2008-12-04_PERMIT FILE - C1994082 (14)

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sediment load, average annual R, and the number of storage years (Y). R is / the rainfall erosivity factor. The equation for required sediment storage f / is: ,,,/// Required Storage = (Annual R)(# years)(Storm Sediment Load) Storm R The TR-37 SEDCAD4 demonstration used an annual R value of 10. This value is the same for what has been used to determine sediment loading in other recent runoff demonstrations at the Yoast Mine. A sensitivity analysis was conducted varying the number of storage years (Y) to maximize sediment storage capacity. Because Pond O11A was designed and constructed to serve as a pre-settling basin for Pond 011, there are no effluent limitations for pond discharge. Consequently, re-suspension of settleable solids is not an issue as long as the pond discharge does not adversely impact Pond 011. The number of storage years (Y) was increased in the sedimentology sub-routine until the model results indicated there was approximately two (2) feet of storage room in the pond (the elevation difference between calculated top of sediment storage and principal spillway invert elevation). This elevation difference corresponds with what is recommended by Warner et al (1998); however, as indicated above, this is a pre-settling basin for downstream Pond 011 and this allows for flexibility in the elevation difference. Reclaimed Condition Storage CapacitV• Results of the TR-37 SEDCAD4 demonstration shows the storm runoff volume is 0.63 acre-feet (ac-ft) for the 10-year, 24-hour event. A user input storage volume that maximizes the available sediment storage in accordance with Warner et al., (1998) guidelines was input into the model for Pond 011A. The following summarizes Pond 011A storage capacity for the TR-37 10-year, 24-hour event SEDCAD4 demonstration. Principal spillway invert elevation (ft msl) = 7,428.1 ft Total pond storage capacity (at principal spillway elev.) = 1.8 ac-ft Storm pred icted ef fluent peak settleable concentration = 0.0 ml/1 Predicted annual s ediment loading = <0.01 ac-ft Predicted sediment storage capacity = 1.2 ac-ft Predicted maximum sediment storage elevation (ft msl) = 7,426.0 ft Number of storage years (Y) = 200 years Spillways. In accordance with Rule 4.05.9(2), Pond 011A was designed and constructed with both a principal and emergency spillway. Exhibit 13-5C shows spillway locations and specifications. The principal spillway consists of a 12-inch CMP riser pipe that drops into a 12-inch CMP barrel pipe. The spillway will keep the pond level at the 7,428.1 elevation. The SEDCAD4 demonstration in Appendix 13-19.1-A shows, in accordance with Rule TR-37 13-19.1-3 Revised 08108