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Response to DRMS Adequacy Review(2)—Cotter-9 Mine Reclamation Plan Amendment <br /> d. Please note channels expecting erosive channel velocities...The August 1, 2013 <br /> response is partially adequate. Please review the revised Attachment A for <br /> missing armoring calculations. <br /> All engineered channels with velocities exceeding 5 feet per second (fps) are <br /> shown to be armored with appropriately sized material. Design velocities can be <br /> seen on the worksheets in Attachment 2. Riprap was sized using the Federal <br /> Highway Administration HEC-11 methodology(Chapter 4: Design Guidelines for <br /> Rock Riprap). This analysis and copies of the relevant portions of the FHA <br /> guidelines are included in Attachment 3. <br /> 4. Page ESWMP-7, section 7.4 paragraph and Retention Pond...The August 1, 2013 <br /> response is partially adequate. The following items need to be addressed: <br /> a. There is a discrepancy between the "Worksheetfor Weir Throat Sect 1.0% Trap. <br /> Channel"and the grades depicted on Sheet 4 of 10. The drawing appears to have <br /> a relatively constant -10%slope through the throat section and the chute. Please <br /> explain the 1% throat used in the analysis. <br /> The revised details now show that the throat has a 1%slope and the chute has a <br /> 10%slope. The flow analysis uses these slopes accordingly. <br /> b. The response does not address the "DWR's requirement to release retained <br /> stormwater within 72 hours". The DRMS previously suggested the Operator <br /> consider a low level outlet be designed into the pond in a case a call is put on the <br /> Dolores River, the Operator can comply with the DOWR requirements. Please <br /> provide a response to this comment. <br /> We have revised the retention pond details to incorporate a spillway capable of <br /> passing the 100-year runoff. The bottom of the spillway channel (and bottom of <br /> rock) has been set slightly above the elevation of the 10-year storm storage with <br /> rock armoring the outlet channel bottoms and sides (rock is on top of the channel <br /> bottom, above the 10-year elevation.) Analysis for a broad-crested weir of the <br /> dimensions specified was used to verify the capacity. The weir is capable of <br /> passing more than the projected 100-year event with the pond providing retention <br /> of the 10-year volume. Analysis was completed using the 24-hour rainfalls <br /> provided by the NOAA Atlas for the area. As suggested, a 4" drain is now more <br /> clearly shown in the pond bottom to provide complete drawdown of the pond in <br /> accordance with the suggested release, and to avoid conflict with possible <br /> Colorado Division of Water Resources District 63 appropriations. These revisions <br /> are shown in the enclosed drawings included in Attachment 4. <br /> 3 <br />