2012-04-18_REVISION - M1984014 (13)

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Response to DRMS Adequacy Review — Cotter JD -8 Mine Reclamation Plan Amendment 4. Page 7, Section 2.4. The last sentence of the first paragraph discusses passing the remaining storm volume across the preexisting roadway through a culvert. The applicant must: a. show the location of the proposed culvert on Figure 2, and b. Provide analyses and designs to demonstrate the proposed culvert has sufficient capacity to pass the 100 year peak flow. Cotter response: The culvert location is shown in Figure 2 of the revised Drainage Plan and the capacity analysis is provided in Section 3.2, Table 5 and Attachment 1 of the Revised Drainage Design Plan, dated March 25, 2012 in Attachment 4 (see Revised Supporting Documentation for Exhibit U.) 5. Page 7, Section 2.4. The last sentence of the second paragraph suggests the applicant will use silt fence to retain sediment should the pond not be able to be constructed as designed and the pond overtops. In essence the applicant is intending to use silt fence to extend the elevation of the retention pond crest. This is not acceptable. Silt fence is not intended to be used as an embankment and will fail rather quickly if used for this purpose. The applicant must commit to either increasing the area of the pond or building a higher engineered embankment to provide the required storage. Cotter response: The silt fence was not intended for use as an embankment. However, the revised Drainage Design Plan in Attachment 4 includes constructing the pond with a larger area at a limited depth. No discharge or spillover of sediment is anticipated that would require downslope sediment retention. 6. Page 7, Section 2.4 (Routing Capacity). DRMS did not find any discussion related to required spillway capacity or spillway design for the existing drainage conditions. Please provide analyses and designs to demonstrate the retention pond spillways have the capacity to pass the peak flow resulting from the 100 year, 24 -hour design storm. Cotter response: Spillway capacity and design for the existing drainage conditions are addressed in Section 4.3 and Table 10 of the revised Drainage Design Plan, dated March 25, 2012 in Attachment 4 (see Revised Supporting Documentation for Exhibit U). 7. Page 9, Section 3.2 (Routing Capacity). DRMS did not find any discussion related to required spillway capacity or spillway design for the operational and post- mining drainage conditions. Please provide analyses and designs to demonstrate the retention pond spillway has the capacity to pass the unattenuated peak flow resulting from the 100 - year, 24 -hour design storm. Cotter response: Spillway capacity and design for the operational and post mining drainage conditions are addressed in Section 5.3 and Table 15 of the revised Page 18