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2002 Seneca IIIG South Expansion * Drainake and Sedimen! Conlrol Pion <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 mn-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 Muskingum'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 ace 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 fox the design and evaluation of the Seneca IIW sedimentation <br />ponds. <br />2.3 RETURN PERIOD/PRECIPITATION <br />A precipitation amount is required fox the appropriate return period. The following return periods and <br />precipitation events were used for the Seneca IIW (South Expansion) mine site: <br />0-year, 24-hour storm event 7.7 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 axe slow, medium, and fast. A <br />slow response corresponds to a forested area or an area with a number of obstmctions. 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 />MW'H * 165 S. Union Boulevard, Suite 410 * Lakewood, Colarodo 80228 * (970) 763-5740 <br />v:~uwLS....,/tr..o nu'r..nlrxm/Am,er,a n-sworn./ue <br />x~e•m Vm ki <br />