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POSTMINE LAND USE <br />The proposed/approved postmine land use for the Seneca. II mine area is primarily for livestock <br />grazing and secondarily as wildlife habitat. The permanent stock ponds will enhance both postmine <br />land uses. <br />POND DIMENSTIONS <br />The dimensions and capacity of permanent stock ponds are shown on the attached stock pond as-built <br />drawings. As shown, all embankments are less than five (5) feet in height as measured from the <br />upstream toe of the dam to the invert of the emergency spillway. The combined slope of all the <br />embankments is less than 5H: I V with no slopes greater than 2H:1 V. All the capacities are less than <br />two (2) acre-feet. All stock ponds have emergency spillways that consist of a single open channel. <br />None of them have a principal spillway or dewatering pipe. <br />SPILLWAY CAPACITIES <br />The stock ponds were analyzed to determine if the spillways for the permanent stock ponds have the <br />capacity to safely pass the peak discharge from the 25-year, 24-hour storm event. In addition, rock <br />riprap requirements were also evaluated. <br />The peak discharge, spillway capacity and rock riprap requirements were determined using <br />SEDCAD4. The peak discharge was determined using the same methodologies and assumptions <br />outlined in the postmine channel SEDCAD demonstrations. A curve number of 71 was used for this <br />evaluation which represents a hydrologic soil group B with a fair herbaceous cover. All hydrology <br />inputs and results are shown on the attached SEDCAD4 computer printouts. <br />To determine spillway capacity, capacity tables were developed based on the existing spillway <br />dimensions using the pond subroutine in the SEDCAD model. The SEDCAD results are attached. <br />Using these tables and the peak flow from the 25-year, 24-hour storm event, the depth of flow and <br />freeboard were determined for each stock pond. The results are shown on the table below. <br />Rock riprap requirements were determined using either the PADER or OSM method found in the <br />channel utility subroutine of the SEDCAD model. The results are summarized on the following table. <br /> <br />MR-61 2 Revised 07/09