2008-12-04_PERMIT FILE - C1994082 (32)

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applicable rules and regulations. The number of storage years (Y) was increased in the sedimentology sub-routine until the effluent settleable solids concentration in Pond 10 discharge exceeded the water quality criterion (0.5 milliliters per liter [ml/1]), or the model indicated there was less than one (1) foot of storage room in the pond (the elevation difference between calculated top o£ sediment storage and principal spillway invert elevation). The storage years (Y) value was then set at the year proceeding the year the settleable solids concentration exceeded the effluent limitation or foz the last year the model indicated there was sufficient storage for the design storm sediment yield. The SEDCAD4 model results present the maximum sediment storage while retaining capacity to store the design storm runoff and maintain compliance with applicable rules and regulations. Storage Capacity. Results of the TR-30 SEDCAD9 demonstration show the storm runoff volume from the 10-year, 24-hour event to be 2.17 acre-feet (ac-ft). There will be no discharge through the emergency spillway during the 10-year, 29-hour event. In addition, there will be adequate freeboard between the water surface elevation and top of embankment for both the 25-year and 100- year, 24-hour events. The Following summarizes storage capacity for the TA- 30 10-year, 29-hour event SEDCAD9 demonstration. Total pond storage capacity (at dewatering orifice elev Principal spillway invert elevation (ft msl) _ Storm predicted effluent peak settleable concentration Predicted annual sediment loading = Predicted sediment storage = Predicted maximum sediment storage elevation (ft msl) _ Number of storage years (Y) _ 9.37 ac-ft 6,769.0 ft 0.0 ml/1 0.09 ac-ft 7.2 ac-ft 6,783.1 ft 80 years The 25-year and 100-year, 29-hour storm event SEDCAD demonstrations present conservative top of sediment elevations. Because these demonstrations assume the principal spillway is clogged, the sediment storage elevation is over estimated. While the calculated sediment storage elevations for the 25-year and 100-year events are above the principal spillway invert elevation (6-inch de-watering orifice), the respective sediment storage elevations are less than the top of the 36-inch CMP riser portion of the principal spillway. Spillways. In accordance with Rule 4.05.9(2), Pond 010 was designed and constructed with both a principal and emergency spillway. Exhibit 13-9A shows spillway locations and specifications. The principal spillway consists of a 6-inch diameter dewatering orifice cut into a 36-inch CMP riser which drops into the 29-inch CMP barrel. This spillway will keep the pond level at the 6,789.0 elevation. The SEDCAD9 demonstration in Addendum 13-3A-A shows, in accordance with Rule 9.05.9(2)(b), there is no discharge through the emergency spillway during the 10-year, 24-hour event. TR-35 13-3A.4 Revised OB/D7