2011-04-18_REVISION - M1981185 (15)

Home Browse Search

Response to General Stormwater Comments on March 01, 2011 Adequacy Issue #5 Attachment T-4 p. 1 footnote: The footnote states the contributing drainage area to all road ditches is limited to 30 acres. A comparison of Figures SWMP-1 and SWMP-2 suggests there is more than 30 acres contributing runoff to the existing access road between May Day #2 and the Lamb Portal. Please provide a narrative and a figure if necessary to explain how the 30-acre limit will be achieved everywhere. a. Note that a quick calculation using a 2.7 inch 10-year storm depth, curve number of 65, and Wilkes, et al "Peak Flows in Colorado" yields an estimated 12 cfs from 30 acres with a greater than 16 percent slope. This is a considerable amount of flow for a roadside "V" ditch with 1 H:1 V side slopes at a 10 percent grade (as suggested in Table SWMP-I Access Road Ditch). DRMS anticipates a riprap blanket two feet thick with 12 inch diameter rock will be required to protect this type of ditch under these circumstances and need to be 1.3 feet deep (plus freeboard). Also note that ditches with 1 H:1 V side slopes will require frequent maintenance as they tend to collapse in on themselves with or without flow. The minimum accepted ditch side slopes are typically 211:1 V, with 2.511:1 V preferred. Please address these concerns in your response. b. Ditch riprap should be designed for both stability and capacity (i.e., minimum and maximum expected roughness, respectively), consider the minimum and maximum expected design gradients for each reach, and use methodology appropriate for the design gradient(s). Response #5 Road drainage ditches and associated debris basins have been located to provide hydraulic relief from drainage areas equal to or less than 20 acres. The 10 year storm event of 2.7 inches has been adopted with a runoff curve number of 58 for upland drainage and 77 for the La Plata floodplain and terrace deposits. Wildcat has proposed road ditches with side slopes of 1.1 because of the narrow preexisting roads cut into solid bedrock. Maintenance will be required as noted and Wildcat has committed to maintaining roads and BMPs. Wildcat Mining Corporation preexisting road grades exceed 15% and will require the installation and maintenance of riprap and other BMPs to maintain the hydraulic integrity of all constructed structures. The road foundation is cut into hard competent rock and inside road ditches will be constructed with necessary drop and appropriately sized riprap. In a some areas, riprap in ditches may have to be rebar reinforced. Adequacy Issue #6 Exhibits C-5 and C-8, vegetation and soils, respectively, only provide information within the permit boundary. Run-on from upgradient contributing drainage basins (as delineated in Figure SWMP-2) require soils and vegetation information to estimate storm water peak flows and/or volumes. Please provide soils and vegetation information for all contributing drainage basins. May Day Idaho Mine Complex I I2(d) Permit Application Revised: April 14, 2011