2013-05-01_REVISION - M1978116 (9)

Home Browse Search

Response to DRMS Adequacy Review (2) — Cotter SM -18 Mine Reclamation Plan Amendment to adjust channel depths. Calculated velocities were all less than 5 feet per second indicating no need to further armor channel sides for erosion protection. Revised calculations and details are included in Attachment 6 along with site photographs of the existing channels in several locations indicating present conditions. 5. Page ESWMP -6 and 7, sections 7.4 and 7.5 and Retention Ponds Drainage Design Plans (Sheet I and 5 of 5). The 100 year, 24 hour runoff volume criteria used for sizing storage in the ponds is acceptable. However, spillways are necessary to pass runofffrom successive storms as there is no way presented in the Retention Pond design plan to drain the ponds via gravity. As such, the emergency spillways for each of the ponds need 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 all three spillways have the capacity and armoring revetment 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) District 63 water commissioner (Tom Brigham) regarding the status of the Dolores River appropriations. DWR's requirement to release retained stonmwater - within 72 hours is seasonal and is subject to change.) 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 spillways capable of passing the 100 year runoff. The solid bottom of the spillway channel (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, allowing greater volumes to drain by overflow, or by filtering through the rock but retaining the 10 year event). Analysis for a broad - crested weir of the dimensions specified was used to verify the capacity. The weir is thus 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 provided by the NOAA Atlas for the area. A 4 -inch drain has been shown in the pond bottom to provide complete drawdown of the pond in accordance with the suggested release and to avoid conflict with possible Colorado Division of Water Resources District 63 appropriations. These drawing revisions are provided in Attachment 7 (sheets 6, 7, and 8) with weir calculations shown in M