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-iiiiiriniiiiii <br /> iii <br />MINED LAND RECLAMATION DIVISION <br />Department of Natural Resources <br />1313 Sherman St.. Room 215 <br />Denver. CO 80203 <br />303 866-3567 <br />Fn%. 303 8328106 <br />DATE: April 18, 1990 <br />T0: Gregg R. Squire <br />FROM: John T. Doerfer <br />RE: Review of Hydrologic Adequacy Response, Amendment No. 1, <br />Eagle-Gypsum Mine, File No. M-84-041 <br />~t ~o~~ <br />„~ '~ '.o <br />.:\ ~.~ <br />/B ]6 <br />Roy Fomei <br />Govema <br />Fred R Bama. <br />DrviS~an D~recloi <br />I have reviewed the responses to the Division's hydrologic adequacy review <br />submitted by Jerome Gamba and Associates for the above-referenced amendment <br />received on April 16, 1990. My comments are provided below. <br />Exhibit D - Mine Plan <br />The Division recommends riprap be placed at the culvert discharge points <br />instead of the straw bales as proposed. <br />2. The inclusion of riprap at the exit of the 9-foot culvert and exclusion <br />of the anti-seep collar is acceptable. <br />3. A dam with the 100-year spillway excavated into adjacent bedrock will <br />function adequately for the intended purpose of sediment control. The <br />design does not include a principal spillway but this is acceptable based <br />on the 10-foot maximum height of the dam as proposed. The following <br />points should be addressed: <br />An estimate of sediment storage volume provided for each basin <br />should be provided. As we discussed, the Universal Soil Loss <br />Equation (USLE) can be used as referenced in Technical Note 50, <br />USDA-SC S, Revised February 1984. From our previous experience, <br />approximately three (3) 1D-year, 24-hour storms correspond to one <br />(1) year of annual soil loss as estimated by the USLE. The <br />applicant should commit to clean out the basin from accumulated <br />sediment when one-half (ljt) this sediment volume is reached. In <br />addition, it is recommended the applicant install a marker in each <br />basin indicating this clean out level and report when cleaning was <br />performed and the volume e~ sediment removed in the annual report. <br />The spillway cross-section and depth of flow should be specified at <br />the design flow condition for each basin. Any riprap necessary at <br />the transition from the than;lel excavated in bedrock to the natural <br />channel should be include) i. the design. <br />STATE OF COLORADO <br />