2009-09-25_PERMIT FILE - M2009076 (22)

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• n U • The Peak Discharge spikes for a two hour period, as illustrated below. Using the above data, it has been determined that a perimeter diversion ditch with the following parameters will be sufficiently adequate to safely pass a 100 year/24-hour storm event and limit damage to the Tailings Impoundment: • Diversion Ditch - Rip-rap will be installed for a channel width of 6 feet and depth of a least 2 feet (open trapezoid cross-section) at each end of the impoundment benches, at the intersection with the natural grade, to provide a permanent stormwater control. This will be constructed in phases as the impoundment expands. The existence of a natural diversion channel with similar dimensions will also be acceptable for stormwater diversion as is found near the bottom half of the impoundment area. This natural channel is incorporated into the sedimentation pond. Also during a 100 year/24-hour event, a Peak Discharge into the Sedimentation Collection Pond will be produced in the amount of +/- 20 cfs (9,000 gpm). The rip-rapped discharge weir (12' long) is more than capable of passing this peak flow. • Stormwater/Sedimentation Collection Pond - This structure is designed to collect stormwater runoff from the Tailings Impoundment Affected Area and allow any carried sediment a chance to settle for future removal. Sizing has been done to allow for containment and settling of a 10 year/ 24-hour storm event (2.6" as reported by NOAA Atlas 2, Vol. III) and controlled passage of a 100 year/ 24-hour storm event (3.8" as reported by NOAA Atlas 2, Vol. III). This catchment and settling basin will be constructed down gradient of the disposal cell to intercept and settle sediment from surface water run-off in the event of such an occurrence. Leachate shall be collected and returned by pumping to the impoundment and/or water storage above the mill. Impacts to surface water will be controlled as per the stormwater management plan (Exhibit T) and design details can be found in the Tailings Impoundment Design Drawings (Exhibit E). 100 year/24-hour Storm Event, Peak Discharge WI M55 OAM".w pn Projoct MUMI Gulch MIBSito SMweaa MpQaAaM. Ta•W& NOW) GWM IWYT C'DO wWft aW Sdt pWV MM&IVMWY DOOMD teWerNe Reawae WPraj dWW9 Paps AppN®aMMrpF P-55 TrYd ML ilp9wllmr - Trld i1 TaiY,p; -- ------------------------------------------------------------ ------------ ---- --------------------- -------------------------- -------- 4 n? TI .O( • A 10 year/24-hour event will produce the following Peak Discharge and Runoff at the Sedimentation Collection Pond, most of which (95%) is derived from the 8.085 Impoundment Area: • Peak Discharge = +/- 7.8 cfs (3,500 gal./min.) 17