5.3. & 5.4. PCSR 06-09-2009 - Laserfiche WebLink

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Routing the 25-year, 24-hour storm event through the sedimentation basins provides the • following information. Sedimentation basins design incorporates sediment storage equal to 0.5 inch from the final landfill cover. Peak Peak Peak Top Initial Peak Storage From Pond Inflow (cfs) Outflow (cfs) Elevation Elevation Storage (ac- Storm (ac-ft) (ft) (ft) ft) P-1 171 7.4 995.8 998 2.5 12.9 P-2 161 6.1 1013.5 1020 3.0 8.4 The calculations above demonstrate that the sedimentation basins manage the runoff from a 25 year 24 hour event with over 2-feet of freeboard in P-1 and over 6-foot of freeboard in P-2 therefore; the design of the sedimentation basins is acceptable. Routing the 100 yr-24 hr storm event through the storm water system results in the following: Peak Peak Peak Top Initial Peak Storage From Pond Inflow (cfs) Outflow (cfs) Elevation Elevation Storage (ac- Storm (ac-ft) ft) (ft) ft) P-1 213 9.4* 997.7 998 2.5 16.8 P-2 227 7.0 1014.8 1020 3.0 13.2 * 1.3 cfs is routed through an emergency overflow to the southwest wetland (0.40 ac-ft) The emergency overflow for Sedimentation Basin P-1 is utilized during the 100 year event, which is a standard design characteristic for sedimentation basins. The emergency overflow is not utilized during the 50 year event. Discharge from P-1 to the wetland west of the site has been designed to minimize impacts by limiting bounce in the wetland, restricting flow rates, and treating stormwater prior to discharge in accordance with Section D of Appendix A of the General Permit for Construction Activity. This wetland receiving discharge from the site currently has two existing channel outlets, both of which eventually drain into Rice Lake. The north channel outlet, currently located on property • Ia3230\32300g001\Reports\Application\Drainage Calculations -Surface Water Management.doc