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Response to DRMS General Stormwater Comments — Cotter JD -9 Mine Drainage Design Plan c. Please review Attachment A for additional channel segments identified by the DBMS that are not included in the analyses provided, and submit analyses for these segments. The additional channel segments identified in the review "Attachment A" have been added for analysis. Results of these analyses are included on the worksheets included in Attachment 3 of this response. d. Please note that channels expecting erosive channel velocities will need to be armored with appropriately sized revetment or constructed in non - erodible material, such as bedrock. All engineered channels with velocities exceeding 5 feet per second (fps) are shown to be armored with appropriately sized material. Design velocities can be seen on the worksheets in Attachment 3. Riprap was sized using the Federal Highway Administration HEC -11 methodology (Chapter4: Design Guidelines for Rock Riprap). This analysis and copies of the relevant portions of the FHA guidelines are included in Attachment 4. 4. Page ESWMP -7, section 7.4 paragraph and Retention Pond Drainage Design Plan (Sheet 4 of 7). The 100 year, 24 -hour runoff volume criteria used for sizing storage in the pond is acceptable. However, a spillway is necessary to pass runofffrom successive storms as there is no way presented in the Retention Pond design plan to drain the pond via gravity. As such, the emergency spillway for the pond needs to be designed to convey 100 year peak flow, assuming the ponds are full (to the spillway invert elevation) at the onset of the design storm. Please provide analyses and designs to demonstrate the spillway has the capacity to pass the peak flow resulting from the 100 year, 24 -hour design storm. (NOTE - The DRMS checked with the Colorado Division of Water Resources (DOWR) Distinct 63 -water commissioner (Tom Brigham) regarding the status of the Dolores River appropriations. DWR's requirement to release retained stormwater within 72 hours is seasonal and is subject to changed) The DRMS suggests the Operator consider a low level outlet be, designed into the pond in case a call is put. on the Dolores River, the Operator can comply with the DOWR requirements. We have revised the retention. pond details to incorporate a spillway capable of passing the 100 -year runoff. The bottom of the spillway channel (and bottom of rock) has been set at the elevation of the 10 -year storm storage with rock armoring the outlet channel bottoms and sides (rock is on top of the channel bottom, above the 10 -year elevation). Analysis for a broad - crested weir of the dimensions specified was used to verify the capacity. The weir is capable of passing more than the projected 100 -year event with the pond providing retention of the 10 -year volume. Analysis was completed using the 24 -hour rainfalls