2013-01-28_REVISION - C2010089 - Laserfiche WebLink

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gates are in the closed position when a major storm occurs. The rationale for this modeling methodology is also explained in Appendix 2.05.3(4) -1 of the New Horizon North permit application package, beginning on page 4. WFC Response: SEDCAD run evaluations for the following four cases, including 10yr -24hr event with 2 - 6" weep holes wide open and weep holes closed, and 25yr -24hr event with 2 - 6" weep holes wide open and weep holes closed were run and the results are included with this application. In both 10yr -24hr and 25yr -24hr precipitation event cases where the weep holes are fully opened, the settleable solids concentration discharge is less than the 10yr -24hr event effluent limit of 0.5m1 /I. All four SEDCAD run cases, including weep holes closed, result in no discharge through the emergency spillway. 2. The as -built designs for pond NHN -001 include a revised Eroded Particle Size Distribution (EPSD), labeled "New Horizon Mine 1 ". This EPSD differs from the previously approved EPSD for NHN -001, and is also dissimilar to any of the EPSDs in the approved New Horizon Mine 1 and 2 designs also labeled "New Horizon Mine 1 "). Please explain the basis for this distribution. If this particle distribution is based on site soil samples, please provide a copy of the laboratory report used for the distribution. WFC Response: The ESPD in the four SEDCAD runs included with this application has been revised to match the previously approved ESPD for Pond NHN -001. 3. Page 5 of Section 2.05.3(4) of the approved permit application package notes that mine water, at a rate of 1 -2 cfm, may be pumped from the pit to Pond 001, 002, or 003. This pit water inflow does not appear to be accounted for in the SEDCAD design, as required by Rule 4.05.6(3)(a). Please either revise the SEDCAD design to include this base flow /permanent pool (Rule 4.05.9(2)), or state that pit pumping will only occur when the sediment pond is at or below the weep hole elevation and that the pond will be dewatered within 24 hours. It should be clear that in no case will pit water pumped into the pond result in the passage of runoff from the 10 -year 24 -hour event through the emergency spillway (Rule 4.05.9(2)(b)). WFC Response: All four SEDCAD run cases included with this submittal were run using a designed null structure #1 with watershed information set to simulate approximately 2 cfs (1.96 cfs) of pit water pumping, with the initial pool elevation at 5,671.5 ft., the gated valve elevation. SEDCAD calculated no discharge through the emergency spillway for all four cases. 4. The placement of the primary spillway in the as -built configuration of pond NHN -001 differs from the design. The spillway has been placed west of the approved location, closer to the inlet of Ditch NHN -001 West. This location may result in short- circuiting of the pond. Please indicate, in accordance with Ride 4.05.6(6), how WFC will ensure that the pond is not short- circuited with Ditch NHN -001 West inflows. WFC Response: The approved Pond NHN -001 principle spillway was located approximately 290 feet from the inlet of Ditch NHN -001 West. The as -built principle spillway was relocated to accommodate alignment with the existing county road culvert, and is approximately 215 feet from the inlet of Ditch NHN -001 West. This distance is closer than the approved design of Pond NHN - 001, but exceeds the distance approved by DRMS in the design for Pond NHN -002, which shows 2