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Mr. Richard Atkinson May 14, 1981 Page Two by use of the formula given in Haan and Barfield: D-DA DV DC DP where: D =delivery factor DA =area factor DV =vegetative factor DC = channelization factor DP = pit factor • For D we had used factors from 0.27 to 0.30 taken from figure 5.23 on page 223, Haanp'and Barrield; Dv had been computed of between 0.12 to 0.13, but used in the equation ah 0.60 due to stated limitation that the product of DpD should be at least equal to the lower of D or D for normal vegetation. Va" fues of 1.0 had been used for factors DC and DP in the formula. In review of the literature, discussioru with others working with sedimentology in Colorado and with Mr. Domigue we conclude that in light of the limited know- ledge in Western wvtersheds that it is appropriate and conservative to use values less than 1 for only the area factor and to use 1 for the other 3 factors in the Haan and Barfield formula, With this assumption the recomputed sediment delivery ratios compared with the previous sediment delivery ratios for the mining area detention ponds are: Sediment Delivery Ratio Detention Pond Previous Calculation Revised Calwlation Taylor Gulch Tributary 0.17 0.26 Streeter Gulch 0.16 0.25 Gulch A 0.18 0.29 The effect of the revised sediment delivery ratios on the calculated sediment yield and pond designs is discussed at the conclusion of the response to USLE CP Factors. 3. USLE CP Factcrs: The Adequacy Review questioned the selection of C factor for straw mu c ing from USDA Technical Note 50. The selected value from this publication was, C = 0.02 for 4 tons of straw mulching per acre. The soil loss was then computed using this value of C at 3.17 tons per aQe per year. Next the total sediment yield for 3 years was computed for each of the sedimentation ponds by converting tans of sediment to acre feet of sediment and multiplying by the drainage area c•f each pond and by 3 years. C~ J