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?000 CYD/LJ Tmrntrmile * Paul G EnnLm!!ou 2 <br />performed by TCC Eor the mine sne. <br />/ 10-year, 24-hour storm event 7.8 in. <br />~~ / 100-}'ear, 24-hour storm event 2.G 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 ntn-otf <br />response. The hydrograph responses available in the SEDCAD4 model are slow, medium, and East. A <br />slow response corresponds to a forested area or an area wuh a number of obstmctions. A Fast response <br />corresponds to an tin-vegetated or poorly protected area. A medium response corresponds to average <br />conditions and was used for the TCC site. <br />~y. ~ The intemal convolution incremen[ is 0.05 hours, and values are saved at the user-specified interval of <br />tt'' 0.1 hours or greater. A convolution increment of 0.1 was specified for the subject site. It should be <br />noted that a time of concentration less than 0.125 hours bypasses the umt hydrograph technique and <br />instantaneous run-off is assumed. <br />2.5 Drainage Basin Area <br />The drainage basin areas were determined by measuring directly Erom scaled drawings. The watersheds <br />(r were located based on area topography and separated by conditions representing various stages of <br />disturbance. Subwatersheds were segregated into areas representing reclaimed conditions, coal <br />stockpde areas, disturbed areas, and ponds. (Refer to SEDCAD4 printouts.) Time of concentration <br />and subwatershed routing, as necessary, was estimated for each subwatershed. <br />2.6 Time of Concentration, T~ <br />• The time of concentration was calculated using the NRCS upland method (NAGS, 1986). The <br />hydraulic lengths, drainage heights and associated slopes were determined from scaled drawings. The <br />Vr ~ values calculated for each stnumre are shown on a subwatershed basis in the SEDCAD4 printouts. <br />Vr Overland flow conditions were assumed for a mawnum of 400 feet. <br />2.7 Muskingum Routing Parameters, K, X <br />The Muskingum routing parameter were also calculated using the NRCS upland method. The <br />hydraulic lengths, drainage hetghts and associated slopes were determined from scaled drawings. The <br />~~ values calculated between each structure and/or subwatershed are shown in the SEDCAD4 printouts. <br />2.8 Curve Number, CN <br />The run-off curve number is a factor relating the amount of rainfall to the amount of tun-off for a <br />given area. Curve numbers were assumed based on supplied vegetation and soils information. \X/Fterc <br />no vegetation or soils data was available, average conditions were assumed. \~'here multiple conditions <br />existed within the watershed, a weighted average curve number was assigned. A summary of the curve <br />number used for this evaluation is presented in Table l.l, Pond G \~'aterhed Curve Numbers. <br />• <br />teomen' ~~~atson ~linmg Group ' 165 S. Union Boulevard, Suice 410' Lakewood, Colorado 80228 ' (303) 763-5140 <br />