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Technical Revision 90-06 <br />Sediment Control Plan <br />• Permanent Impoundments <br />In Technical Revision 87-3, CYCC requested that CMLRD approve the Middle Creek (M3- <br />N) and Mine 3 (M3-S) detention basins as permanent structures. These two ponds were <br />requested as permanent structures in the original permit application. In CMLRD's findings <br />of compliance, they identified several deficiencies and recommended that CYCC submit <br />a technical revision at a later time addressing these concerns. This was again mentioned <br />in your letter of April 4, 1985. Therefore, this section of TR 87-3 has been arranged to <br />address the deficiencies identified in the Findings Document. Also included is Appendix <br />A, as verification of CYCC's water rights. <br />1. The ponds must have an expected useful life of at least 25 years as stated in SCS <br />Public Standard 378. Based upon USLE yields calculated by the applicant the expected <br />life would be less than 25 years. The USLE sediment yields were calculated using a <br />cropping factor after mining of 0.042. This number is based on a 60% ground cover. <br />• That number cannot be justified in view of the fact that the reference areas had an <br />average cover of around 30°k. Also, a weight conversion factor of 90 pounds/cubic foot <br />was used compared to 62.4 pounds/cubic foot in the applicant's MUSCLE calculations. <br />RESPONSE: <br />The revised 25 year sediment year calculations found in Table TR-86-3-1 show that <br />these ponds do have an expected useful life of at least 25 years. All the factors for these <br />calculations, except the cropping factor, are taken from the previously approved pond <br />design. As can be seen in Table TR-86-3-1, the pond capacity minus the 25 year <br />sediment yield still leaves a storage volume of .08 and 16.46 acre feet for ponds M3-N <br />and M3-S. These calculations assume that these ponds will be cleaned until bond release <br />of the reclaimed tributary areas at which time cover will equal that on the reference areas. <br />Justification for use of a cropping factor of 0.10 at the time of bond release is based on <br />40% ground cover. This cover value is a very conservative estimate based on the <br />• average of three years of data from two reference areas with the poor year averaged in <br />again (Table TR-86-3-2). These two reference areas are similar to more than 90% of the <br />pre-mine vegetation that has been disturbed. The low cover values in 1980 which are <br />TR 90-06 Exhibit 3 - 32 Revised 11/16(90 <br />