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<br />Calculation Methods <br />Sediment ponds were sized using the following method: <br />1, Drainage area was estimated by planimeter from topographic maps. <br />2. The soil type was taken from Exhibit 23 and the hydrologic soil group <br />selected from Table A or B, (Exhibit 23 is located in Tab Section 6) <br />3. The ground cover was conservatively estimated by visual observation. <br />4. The curve number was determined from figure D. All areas were from the <br />sage-grass complex. <br />5. Precipitation for the appropriate event was selected from Figure S-7. <br />6. Direct runoff was determined from the appropriate column in Table S-3. <br />7. Runoff volume was then calculated by: <br />Volume of Runoff = (Inches of Runoff)(Acres) <br />12 inches/foot <br />S. Sediment volume was calculated by the Universal Sail Loss Equation as <br />follows: <br />A = RK(LS)CP <br />• Where A = Soil loss in tons/acre/year <br />R = Rainfall factor (Figure 8) <br />K = Soil erodibil:ity factor (Figure C) <br />LS = Length-slope factor (Table 6) <br />C = crop management factor (Table H) <br />P = Erosion control practice factor <br />(this factor was not accounted for so P=i in ail cases) <br />The Universal Soil Loss Equation does not account for deposition of sedi- <br />ment across the watershed slope, therefore, the design should be conser- <br />vative. <br />9. Peak discharge was determined using the methods in Appendix D and Appendix <br />E from "Methods for Determining Peak Flows in Colorado" as follows: <br />Using the equation S =0.25Z(LC25 + LC50 = LC75) <br />DA <br />Where 5 = Slope in ft/ft <br />Z = Height from high point to low point <br />DA = Drainage area in square feet <br />LC25, LC50, LC75 = The length of three contours located <br />at 25, 50, and 75 percent of the <br />.watershed height (Z). <br />• Using the hydraulic length of the watershed, an equivalent drainage <br />area was selected from figure E-l. <br />I^JF7 - 1 <br />(PJew 10/81) <br />