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e ~ -~, ~, <br />UTAH INTERNATIONAL INC. <br /> <br />BOX 187 - CRAIG- COLORADO 81625 <br />303-824-4407 <br />April 6, 1981 <br />• <br />Mr. Jerry Zimpfer <br />Colorado Mined Land Reclamation Division <br />Room 423, Centennial Building <br />1313 Sherman Street <br />Denver, Colorado 80203 <br />Dear Jerry: <br />I have recently reviewed your memo to Rich Dominque critiquing Trapper <br />Mine's proposed criteria for sediment volume in our sedimentation dams. As you <br />have aptly noted, sediment generation is not always directly in proportion to <br />precipitation. The studies you cite, however, are for undisturbed drainages and <br />the phenomenon of decreasing sediment yield with increasing precipitation is <br />largely explained by the accompanying increase in vegetation (see attachment). <br />The sediment storage criteria we are describing, however, is based on runoff from <br />disturbed and denuded areas- The relationship between sediment yield and preci- <br />pitation would therefore be expected to be directly proportional. Our assump- <br />tions, then, remain valid since we are comparing sediment yield from disturbed <br />lands in Kentucky per Curtis (1974) to expected yields from disturbed lands in <br />Colorado. Your reviev further fails [o account for [he data.described from the. <br />Black Mesa Mine in our estimate of probable sediment yield, nor does it acknow- <br />ledge the calculations using site specific data following the EPA's publication, <br />"Preliminary Guidance for Estimating Erosion on Areas Disturbed by Surface Mining <br />Activities in the Interior Western United Slates". <br />We have provided three separate methods of calculating sediment storage vol- <br />ume for Trapper Mine, each of which corroborates the other. The methods use site <br />specific precipitation data, site specific TSS data, and site specific runoff <br />data. <br />Jerry, I would appreciate [he opportunity to discuss [his with you further <br />if there are still questions regarding [he validity of our estimates. <br />Sincerely, <br />Wayne E. Sowards <br />Assistant [o Mine Manager <br />Trapper Mine <br />n <br />U <br />l.'ES/ js <br />cc: Rich Dominque <br />Carol Pahlke <br />Hike Savage <br />File <br />bcc: R. C. Diederich <br />R. E. Nelson <br />G. M. Stubblefield <br />N. Krishnamur[hi <br />K. W. Watson <br />