2013-10-30_REVISION - M1977310 (3)

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Whetstone AssociatesS Technical Memorandum professional engineer registered in Colorado is required. The drawings should address the following. a. The expected depth to erosion resistant material on the northwest side of the berm, of which the design is said to take advantage. b. Based on Figures 4 and 5, the width of the existing berm is approximately 10 —1 S feet wide. The second paragraph and Table 17 state the spillway will be 10 feet wide. The last paragraph states the spillway will be constructed on the northwest side of the berm, rather than the middle of the berm; and Figure 13 suggests a "V" shaped spillway. Please provide a plan view, longitudinal and lateral cross- sections on the drawings showing how the spillway will be constructed and protected to prevent head cutting the berm in the event the spillway is utilized during the 100 year, 24 -hour design storm event. The design also needs to provide spillway and spillway channel protection analyses demonstrating the protection is adequately sized. c. There is no way presented in the SWRP design plan to drain the pond via gravity. 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. (NOTE – The DRMS checked with the Colorado Division of Water Resources (DWR) District 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 change.). d. Please address the reclamation /post mining plan for the SWRP. The DBMS strongly encourages breaching the embankment upon closure unless the landowner has a use for the pond (e.g., stockpond) and intends to maintain it. extensive Cotter response: The revised DDP contains six engineering drawings (Plates) that have been signed and stamped by a Professional Engineer registered in the state of Colorado. The drawings address the four issues (a -d) as follows: a. The design for the planned SWRP includes a new spillway located at the existing lightly traveled road leading toward the lysimeter. The design does not rely upon natural erosion - resistant material within the spillway approach or beneath the traveled roadway. Instead, the SWRP capacity is significantly expanded, such that the overflow velocity from the pond is 0.42 ft /sec, which will result in very little scour. Rock protection will be installed on the spillway crest and exit section. The required rock size is D50 = 0.75 inches, based on the 0.42 ft /sec overflow velocity and a suitable factor of safety. b. The planned spillway is a 10 -ft wide rock -lined trapezoidal spillway. The plan view, longitudinal cross section, and perpendicular cross section are provided in the revised DDP. The engineering design drawings have been signed and stamped by a professional engineer registered in the State of Colorado. 4148A.131030 8