2012-04-18_REVISION - M1984014 (13)

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Response to DRMS Adequacy Review — Cotter JD -8 Mine Reclamation Plan Amendment Again, the depicted road location has been revised based upon additional site inspection and the associated flows paths confirmed as shown in the revised Figure 1. a. Also note the drainage system is required to pass or convey the runoff from the 100 year, 24 -hour storm. As such, stormwater channels in the permit area must be able to convey the peak flows resulting from the 100 year event not stored as part of the 10 year storage capacity. It may be easier to demonstrate the capacity to divert the entire 100 year peak flow. Cotter response: An analysis of the water tank access road ditch and the following culvert beneath the mine access road has been performed to demonstrate sufficient capacity to convey the 100 -year 24 -hour storm event. No storage is proposed. 2. Page 5, Section 2.2. Design Criteria. The third paragraph discusses design criteria for riprap -lined channels. The reviewer could not find any peak flow analyses /calculations or channel designs in Drainage Design Plan or the EPP. Please provide analyses and designs to demonstrate the drainage ditches /channels have the capacity to pass the peak flow resulting from the 100 year, 24 -hour design storm. Cotter response: Descriptions and design parameter tables identifying peak discharge and storm flow structure construction requirements are now included in Sections 3.3 and 4.3 of the revised Drainage Design Plan in Attachment 4 (see Revised Supporting Documentation for Exhibit U). The calculations demonstrate sufficient capacity and address the need for erosion protection (riprap /gravel mulch). 3. Page 7, Section 2.4 and Table 6. There are some inconsistencies and errors in the table: a. The stormwater pond area in the first paragraph (8,050 ft is inconsistent with the pond area in Table 6 (8,047 ft DRMS acknowledges this is a rounding difference, but requests consistent numbers be used. Cotter response: The text has been corrected so that any rounded values are reported consistently between the text and the tables in the revised Drainage Design Plan. Tables 6 and 7 are now Tables 7 and 8, respectively, in the revised Drainage Design Plan in Attachment 4 (see Revised Supporting Documentation for Exhibit U). b. The volume in cubic feet, in Table 6 is in error when compared to the volume in acre ft. and gallons. It is also identical to the volume in Table 7. Please correct the volume. Cotter response: The volumes have been corrected. Tables 6 and 7 are now Tables 7 and 8, respectively, in the revised Drainage Design Plan in Attachment 4 (see Revised Supporting Documentation for Exhibit U). Page 17