outfall pipe. 2. Secondary Emergency Overflow — This is a 48" that bypasses the control structure to discharge directly to the outfall pipe. The emergency overflow elevation is set at the water surface elevation achieved when the undetained 100-year storm is flowing into the primary overflow. Water Surface Elevation Calculations Design 100-year water surface elevation = 420.60' Post Development 100-year, undetained flow = 5.18 cfs Per Figure 1-5.16 Riser Inflow Curves (see Miscellaneous Forms and Worksheets) 0.50 feet of head is required for 5.18 cfs into an 18-inch riser. Therefore the 100-year overflow elevation = 421.10' This provides 1.1' of freeboard above the primary overflow water surface elevation to the pond berm. The primaryoverflow is through the jailhouse window in the control structure manhole. g J This has been modeled as a rectangular weir with a maximum depth of 6 inches. Per the calculations below, the jailhouse window needs to be 52 inches wide. I I I I I PACLAND Job# 50108001 Page 23