Laserfiche WebLink
Response to DRMS Adequacy Review (4) — Cotter LP -21 Mine Reclamation Plan Amendment <br />be evaluated for both stability and capacity, i.e., minimum and maximum <br />expected roughness, as well as minimum and maximum design slopes. For <br />example, an excavated earth channel, after weathering would be expected to have <br />a minimum n = 0.018 (use to evaluate stability or maximum expected velocity); <br />and a maximum n 0.025 (use to evaluate capacity). In addition, the DRMS <br />requires channel freeboard be evaluated: channels shall be designed with a <br />minimum of 0.5 feet of freeboard unless the velocity head (V2 12g) is significant, <br />then the minimum required freeboard is half the velocity head, or v2 14g. <br />The channels have been evaluated for stability and capacity using a range of <br />roughness coefficients and slopes, as requested. Results are provided in the <br />worksheets enclosed in Attachment 3. <br />c. Please provide a rationale for the selected roughness coefficients, and evaluate <br />each designated channellditch design slopes (minimum and maximum) for both <br />capacity and stability. <br />Manning's N values were selected from tables provided in Attachment 2. <br />These values are used in the worksheets shown in Attachment 3. <br />d Please design all the ditches with the appropriate freeboard and provide <br />channel design depths for construction. <br />All ditches are designed to provide at least 0.5' of freeboard, and normal flow <br />depths are shown on the worksheets in Attachment 3 and on the drawings in <br />Attachment 8. <br />5. Page ESWMP -6, second paragraph and Retention Pond 50: Grading and Details (Sheet <br />5of5). <br />a. The 100 year, 24 -hour runoff volume criteria used for sizing storage in the pond <br />is acceptable. However, a spillway is necessary to pass runoff from successive <br />storms as there is no way presented in the Retention Pond design plan to drain the <br />pond via gravity. As such, the emergency spillway for the pond needs to be <br />designed to convey 100 year peak flow, assuming the ponds are full (to the <br />spillway invert elevation) at the onset of the design storm. Please provide analyses <br />and designs to demonstrate the spillway has the capacity to pass the peak flow <br />resulting from the 100 year, 24 -hour design storm. <br />(NOTE — The DRMS checked with the Colorado Division of Water Resources <br />District 60 water commissioner (Aaron Todd) regarding the status of the San <br />Miguel River appropriations. Mr. Todd stated that the San Miguel River is not <br />currently over appropriated and as such, DWR has no current requirement to <br />release retained stormwater within 72 hours. He also indicated this is subject to <br />change.) <br />3 <br />