2012-02-17_PERMIT FILE - M2011040

Home Browse Search

Specific Comments: 4. Page 21 (EPP), and Figures Exhibit U -1 and Exhibit U -3 - Orepad Area. Paragraph U (6.a.1) indicates any runoff volume from the orepad area exceeding the 10 -year, 24 -hour storm volume will overflow to the decline and into the underground workings. Neither Exhibit U -1 nor U -3 shows a drainage path from the Orepad Pond to the decline. Please provide details for this channel and design calculations demonstrating that it can pass the peak flow resulting from the 100 -year, 24 -hour storm. Response - Design information added to sheet. a. No stage- storage or storage volume capacity information is provided for the Orepad Pond (as is for the South Pond on Exhibit U -2). Please provide this information for the Orepad Pond. Response - Stage storage information has been added 5. Page 23 (EPP), and Figure E.I - Primary Waste Pile. There is a discrepancy between the Primary Waste Pile area shown on Figure E.I (1.130 acres) and that on page 23. section U(6.b.1) as 1.146 acres. Please correct the discrepancy. Response - Acreage has been corrected 6. Page 29 (EPP), and Figures Exhibit U -3 and Exhibit U-4 - North and Central Ponds. The second paragraph indicates a storage capacity of 4,624 and 5,837 cubic feet for the North Detention Pond and the Central Detention Pond, respectively. However, no stage- storage or information is provided for either pond on Exhibits U -3 and Exhibit U-4, respectively (as is for the South Pond on Exhibit U -2). a. Please provide this information for the North Detention Pond on Exhibit U-4. b. Please provide this information for the Central Detention Pond on Exhibit U -3 Response - Stage storage information has been added to the appropriate exhibit sheets. 7. Exhibit U-1 - Overall Plan, calculations and methodology. a. The 10 -year runoff volume appears to utilize a method involving a 10 -year coefficient, C10. Please provide a source for the methodology and rationale for the selected coefficients. Alternatively, use the widely accepted TR -55 methodology used for the peak flows and provide rationale for the curve numbers (CN) selected other than the "Arid Rangeland, Pmyon- Juniper, Poor Condition ". Note: other CNs will be needed to represent the roads, industrial areas, waste dump tops and waste dump out slopes. An estimate of the acreages for each and the selected CN. Response - Source for the original CIO coefficient is the Rational Method which is still widely accepted As the coefficient represents the impermeability of a site's surface, it is a logical extrapolation to presume that for a 24 -hour storm event that applying the coefficient to the total depth of precipitation will provide the total anticipated runoff. Previously DRMS had found this acceptable. In response to your request the total runoff volumes have been recomputed using the SCS formula. CN numbers and rationales are being provided Due to the primarily sandstone nature of the waste material it would be classified as a Class A soil group with a CN significantly lower than that for `Arid Rangeland, Pinyon- Juniper, Poor Condition.' As the plan is to reclaim the dumps as the mining progresses, the topsoiled areas of the dump faces will also have lower CN than for the original site as there will be a deeper base below the topsoil (if nothing else putting it into a Class C classification) and it will be mulched and seeded. Therefore CN numbers have been selected based on the existing conditions with modifications for areas of disturbance.