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

I I Table 1 Total Site Discharge Event Q(cfs) 2 yr 0.21 10 yr 1.28 100 yr 3.01 II Table 2 Powder Mill Gulch Discharge Orifice Diameter Area Elevation S2yr Q2yr Sioyr Q10yr Slooyr Qlooyr (in) (sf) (ft) (ft) (cfs) (ft) (cfs) (ft) (cfs) - 1 1.73 0.016324 507 510.56 0.153243 510.75 0.157279 511.08 0.164053 2 8.22 0.368528 510.56 510.56 0 510.75 0.79925 511.08 1.322231. 3 6.89 0.25892 510.75 510.56 0 510.75 0 511.08 0.740044 Total Q: 0.15 0.96 2.23 Qdesign 0.16 0.95 2.23 Table 3 18"Storm Sewer Stub Discharge IL--I Orifice Diameter Area Elevation Styr Q2yr S10yr Qtoyr Stooyr Q10oyr (in) (sf) (ft) (ft) (cfs) (ft) (cfs) (ft) (cfs) II 1 1 0.005454 507 510.56 0.051202 510.75 0.052551 511.08 0.054814 2 4.87 0.129356 510.56 510.56 0 510.75 0.280541 511.08 0.464111 3 4.09 0.091238 510.75 510.56 0 510.75 0 511.08 0.260775 Total Q: 0.05 0.33 0.78 Qdesign 0.05 0.33 0.78 Water Quality Treatment The water quality pond can be changed to conform to the water quality objectives as outlined in section 3-3.5 of the COE Stormwater Management Manual with a simple modification. The design parameters for a wet pond are maintaining>_ 15%of the six month storm(49,581 x 0.15 = 7,437 cf) and a maximum average depth of 2.0 ft in the treatment cell and have a minimum depth of three feet in the open water cell. By increasing the elevation of the treatment cell bottom from 504.5 ft to 505 ft,the average depth in the cell becomes 2.0 ft. The volume of the treatment cell is reduced by 1,143 cf to 10,166 cf,which is greater than the required minimum of 7,437 cf. Finally, by reducing the water quality pond by 1,143 cf from the original proposal,the volume remaining is 64,347 cf,which is still more than adequate to retain the six month storm event under developed conditions. Other Permits The required NPDES construction permit has already been issued by DOE. C:\My Documents\Northshore(97-070)%orthshore supplemental.doc