2011-03-01_REVISION - M1981185 (21)

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May Day & Idaho Mill Complex - General Stormwater Comments Page 2 March 1, 2011 discussed in the SWMP, Attachment T-4. Please indicate in the narrative and/or Figures SWMP-1 and SWMP-2 if and where these BMPs will be used, or remove them if they will not be used. 4. Precipitation amounts: Several Tables (e.g., Table A - Drainage Basin Flow Analysis- Upper Deadwood Gulch Drainage) include references to Technical Paper 40. Please note that TP-40 is not applicable to western Colorado. NOAA Atlas II, Volume 3 or the other listed reference (Peak flows in Colorado by Wilkes, et al) should be used for storm depth estimations. Appropriate references indicate the 10-year, 24-hour storm depth for the site to be approximately 2.7 inches, not the 2.5 inches listed in Table A. 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 1H:1V 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 2H:1 V, with 2.5H: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). 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 stormwater peak flows and/or volumes. Please provide soils and vegetation information for all contributing drainage basins. 7. Please clarify the meaning of, or remove "Sq feet" under the column labeled "Rainfall Amount" and "Acres" under the column labeled "Runoff' as necessary on page 1 of Tables SWMP A, B, C, D, G, an H. 8. Culvert Designs - Tables SWMP D, E and F: Page one of each table references Haested's CulvertMaster software package. However, it appears that Haested's F1owMaster software was used to perform the culvert analyses. A full-flow pipe analysis is not appropriate for culvert design or analyses. This approach fails to account for head loss at the culvert entrance and outlet. It should be noted that a considerable amount of head is required to force a culvert on a relatively steep slope (e.g. 3 % in Tables SWMP D & F) to flow full. Standard engineering practice for allowable head at a culvert mAmin\tc1 \m-1981-185 maydayidaho\stormwatercomments_mem01 mar11.docx