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~. Mr. William T. Davis - 3 - June 7, 1989 The statement on page 5 of Section 2.05.3(1) and (2) that "the sediment pond will be converted to a stock pond when all mining activities cease" should be revised to indicate compliance with the requirements of Rule 4.05.6(9), i.e. when reclamation has occurred, revegetation standards are met, and drainage entering the pond meets applicable federal and state quality standards. If the pond is proposed to be retained permanently, Rule 4.05.6(9) also requires that it meet the requirements of Rules 4.05.9 and 4.05.17, Information demonstrating this compliance also needs to be provided. Ditch designs should be specific for all sites, whether diversion or containment ditches. The designs must indicate peak flow estimates, flow velocity estimates, ditch labels, ditch locations, ditch dimensions, and riprap criteria for areas with erosive flow velocities. The application indicates all ditches will be sized fora 100 year-24 hour peak flow. Rule 4.05.3 only requires sizing for a 2 year-24 hour peak flow. The design discharge is given in the application narrative as 5.62 c.f.s. but this does not agree with any figure provided in Exhibits 5 thru 8. The Division's calculations for a 2 year-24 hour peak flow for Phase 1 and Phase 2 are 13.4 c.f.s. and 8.1 c.f.s respectively. These values are for actual mine phase areas and do not include flat areas below the mine phases which also must be accomodated. The 100 year, 24 hour peak flow calculated for Phase 1 and 2 are 34.6 c.f.s. and 20.9 c.f.s., respectively. Culvert sizing must also be included with the required drainage control plan, as well as provisions to protect any culverts which cross Engleville Gulch from flood flows. The applicant must plan fora containment ditch to be located in the northwest section of the permit area. In designing the pond, the application provides runoff volume numbers under Section 2.05.3(4) that are different from numbers provided by Applied Hydrology Associates. Some of the difference is explained by the fact that the same acreage values were not used, but, even considering this, a large difference exists. The Applied Hydrology Associates figures are reasonable and should be used. - The USLE calculations contained in Section 2.05.3(4) incorrectly use the K, LS, and CP variables. The SCS K value for the local soil is .37 not .26. The LS factor for the site is much higher than the .3 value used; the C factor cannot exceed 1.0 (1.67 is used). In general a combined CP factor of 1.0 is used for disturbed areas. Division sediment yield calculations are higher than the value provided and it is recommended that the 0.1 acre-foot/disturbed acre rule be used. This would allow for three acre-feet of storage.