2009-09-04_PERMIT FILE - C1982056 (4)

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• 10 -Year, 24 -Hour with Pond Full with Vegetated Perimeter Drainage Ditches — The sediment pond is assumed to contain three (3) years of sediment storage and have a permanent pool at the invert of the emergency spillway at the start of the design storm. The perimeter drainage ditches are modeled as vegetated channels. This analysis was completed to show that the freeboard in both temporary perimeter drainage ditches (relative to the maximum depth of water) is at least 0.3 feet, and that discharge from the sediment pond will comply with NPDES permit limits. From the SEDCAD model, TSS in runoff entering the sediment pond is estimated at 2,428 mg /L and the SS inflow concentration is 1.70 mL/L. Outflow sediment concentrations from the sediment pond are 40 mg/L and 0.00 mL /L, respectively. 25 -Year, 24 -Hour Spillway with Pond Full with Erodible Perimeter Drainage Ditches and Erodible Spillway Channel — The sediment pond is initially full to the spillway invert. The perimeter drainage ditches are modeled as erodible channels. The 25 -year, 24 -hour runoff peak is then routed through the spillway to demonstrate that the spillway design is adequate to convey the flow without scour, maintaining at least one (1) foot of freeboard. The SEDCAD results show that the peak water surface elevation for flows over the emergency spillway is 6,734.61 feet, 1.08 feet below the top of dam elevation of 6,735.69 feet. The downstream spillway channel will pass the 25 -year, 24 -hour peak flow of 6.99 cfs with a peak velocity (2.28 fps) less than the limiting design velocity of 5 fps and a maximum flow depth of 0.25 feet with at least 0.3 feet of remaining freeboard. • 25 -Year, 24 -Hour Spillway with Pond Full with Erodible Perimeter Drainage Ditches and Vegetated Spillway Channel — The sediment pond is initially full to the spillway invert. The perimeter drainage ditches are modeled as erodible channels. The 25 -year, 24 -hour runoff peak is then routed through the spillway to demonstrate that the spillway design is adequate to convey the flow without scour, maintaining at least 1.0 feet of freeboard, and that discharge velocity in the spillway channel that does not exceed the maximum permissible velocity. Additionally, the maximum flow depth in the channel must retain a freeboard of at least 0.3 feet. The SEDCAD results show that the peak water surface elevation for flows over the emergency spillway is 6,734.61 feet, 1.08 feet below the top of dam elevation of 6,735.69 feet. The downstream spillway channel will pass the 25 -year, 24 -hour peak flow of 6.99 cfs with a maximum flow depth of 0.74 feet with at least 0.3 feet of remaining freeboard. Summary of SEDCAD Model Results The runoff volume resulting from the 10 -year, 24 -hour event is 0.7822 acre -feet. By subtracting this number from the volume of the full pond (1.246 acre -feet at the spillway invert), we have determined that the pond must not contain more than 0.4638 acre -feet (elevation 6,732.40 feet; pond depth 1.87 feet) to maintain the pond capacity to still contain runoff from the 10 -year, 24 -hour event without spilling. If the initial pond volume contains three (3) years of sediment storage with no pooled water, the 10 -year, 24- hour runoff volume of 0.7822 acre -feet is completely contained in the pond with 1.0 feet of freeboard before overtopping the spillway. If the pond is initially full at the start of the 25 -year, 24 -hour precipitation event, the single spillway can pass the peak flow of 6.99 cfs with a maximum depth of 0.41 feet in the spillway with 1.08 feet of freeboard, and no channel scour (velocities are non - erosive) at the spillway. The downstream spillway channel is also stable at the same peak velocity with a maximum depth of 0.74 feet and at least 0.3 feet of remaining freeboard. With the pond completely full at the start of the runoff event, discharge from the sediment pond during the 10 -year, 24 -hour storm event will meet NPDES limits for allowable sediment concentrations (TSS and SS) regardless of the condition of the . perimeter drainage ditches (earthen erodible channels or vegetated channels). A summary of pertinent pond data is displayed in Table 3. TCC 6MN Intake Shaft Facility 5 Water & Earth Technologies, Inc. Drainage and Sediment Control System Design April 24, 2009