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SUMMARY <br />In summary, the tailing impoundment currently has the capacity to fully contain the <br />runoff resulting from the PMP event and can safely store this runoff for an extended period. <br />The design storm used in this analysis was the result of a study performed by AWA that <br />determined that a 5.6 -inch storm in three hours would be appropriate for the Henderson Mill <br />Site. The configurations of the facilities at the mill are such that the flood runoff from this <br />storm must be temporarily detained, primarily in the large tailings impoundment. Upon <br />abandonment of the decant/spillway system, the surcharge storage volume available in the <br />tailing impoundment and East Branch Reservoir must be maintained at a minimum of 3,582 <br />acre - feet This volume corresponds to the calculated PMF inflow to the pond with <br />allowances for inflows from the failure of Ute Creek Reservoir, inflows from baseflow, and <br />infiltration into the pond tailing sands. It is important to note that any excess capacity in East <br />Branch Reservoir below the level of the emergency spillway would reduce the flood volume <br />that would flow into the tailing pond, up to a maximum of 676 acre -feet, which is the total flood <br />inflow to that reservoir. <br />We recommend that the available storage capacity in the tailing pond also include an <br />allowance for six inches of residual freeboard. Six inches of residual freeboard, with an <br />average beach slope of 0.7 to 1.0 percent, would leave about 50 to 70 feet of crest width at <br />the maximum water level. The dam crest elevation, which is continuously increasing as <br />tailing is deposited, should be the minimum elevation along the crest of the dam, as <br />determined by field survey. <br />With the abandonment of the existing decant/spillway system, it will be necessary to <br />pump the floodwaters from the impoundment after the storm has passed. This would be <br />accomplished using the proposed barge pumping system, along with "on- site" and imported <br />portable pumps. Depending on the capacity of imported portable pumps, dewatering of the <br />system would take several weeks. Approximately 5,000 gpm of imported pumping capacity <br />would be required to dewater the pond in about 6 weeks. The pond could be dewatered with <br />only the pump barge system and on -site pumps, (i.e. without any imported pumps) in about <br />18 <br />