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Response to DRMS Adequacy Review (4) — Cotter LP -21 Mine Reclamation Plan Amendment <br />loam in the making). The mine site was visited on multiple occasions by <br />experienced personnel. During these visits, part of the observations included <br />evaluation of the vegetative cover and the general soils types found there for <br />future quantification of runoff. These observations were not only performed <br />for the specific mine site, but also for the probable offsite watersheds thought <br />to affect the area. Broad -based soil evaluations, such as the one included in <br />Exhibit I of the original permit amendment documents, tend to cover very <br />large areas of a regional analysis. Our selection of CN and Manning's "N" <br />coefficients involves looking at a variety and range of possible values found <br />in several reliable and respected resources such as: tables from the SCS TR- <br />55 Manual (Urban hydrology for Small Watersheds), the National <br />Engineering Handbook (Section 4, Hydrology), V.T. Chow (Open Channel <br />Hydraulics), and the Mesa County Stormwater Management Manual which <br />includes tables from many of these sources. Copies of the tables referred to <br />here are included in attachment # 2 of this response. We feel that our <br />original designations of CN values for the areas are reasonable and <br />applicable. <br />b. Please provide some narrative supporting the selection of HSG `B "for the onsite <br />subbasins (ON 30 and PN 40). <br />Waste rock is in place over a majority of the onsite basins 30 and 40. As a <br />result, the onsite basins (30 and 40) are generally more homogenous than the <br />offsite basins and typically consist of waste rock with small areas of <br />undisturbed sandy material. It is our opinion that the originally selected <br />value of 75 is actually high. Mined waste rock comes from strata consisting <br />of very high percentages of sandstone and sandy material. It has been broken <br />up by the mining process and appears to be a fairly free - draining material <br />even when stockpiled. Selection of CN is rarely based strictly on any one <br />specific category or classification, but more typically by a comparison of <br />several categories of material with similar traits. <br />4. Page ESWMP -6 <br />a. Peak flow summary table: no PondPack results are provided for the Qloofor <br />subbasins Offsite 10 and Offsite 20 in the Attachments. Please provide the <br />computer model results for these two subbasins. <br />PondPack results are now provided in the enclosed Attachment #1 for all <br />basins. <br />b. Second paragraph and FlowMaster output pages. A Manning's n = 0.035 is used <br />for the design analysis. However, no rationale is provided for the selected <br />• roughness coefficient, which implies a rough cut in bedrock or rock in the channel. <br />Because channel roughness is seldom uniform, the DRMS requires channels <br />2 <br />