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• tL = 0.6tc <br />and tL = L0.8 (S+1 0.7 <br />1,900 Y 'S <br />where: L = hydraulic length of watershed in feet (4,130') <br />tL = lag in hours <br />S = 1,000 -10 (where CN is the retardance factor and is <br />CN equivalent to the runoff curve number) <br />(CN = 78) <br />Y = average watershed land slope in percent = 16.7% <br />tL = (4130')0'8 (2.82+1)0.7 <br />1,900 (16.7)0.5 <br />tL = 0.26 <br />0.26 = 0.6tc <br />• ~ and tc = 0.26 <br />0.6 <br />tc = 0.43 hrs <br />Sediment Volume <br />The sediment volume was estimated using the Universal Soil Loss Equation <br />(USLE). From the USLE, a soil loss rate was calculated, a delivery ratio <br />was applied to obtain the sediment yield, and a sediment density was used <br />to obtain the sediment volume. <br />The equation for the USLE is A = RKLSCP; where: <br />A = soil loss in tons per acre per year; <br />R = a measure of the erosive forces of rainfall and runoff; <br />K = soil erodibility; <br />L = slope length; <br />S = slope gradient; <br />• C = cover and management factor; <br />P = erosion control practice factor. <br />7-6e-7 <br />