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-16- Mr. Richard Atkinson May 14, 1981 Pcce Two x by use of the formula given in Haan and Earfield: D = DADVDCDP where: D =delivery factor DA =area factor DV =vegetative factor DC = . chanrelization factor DP = pit factor For D we had used factcrs from 0.27 to 0.30 taken from figure 5.23 on page 223, Haan'Qand E?vrfield; Del had been computed at between 0.12 to 0.13, but used in the equation at 0.60 due to stated limitation that the prcduct"cf DpD should be at least equal to the lo•,ver of D cr D for namcl vegetrticn. Va" fues of 1 .0 hcd besn u<ed for factors DC end DP in the formula. . In revie•d of tine iteratvre, discussions v+ith cth~s wcricing wit sedimento oay ` in Colorado erid with h1r. Domiyue"we conclude ~,,.+ in licit of the limited know- ledge im Western vrotersheds that it is eppropricte cra cnservative to use vale: s less than 1 for only the Dreg factor and to use 1 for the e ner 3 factors in the Haan and Earfield formula. With this assumption the recomputed sediment delivery ratios compered with the previous sediment delivery ratios fa the mining area detention ponds are: Sediment Delivery P,otio Detention Pond Previous Cclculation Revised Calalation 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 calculeted sediment yield and pand designs is discussed at the conclusion of the response to USl_E CP Factors. 3. LISLE CP rectors : The Adequacy Review questioned the selection of C factor for straw mu c ing from USDA Technical Note 50. The selected velue from this publication vros C = 0.02 for 4 tons of straw mulching per age. The soil loss wos then computed using this value of C at 3.17 tons per Dore per year. Next the total sediment yield for 3 years was computed for each of the sedimentation ponds by converting tons of sedimenfi to acre feet of sediment and multiplying by the drainage area of each pond and by 3 years.