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Response to DRMS Adequacy Re,.Tiew (2) — Cotter SM -18 Mine Reclamation Plan Amendment <br />tables from the SCS TR -55 Manual (Urban Hydrology for Small <br />Watersheds), the National Engineering Handbook (Section 4, Hydrology), <br />V.T. Chow (Open Channel Hydraulics), and the Mesa County Stormwater <br />Management Manual which includes tables from many of these sources. <br />Copies of the tables referred to here are included with this response in <br />Attachment 5. In addition, soil descriptions contained within the USDA Soils <br />Report section of our overall document (Exhibit I) contain the following for <br />Type 23 (Bodot, dry -Ustic Torriorthents complex): "Drainage Class: Well <br />drained" and "Capacity of the most limiting layer to transmit water (Ksat): <br />Moderately low to moderately high." In summary, after careful <br />reevaluation of the possible CN values for this site, we believe the original <br />values selected in our drainage report are accurate and applicable based on <br />the aforementioned subtementc and tables included in the e_� sures- <br />3. Curve number data. Curve number data is only provided for "OFF 10" and "OFF 20" <br />(pages ESWMP -1 4 and 15). Please provide similar information for onsite basins 10, 20, <br />30, and 40. <br />Offsite CN values were shown in the original report because they were <br />composite values derived from multiple surface types. The onsite basins (10, <br />20, 30, and 40) are generally more homogenous and typically consist of waste <br />rock with areas of undisturbed sandy material. The value of 75 is derived <br />from interpolation of the enclosed Tables (Attachment 5). No composite <br />calculation was performed. It is Cotter's opinion that the originally selected <br />value of 75 is conservatively high. Mined waste rock comes from strata <br />consisting of very high percentages of sandstone and sandy material. It has <br />been broken up by the mining process and appears to be a fairly free - <br />draining material even when stockpiled. Selection of CN is rarely based <br />strictly on any one specific category or classification, but is more typically <br />based on a comparison of several categories of material with similar traits <br />such as those found within the tables of Attachment 5. Cotter believes that <br />the original value of 75 is accurate and applicable. <br />4 <br />