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Peabody Sage Creek Mine — Drainage and Sediment Control Plan <br />2.1. Hydrologic Calculations <br />SEDCAD4 calculates runoff and peak flow via a numerical modeling technique based on <br />user inputs of a design storm event (i.e., rainfall volume and duration, selection of rainfall <br />distribution and convolution increment). Hydrographs are developed on a subwatershed <br />basis with inputs of area, time of concentration, SCS curve number and the selection of one <br />(1) of three (3) dimensionless unit hydrograph shapes. Calculated runoff is routed to the <br />various structures input into the model. Muskingum routing parameters are used to route <br />channel runoff between structures. Input values are shown in the SEDCAD4 demonstrations <br />presented in the accompanying design demonstrations and are explained in the following <br />sections. <br />2.1.1. Precipitation Distribution <br />A precipitation distribution is input to model the run -off hydrograph. SEDCAD4 allows the <br />user to choose between the Natural Resources Conservation Service (NRCS- formally SCS) <br />type I, type IA, type II, Type III, or user defined storm unit mass curves. The type I1 <br />distribution was input as an estimation of 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, <br />ditches and culverts. The following return periods were utilized in accordance with Rules <br />4.05.3 and 4.05.9. Precipitation amounts were determined from site specific climatological <br />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 <br />run -off response. The hydrograph responses available in the SEDCAD4 model are slow, <br />medium, and fast. A slow response corresponds to a forested area or an area with a number <br />of obstructions. A fast response corresponds to an un- vegetated or poorly protected area. A <br />medium response corresponds to average conditions and was used for areas that were <br />reclaimed and revegetated. <br />The internal convolution interval is 0.05 hours, and values are saved at the user - specified <br />interval of 0.1 hours or greater. A convolution interval of 0.1 was specified for this <br />modeling. It should be noted that a time of concentration less than 0.125 hours bypasses the <br />unit hydrograph technique and instantaneous run -off is assumed. As explained in the <br />SEDCAD4 Design Manual and User's Guide (Warner et al., 1998), the time of <br />concentration restriction of 0.125 hours is somewhat mandated due to a combination of array <br />size restrictions, minimum internal convolution interval, and the user specified time <br />increment for saving convoluted values. <br />TRO 1 -10 2.05.3 -E2.6 12/10 <br />■ <br />