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January 2002 Seneca II-W/ South E~antion * Drainage and Sediment Control Plan • Page 2 <br />2.0 HYDROLOGIC CALCULATIONS <br />SEDCAD is a comprehensive, computer based model used to evaluate storm water, erosion, and <br />sediment control management techniques. The SEDCAD model calculates run-off and peak flow via a <br />numerical modeling technique based on user inputs of a design storm event (i.e., rainfall amount and <br />duration, selection of rainfall distribution, and convolution increment). Hydrographs are developed on <br />a subwatershed basis with the input of area, time of concentration, SCS Curve Number, and the <br />selection of 1 of 3 dimensionless double triangle unit hydrograph shapes. Routing of hydrographs is <br />accomplished by Musl~ingum's Method. <br />Inputs to the hydrology component of the SEDCAD computer model include: <br />• Precipitation Distribution <br />• Storm Duration <br />• Return Period/Precipitation <br />• Hydrograph Response Shape <br />• Drainage Basin Area <br />• Time of Concentration <br />• Muskingum Routing Parameters <br />• Curve Number <br />Input values are shown on the SEDCAD printouts and explained in the following sections. <br />2.1 PRECIPITATION DISTRIBUTION <br />A precipitation distribution is input to model the run-off hydrograph. SEDCAD allows the user to <br />choose between the SCS type I, type IA, type II, type III, and user defined storm unit mass curves. <br />The type II distribution was input as an estimation of the area storms. <br />2.2 STORM DURATION <br />A storm duration of 24 hours was used for the design and evaluation of the Seneca II-W sedimentation <br />ponds. <br />2.3 RETURN PERIOD/PRECIPITATION <br />A precipitation amount is required for the appropriate return period. The following return periods and <br />precipitation events were used for the Seneca II-W (South Expansion) mine site: <br />10-year, 24-hour storm event 1.7 in <br />25-year, 24-hour storm event 2.1 in <br />2.4 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 SEDCAD 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 unvegetated or poorly protected area. Medium and fast hydrograph responses were <br />chosen for the subject site. <br />The internal convolution increment is 0.05 hours, and values are saved at the user specified interval of <br />MUrI-I * 165 S. Urrian Borr/evard, Suite 410 * Zvkervoarl, Colorado 80228 * (303J 763-5140 <br />m>irrrs..A.:,.rs'.~.._, un-.c~„n./moor/.in..r~,..r risriR,anl~.d,K <br />ii~.„ai iro?,~a <br />