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dlay 7999 Cypnrt TWOrhnd/t'PondD EroLmnon 12 <br />2.1 Precipitation Distribution <br />A preripuauon distribution is input to model the run-oft hydrograph. SEDCAD 4 allows the user to <br />choose between the Natural Resources Consen~adon Service (MRCS -formally SCS) Tppe I, T}'pe IA, <br />Type II, T}'pe III, or user defined storm unit mass cun'es. The Type II distribution was input as an <br />estimation of the TCC area storms. (Barfield e! al, 7981.) <br />2.2 Storm Duration <br />A storm duration of 24 hours was used for the design and eaaluation of Pond D. <br />2.3 Return Period/Precipitation <br />A precipitation amount is required for the appropriate return period. The following renrrn periods and <br />precipitation events were used for the TCC Ivline. These values have been used in previous analyses <br />performed by TCC for the mine site. <br />l0-year, 24-hour storm event 1.8 in. <br />2i-year, 24-hour storm event 2.I in. <br />2.4 Hydrograph Response Shape <br />A unit hydrograph shape is chosen for each drainage area or sub-area modeled to predict dye run-o EE <br />response. The hydrograph responses available in the SEDCAD 4 model aze slow, medmm, and fast. <br />A slow response corresponds to a forested area or an area with a number of obstmcuons. A East <br />• response corresponds to an tin-vegetated or poorly proreaed area. A medium response corresponds to <br />average conditions and was used Eor the TCC site. <br />The internal com-olu[ion mcrement is 0.05 hours, and values are saved at the user-specified inten-al of <br />0.1 hours or greater A convolution increment of 0.1 was specified for the subject site. It should be <br />noted chat a time of concentration less than 0.125 hours bypasses the unit hydrograph technique and <br />ynstantaneous run-off is assumed. <br />2.5 Drainage Basin Area <br />The drainage basin areas were determined b}' measurmg directly from scaled drawings. The watersheds <br />were located based on area topography and separa[ed by conditions representing various stages of <br />disturbance. Subu•atersheds were segregated inro areas representing reclaimed conditions, pre-law <br />reclaimed, refuse pile, mining disturbed, facilities dysnubed, and Pond D. (Refer to SEDCAD 4 <br />printouts.) Time of concentration and subwatershed routing, as necessan', was estimated for each <br />subwatershed. <br />2.6 Time of Concentration, T~ <br />The rime of concentration was calculated using the NRCS upland method (NAGS, 1986). The <br />hvdrauGc lengths, drainage heights and associated slopes were determined from scaled drawings. The <br />values calculated for each stmcture are shown on a subwatershed basis in the SEDCAD ~ printouts. <br />Owcrland flow conditions were assumed for a maximum of 400 feet. <br />l J <br />~lunegomen' 1C'arson \Lmng Group' 1Gi ~. Cnion 6uulecarJ, Sui[e d1U' Lake~cooJ, Colorado 8022d' (3G3) 7G3-SI3U <br />D-6 <br />