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• P = The erosion-control practice factor: is the <br />ratio of soil loss with contouring, strip <br />cropping, or terracing to that with straight row <br />farming up and down slope. <br />The rainfall factor (R) was calculated (Wischmeier, 1974) <br />by using the 2-year, 6-hour rainfall (0.9 inches) far <br />Somerset, Colorado. This calculation yielded a rainfall <br />factor (Rj of 30. The soil erodibility nomograph for <br />farmland and construction sites (Wischmeter, 1971}. The <br />Absarokee loam, which had a percent sand of 37, a percent <br />silt of 30, a percent organic matter of 5.4, a very low <br />permeability and a medium granular structure, was used to <br />find the soil erodibility factor (k). The nomograph <br />yielded a soil erodibility factor of 0.22. The slope- <br />length factor (L) and the slope-gradient factor (S} were <br />determined using Figure 5.15 in Barfield, Warner and Haan <br />• (1981). .Art LS Factor of 2.7 was derived for the 400 ft. <br />slope length and lOX slope. The factors C and P were <br />assumed to be equal to 1.0 to represent no cover and no <br />onsite erasion control. <br />Using the values above i:he LISLE sediment yield is: <br />A = RKLSCP <br />A (30)(.22)(2.7)(1)(1) 17.8 tons/acre/yr. <br />Therefore, sediment yield from the 5.5 acres of disturbed <br />area is 98 tons/yr. or 2063 ft3/yr. Thus an additional <br />4126 ft3 of pond storage will be needed for two years <br />sediment storage. <br />Total pond capacity below the riser exceeds 10,638 ft3 of <br />storage required for sediment yield and runoff from the <br />+ 10 year, 24 hour precipitation event. <br />D-T <br />