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January 2002 Seneca IIW South Exfanrion * Drainage and Sediment Cantro(P(an 1 Page 3 <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 bypasses [he unit hydrograph technique and <br />instantaneous run-off is assumed. As explained in the SEDCAD User's Manual (Schwab and Warner, <br />1987), the time of concentration restriction of 0.125 hours is somewhat mandated due to a <br />combination of array size restrictions, minimum internal convolution interval, and the user specified <br />time increment for saving convoluted values. <br />2.5 DRAINAGE BASIN AREA <br />The drainage basin areas were determined by digitizing directly from scaled drawings. The watersheds <br />were located based on area topography and separated by conditions representing various stages of <br />disturbance. <br />2.6 TIME OF CONCENTRATION, Tc <br />The time of concentration was calculated using the SCS upland method (Soil Conservation Service, <br />1986). The hydraulic lengths, drainage heights and associated slopes were determined from scaled <br />drawings. The values calculated for each stmctuxe are shown on a subwatershed basis in the SEDCAD <br />printouts. 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 SCS upland method. The hydxauGc <br />lengths, drainage heights and associated slopes were determined from scaled drawings. The values <br />calculated between each structure and/or subwatershed are shown in the SEDCAD 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 run-off for a <br />given area. Curve numbers were assumed based on supplied vegetation and soils information. Where <br />no vegetation or sols da[a was available, average conditions were assumed. Where multiple conditions <br />existed within the watershed, a weighted average curve number was assigned. A summary of the curve <br />numbers used fox this evaluation is presented in Table 1. <br />TABLE 1 <br />SENECA IIW (South Expansion Areal CURVE NUMBER <br />Area <br />Condition <br />Cover T e Hydrologic <br />Condition Hydrologic <br />Soil Grou Curve <br />Number <br />Undisturbed As en/Mountain Shrub Good (>70%) B 30 <br /> Good (>70%) C 41 <br /> Good (>70%1 D 48 <br /> Sa ebrush Good (>70%) C 47 <br /> Good (>70%1 D 55 <br /> Mesic Draina a Fair 130%<x<70%) C 81 <br /> Herbaceous Good 1>70%) D 85 <br /> Herbaceous Good 1>70%1 C 74 <br />Disturbed Pit &Pre-Stripped <br />Spoil <br />Partial) Reclaimed Bare <br />Bare/Rough <br />Bare/rou h C <br />C <br />C 91 <br />86 <br />86 <br /> Haul Road Bare C 91 <br /> Reclaimed Good (>70%) C 71 <br />MIDI-] * 765 S. Union Boulevard, Suite 410 * lokeulaad, Colorado 80228 * (970) 763-5740 <br />LL': Y'P/].ms/.Iowa IIY' :CO.q/pRIJ )/M+Asnl I) SbmA V/.M <br />Pm'iM ~/OSV/ <br />