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• Storm Duration <br />A stotm dutadon of 24 hoots was used to model the watershed for the Somerset, Sanborn Creek, and Elk <br />Creek Mine areas. <br />Remrn Period/Precipitation <br />A precpitation amount is required for the appropriate return period The following precipitation amounts <br />were used for the Somerset, Sanborn Creek, and Elk Creek Kline areas: <br />10.yeaz, 2~hour event 2.0 inches <br />25-yeaz, 24hour event 2.4 inches <br />100-yeaz, 24hour event 2.8 inches <br />The preapitation amounts were obtained from the Somerset and Sanborn Creek Mining and Reclamation <br />Plan. <br />Hvdrog_raph Response Shams <br />A unit hydrograph is chosen for each drainage azea of sub-area model to predict the runoff response. The <br />hydrograph responses available in the SEDCAD model aze slow, mediurq and fast. A slow response <br />coaesponds to a forested azea or ur azea with a number of obstructions. A fast response corresponds ro an <br />unvegetated or poorly protected area. Fast and medium hydrograph responses were chosen for disttrrbed <br />and undisturbed azeas, respectively. <br />The rntemal convolunon increment is 0.05 hours and values aze saved at the user specified interval of 0.1 <br />hours or greater. A convolution increment of 0.1 was specified Fot the Somerset, Sanborn Creel:, and Elk <br />Creek Mine azeas. <br />Drainage Basin Area <br />The drainage azeas were deterxruned by direct measurement from l" = l00', 10' contour maps. The <br />watershed areas are shown on blip 2.O~D76. Watershed areas were determined assurnutg all areas draut to <br />• the ponds. <br />