2010-05-07_PERMIT FILE - C2009087A (39)

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The sedimentology subroutine calculates the R storm value as a function of the design storm rainfall amount and distribution. Consequently, the 25 -year, 24 -hour storm event "over- estimates" the • "design" sediment storage requirements. To eliminate this artifact a user input sediment storage value, equal to the volume calculated by the 10 -year, 24 -hour storm event, was used as the calculated sediment storage requirement for the 25 -year, 24 -hour event. Storage Capacity: Results of the design SEDCAD demonstration show that the sump configurations allow for adequate retention time to control the design storm runoff volume from the 10 -year, 24- hour storm event, capturing most of the sediment load and minimizing settleable solids concentrations in the discharge. Furthermore, there will be no discharge through either principal spillway as a result of the 10 -year, 24 -hour event (Rule 4.05.9(2)(b)). In addition, there will be adequate freeboard between the water surface elevation and top of embankment during the 25 -year, 24 -hour event in accordance with Rule 4.05.9(7)(d). The following summarizes the storage capacity for the 10 -year, 24 -hour event SEDCAD demonstration (see Addendum 2.05.3- E2.E -1). S12illways: The Coal Pile Sumps were designed with both a principal and emergency spillway in accordance with Rule 4.05.9(2)(b). Map 2.05.3 -E2.E, Coal Stockpile Sumps, shows spillway locations and specifications. The principal spillway for UCSS is a 12 -inch corrugated straight metal pipe; the principal spillway for LCSS is an 18 -inch corrugated straight metal pipe. Both pipes will be fitted with a T- shaped cap; the "T" is on its side, the barrel is the spillway pipe. • For UCSS, the invert elevation of the barrel pipe is at 6,664.0 feet; the bottom of the "T" is at 6,663.2 feet — water will not flow freely through the principal spillway when the water level drops below the 6,663.2 elevation. • For LCSS, the invert elevation of the barrel pipe is at 6,653.0 feet; the bottom of the "T" is at 6,652.2 feet — water will not flow freely through the principal spillway when the water level 2.05.3 -E2.E4 12/09 UCSS LCSS Total pond storage capacity (at invert of principal): 2.3 ac -ft 2.5 ac -ft Predicted 10 -year, 24 -hour event runoff volume: 0.7 ac -ft 3.1 ac -ft • Principal spillway invert elevation (ft msl): 6,664.0 ft 6,653.0 ft Emergency spillway invert elevation (ft msl): 6,665.0 ft 6,655.0 ft Permanent pool elevation (ft msl): 6,664.3 6,653.5 Predicted peak water elevation (ft msl): 6,664.7 ft 6,654.8 ft Predicted annual sediment loading: 0.5 ac -ft 0.6 ac -ft Predicted maximum sediment storage elevation (ft msl): 6,662.0 ft 6,651.2 ft Predicted sediment storage volume: 1.6 ac -ft 1.7 ac -ft Predicted number of storage years: 3 years 3 years S12illways: The Coal Pile Sumps were designed with both a principal and emergency spillway in accordance with Rule 4.05.9(2)(b). Map 2.05.3 -E2.E, Coal Stockpile Sumps, shows spillway locations and specifications. The principal spillway for UCSS is a 12 -inch corrugated straight metal pipe; the principal spillway for LCSS is an 18 -inch corrugated straight metal pipe. Both pipes will be fitted with a T- shaped cap; the "T" is on its side, the barrel is the spillway pipe. • For UCSS, the invert elevation of the barrel pipe is at 6,664.0 feet; the bottom of the "T" is at 6,663.2 feet — water will not flow freely through the principal spillway when the water level drops below the 6,663.2 elevation. • For LCSS, the invert elevation of the barrel pipe is at 6,653.0 feet; the bottom of the "T" is at 6,652.2 feet — water will not flow freely through the principal spillway when the water level 2.05.3 -E2.E4 12/09