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4.2 Curve Number (CN) <br />Curve numbers reflect the runoff potential of an area (a value of 100 represents an impervious area where <br />100% of the water would run off), and is a function of soil type (a 'C' used in this analysis) and the land ttse. <br />For this analysis, the CN value is (reference [he SEDCAD+ guide): <br />1) For undisturbed portions of [he watershed, overland flow: 63 (for Arid and Semiarid Rangelands, <br />Oak, fair cover). <br />2) For [he pile slopes, overland flow: 91 (no vegetation or cover). <br />4.3 Hydrograph Response Shape and Overland Flow Lengths <br />Hydrograph response shapes reflect the rapidity wi[h which water runs off a watershed, ra[ed in SEDCAD+ <br />az faz[, medium, or slow. A slow response shape would mrtespond to a fores[ed area ur an area with a lot <br />of obstructions, while a fast response shape would correspond to an unvegetated or poorly protected area. <br />These ratings are Cor overland flow. Once overland flow is surpassed, a fazt rating is assigned. <br />Lengths of overland flow (sheet and rill drainage) are direct input parameters in SEDCAD+ for sedimentology <br />analyses, and have an indirett influence on other parameters, such az LUG The maximum permissible lengths <br />of overland flow used within this analysis, with corresponding hydrograph response shape parameters, are <br />presented in the following table. <br />Lengths (in feet) of Overland Flow and Hydrograph Response Shapes (Slow, Medium, or Fast) <br />Vegetative Cover ~ Slopes less than or equal to 25% Slopes over 25% <br />Oak Brush, etc. ; 225 (S) i 175 (M) <br />Bare. Unprotected Soil ~ 125 (M) j 75 (F) <br />4.4 Control Practice Factor <br />The SEDCAD+ Model considers the sediment loss from an area with a given cover and conservation practice, <br />az compared to that of a field in continuous fallow, by introduction of a Control Practice (CP) Faaor. The <br />CP is a function of Canopy Cover and Percent Ground Cover. The CPs used in this analysis were selected <br />from the SEDCAD+ guidelines. <br />A conservation practice of 'Permanent Pas[ure, Rangeland, Idle Land, and Grazed Woodland' was selected <br />as representative of the natural site. Vegetative cover was evaluated as Appreciable Brush. 2 meters tall. 50% <br />Canopy Cover. 60% Ground Cover. The CP value selected was 0.033. <br />For bare, unpro[ected soil, a CP of 0.80 (loose to 12 inches, rough) was used. <br />5. INTERCEPTOR DITCH AND POINT DISCHARGES <br />Design inpu[ parameters include the standard West Elk triangular ditch design of Z (horizontal) [o l (vertictl) <br />side slopes. <br />(Wesi Elk SAE. Lone Pine Gulch Development Wazee Pile, Page 5 of 3) <br />