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Peabody Sage Creek Mine — Drainage and Sediment Con Plan <br />area, time of concentration, SCS curve number and the selection of one (1) of three (3) <br />dimensionless unit hydrograph shapes. Calculated runoff is routed to the various structures input <br />into the model. Muskingum routing parameters are used to route channel runoff between structures. <br />Input values are shown in the SEDCAD4 demonstrations presented in the accompanying design <br />demonstrations and are explained in the following sections. <br />2.1.1 Precipitation Distribution <br />A precipitation distribution is input to model the run -off hydrograph. SEDCAD4 allows the user to <br />choose between the Natural Resources Conservation Service (MRCS — formally SCS) type I, type IA, type <br />II, type III, or user defined storm unit mass curves. The type II distribution was input as an estimation of <br />the area storms. <br />2.1.2 Storm Duration/Return Period/Precipitation Amounts <br />A storm duration of 24 hours was used for the design and evaluation of the ponds /sumps, ditches and <br />culverts. The following return periods were utilized in accordance with Rules 4.05.3 and 4.05.9. <br />Precipitation amounts were determined from site specific climatological records. <br />• 10 -year, 24 -hour storm event <br />1.7 inches <br />• 25 -year, 24 -hour storm event <br />2.1 inches <br />• • 100 -year, 24 -hour storm event <br />2.5 inches <br />2.1.3 Hydrograph Response Shape <br />A unit hydrograph shape is chosen for each drainage area or sub -area modeled to predict the run -off <br />response. The hydrograph responses available in the SEDCAD4 model are slow, medium, and fast. A <br />slow response corresponds to a forested area or an area with a number of obstructions. A fast response <br />corresponds to an un- vegetated or poorly protected area. A medium response corresponds to average <br />conditions and was used for areas that were reclaimed and revegetated. <br />The internal convolution interval is 0.05 hours, and values are saved at the user - specified interval of 0.1 <br />hours or greater. A convolution interval of 0.1 was specified for this modeling. It should be noted that a <br />time of concentration less than 0.125 hours bypasses the unit hydrograph technique and instantaneous run- <br />off is assumed. As explained in the SEDCAD4 Design Manual and User's Guide (Warner et al, 1998), the <br />time of concentration restriction of 0.125 hours is somewhat mandated due to a combination of array size <br />restrictions, minimum internal convolution interval, and the user specified time increment for saving <br />convoluted values. <br />2.1.4 Watershed Area <br />The watershed areas were determined by direct measurement from Map 2.05.3 M4, Drainage and <br />Sediment Control Plan (operations) or Map 2.05.4 M1 Postmine Topography (postmine). <br />2.05.3 -E2.6 <br />