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Leadville Mill — Storrnwater Comments Page 2 January 21, 2014 d. 10 -year, 24 -hour runoff volume for the area(s) contributing to the sediment pond(s). 2. The DRMS requires sediment from potentially impacted areas (i.e., from which EPF contact stormwater originates) be contained. The required storage volume is the runoff volume from the 10 -year, 24 -hour storm. The DRMS found no sediment control ponds, nor runoff volume analyses. a. Please consider a stormwater pond designed to store the runoff volume from the 10 -year, 24 -hour storm for all contact stormwater. The response should include the following: i. Stage- storage table to compare with estimated runoff volume. ii. Drawing(s) (to scale) with contours demonstrating pond capacity, spillway location, and spillway erosion protection. iii. Design drawing(s) and specifications showing embankment and spillway designs. b. If the Operator does not wish to design and construct a sediment pond, then the Operator must demonstrate the proposed BMPs (e.g., disturbance area drainage sediment basins 1 — 4) have adequate combined storage volume to contain the contact stormwater runoff volume from the runoff volume from the 10 -year, 24- hour storm. 3. Figure 6 -2: There is a note just north of the TSF indicating "Disturbance Drains to TSF ". The DRMS understands that three feet of freeboard will be maintained in the TSF. Please demonstrate how the runoff volume from this disturbed area is factored into the required freeboard. The submittal should include a runoff volume analysis and a water balance demonstrating the maximum volume of process water to be stored at any one time in the TSF. 4. Page 6, Second Paragraph: The response indicates the upland diversion "will handle a 200 year, 24 -hr rainfall event'. The precipitation depth (2.47) and statements earlier in the response suggest this should be the 100 -year, 24 -hour event. Please clarify which design storm is intended. The DRMS could not find stormwater hydraulic analyses or design drawings, other than Figure 6 -2. A review of Figure 6 -2 indicates three diversion channels (one along the west side of the mill building and TSF, one along the north haul road running from west to east, and one diverting runoff near the TSF emergency spillway (beginning with the bend at approximately elevation 9715); and three contact water drainages (the labeled "berm" just south of the mill building, the silt fence on the west side of the TSF, and the stormwater line of the south side of the TSF). Also, the outfall from the emergency spillway needs to protected to ensure flows exiting the spillway to not erode the TSF embankment either at the overflow channel or along the toe of the east side embankment. In order for the DRMS to fully evaluate the engineering designs for engineered (i.e., constructed) channels and sediment ponds, please provide the following: a. Channels and Spillways: i. Drawing(s) (to scale) showing diversion channel locations, cross - section geometry for construction (minimum design depth included). m:lmin\tcl\rninerals supportlm- 1990 -057 union milling\strmwtrcmnts _mem21jan14.docx