2012-09-10_REVISION - M1982090 (5)

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Mr. Oswald/M -1982- 090 /TR -04 September 10, 2012 Page 3 of 6 feet per second at flow rates of 1,000 to 3,000 gallons per minute (gpm) respectively. Although the relatively low flow velocity at the pipe outfall doesn't warrant energy dissipation, CBCI will armor the segment of the pond embankment that receives the discharged flows with geotextile liner and clasts >2 feet wide. This armament will ensure the long term stability of the embankment as it is constantly inundated by the discharge. Figure 4 shows how the discharge pipe outlet will be sloped and elevated with respect to the pond. DRMS: Does the length of time water is detained in the sediment pond depend on the permeability of the earthen material lining the pond structure, or on the CMP outfall pipe itself? (Or does the residence time depend on both of these factors ?) CBCI: The calculated detention time included in the TR submittal assumes that there is no loss to percolation through the earthen material, and that the flow rate, volume capacity of the pond, and the elevation of the outfall of the pond are the only factors in determining detention time. Due to the fact that the pond was never equipped with a synthetic or clay liner there is a possibility of loss to percolation. The potential loss to percolation cannot be calculated at this time because the hydraulic conductivity of the earthen material and limestone liner is not known. However, previous operators have stated that the inflow rates and outflow rates of the pond during previous operations were identical, which indicates that loss to percolation, if any, was not measureable. Also, based on the historic discharge permit files, there is no indication from regulators overseeing the discharge that there was a change in flow rates when flows were measured at the portal (prior to pond construction) or when flows were measured at the pond outfall. DRMS: If the volume of detention time of the water in the sediment pond depends on the outfall pipe, please provide design information about the planned elevation of the outfall pipe and the elevation of the water surface at pond capacity. A cross - section diagram will be helpful here also. CBCI: Until excavation of the pond is complete, CBCI will not know the exact slope and depth of the historic sedimentation pond that is proposed to be replicated. Based on the historic discharge permit files and statements made by previous operators, CBCI is anticipating that the capacity of the pond will be the equivalent volume of 12 hours of storage at a flow rate of 1,000 gpm (assuming no loss to percolation). Based on volumetric modeling (Figures 2, 3, and 4), the appropriate elevation to meet the design capacity (720,000 gallons for 12 hour detention time @ 1,000 gpm) is 9,748 feet. The outfall pipe (36" CMP) will be elevated so that the bottom of the pipe will be at 9,748 feet. To ensure controlled overflow in the event that the outfall pipe is blocked, a 6" thick concrete cap, with wings sloping down from the up- gradient earthen dam, will be constructed above the outfall pipe to protect the integrity of the pipe and earthen dam in the event of overflow (see Figure 5). This overflow feature will be elevated 3.5 feet above (elev. 9,515.5 ft.) the designed capacity level, and will be the lowest point of the pond along the earthen dam (other than the underlying discharge pipe). The modeled contours in Figure 2 and the profile views in Figures 3 and 4 provide a conceptual view