2010-11-30_APPLICATION CORRESPONDENCE - C2010088 (2)

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Corey Heaps CAM- Colorado, LLC November 30, 2010 Page 16 Resources. If CAM intends to discharge through the pond bottoms, please contact the Water Quality Control Division to determine if a CDPS Permit is required. 53. Sediment Ponds 41, #2, #3, and #5 appear to be incised, based on the details shown on Maps 13 and 14. No embankments are proposed to be constructed for these four ponds, and dewatering will occur as needed by pumping to Reed Wash. Sediment Pond #4, however, is located immediately adjacent to Reed Wash and has been designed with an emergency spillway. Details of Pond #4 are provided on Map 14. The spillway elevation appears to coincide approximately with the existing ground surface. It appears that an embankment is proposed along the outermost edge of the pond (Section I- P), in order to provide sufficient freeboard above the spillway. It is unclear, from this drawing, what is the intended vertical limit (depth below ground surface) of embankment construction. The fine - grained in -situ soils may be unsuitable material to effectively form the lower reaches of the impounding structure. The Division has determined that a detailed design plan for this structure (Sediment Pond #4) is required, as described in Rule 2.05.3(4)(a)(ii)(A) and (B), to ensure compliance with the safety factor requirements of Rule 4.05.6 and Rule 4.059. 54. The Division has a general concern regarding the sediment pond designs and their location. The long ponds immediately adjacent to the railroad track embankment could cause stability problems over time due to saturation of the embankment fill. Does the design and location of these ponds meet the Union Pacific railroad's requirements governing the construction of embankments? 55. Under Rule 2.05.3(4), in the Sedcad designs, CAM is using 1.4 inches, 1.8 inches and 2.2 inches as the design precipitation events for the 10 yr 24 hour event, 25 year 24 hour event and 100 year 24 hour event respectively. However, referring to the NOAA Atlas 2 "Precipitation- Frequency Atlas of the Western United States ", the Division obtained values of 1.6 inches, 2.0 inches and 2.6 inches respectively. It appears that CAM may have used the NOAA May to October precipitation event figures rather than the NOAA annual precipitation event figures. Please justify the precipitation event values used in the Sedcad designs or revise the Sedcad designs accordingly. 56. Under Rule 2.05.3(4), in the Sedcad designs for the west culverts, a curve number of zero is used for the irrigated wetlands area, citing a ground cover of 100% as justification for that curve number. If the wetlands are saturated or are comprised of standing water, a precipitation event could produce runoff, in spite of the vegetative cover. Please describe the nature of the wetlands as it relates to the determination of the curve number. 57. Please add a more detailed description in Section 2.05.3(4)(a)(ii)(C) on permit text 2.05- 18 and in the Probable Hydrologic Consequences Section 2.05.6(3)(b)(iii) starting on