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Response to DRMS Adequacy Review (4) — Cotter LP -21 Mine Reclamation Plan Amendment Rarely is it possible to establish CN values based on a single category. More typically, a number of different categories are compared when soils at a site are not specifically described by the limited listings found in most tables. The subjective process used for onsite basins 30 and 40 is discussed in the response to #3b, above. 10. Page ESWMP -20. The 11- minute time of concentration (TQ for subbasin ON 40 appears high. All the conceivable hydraulic paths in ON 40 have significant slope lengths 4H.• I V or steeper, yet the steepest flow segment is 9 %. The DRMS expects the TC to be on the order of S to 6 minutes. Please re -check the TC calculation for ON 40. Flow comes off Basin 40 from a high point near the middle of the western boundary. It sheet flows east for 300' dropping 27' (s= 9.0 %). It then flows along a retention berm for 615' dropping at 3.4% slope with a velocity of approximately 2 feet per second. This, and the flow path for Onsite Basin 30, is now shown on the revised sheet 2 included in Attachment 8. The Basin 40 flow path gives a tc calculation of slightly under 11 minutes, compared to the original calculated tc of just over 11 minutes. This, combined with the corrected value of area for Onsite 30 of 2.82 acres, gives a slightly increased runoff for the onsite areas. 11. Pages ESWMP -22 — 34, FlowMaster analyses. a. Referencing Comment #4b above, all engineered channels and ditches need to be analyzed for maximum and minimum design slopes and for both stability and capacity. Please provide the necessary analyses. All engineered channels have been analyzed for velocity and capacity. Results are provided in the worksheets enclosed in Attachment 3. b. Please identify which analysis applies to which channel /ditch using a naming convention consistent between the analyses and the drawings. The channels are named on the plan sheets in Attachment 8 and similarly referenced on the worksheets in Attachment 3. c. ESWMP -34. The DRMS assumes the peak flow for this analysis was derived by adding the peak flows from ON 30 and ON 40. ff this is the case, the design value should be 7.6, instead of 7.5 cfs The unit - hydrograph analysis utilized by PondPack overlays multiple hydrographs for a given design point which generally peak at different times resulting in a total which may not be a simple addition of individual peak values.