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lanumy?000 Cyprur Tmentymile`PonAB EuaLiuhon 2 <br />performed by TCC for the mine si[e. <br />• l0-year, 24hour storm event 1.8 in. <br />25-year, 24-hour storm event 2.1 in. <br />100-year, 23-hour storm event 2.6 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-ofE <br />response. The hydrograph responses available m the SEDCAD4 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 tin-vegetated or poorly protected area. A medium response corresponds to average <br />/ conditions and was used for the TCC site. <br />v// The internal convolution increment is 0.05 hours, and values are saved at the user-specified interval of <br />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 b}'passes the unit 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 from scaled drawings. The watersheds <br />/ 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 />stockpile 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 (NRCS, 1986). The <br />hydraulic lengths, drainage heights and assooated slopes were determined from scaled drawings. The <br />values calculated for each structure are shown on a subwatershed basis in the SEDCAD4 printouts. <br />Overland flow conditions were assumed for a maximum of 400 feet. <br />2.7 Muskingum Routing Parameters, K, X <br />/ The Muskingum routing parameters were also calculated using the NRCS upland method. The <br />,/ hydraulic lengths, drainage heights and assooated slopes were determined from scaled drawings. The <br />values calculated between each stntcmre and/or subwatershed are shown in the SEDCAD4 printouts. <br />2.8 Curve Number, CN <br />/ The inn-off curve number is a factor relating the amount of rainfall to the amount of run-off for a <br />// given area. Cun'e numbers were assumed based on supplied vegetation and soils information. \lUhere <br />no vegetation or soils data was available, average conditions were assumed. \Vhere multiple conditions <br />existed within the watershed, a weighted average curve number was assigned. A summary' of the curve <br />numbers used for this' evaluation is presented in Table 1.1, Pond B \X~atershed Curve Numbers. <br />• <br />hlon[gomery \C~a tson Mining Group ' 1G5 S. Union Boulevard, Suiee 4l0 ' LaAewwd, Colorado 80228 " (303) 7G3-51-0D <br />