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The SEDIMOT II hydrology and sedimentology computer model was used to calculate <br />runoff from the 10 -year, 24 -hour and 25 -year, 24 -hour storm events. Results of <br />the SEDIMOT II modeling are presented in Appendix 7 -6a. <br />Exhibit 7 -8 indicates that the permanent pool elevation is 6,611.2 feet, with a <br />designed pond storage capacity of 267.4 acre -feet. Consequently, there is <br />267.4 acre -feet of sediment storage available, which equates to 52.7 10 -year, <br />24 -hour precipitation events. The SEDIMOT modeling indicates that peak runoff <br />volume and inflow for the 10 -year, 24 -hour storm event are 11.9 acre -feet and <br />19.3 cubic feet per second (cfs), respectively. The peak stage is 6,612.0 <br />feet. This is 2.0 feet below the top of the impoundment (stage of 6,614 feet), <br />which is the cord elevation for the haul road. The top of impoundment design <br />elevation is conservative because the haul road is lined with a MSHA berm. The <br />average settleable solids concentration in pond discharge during the peak 24- <br />hour period is 0.0 milliliters per liter (ml /1). The Water Quality Control <br />Commission (WQCC) settleable solids effluent limit is 0.5 ml /1. The SEDIMOT <br />modeling shows that there is sufficient storage capacity available, and that <br />the settleable solids effluent concentration is less than the applicable water <br />quality standard. Therefore, Pond 002 as constructed and operated meets the <br />intentions of Rule 4.05.6(3)(a). <br />j The Pond 002 spillway system has an invert elevation of 6,611.2 feet. The <br />spillway channel is constructed in native material; in addition, the channel is <br />lined with vegetation and riprap, with a Manning's roughness coefficient of <br />0.055. The channel is a minimum of three feet deep. The roadside MSHA berm <br />provides additional freeboard. The calculated peak discharge from the 25 -year, <br />24 -hour storm event is 18.9 cfs. The design flow depth is 1.87 feet, with a <br />flow velocity of 1.03 feet per second (see Appendix 7 -6a). The peak stage from <br />the 25 -year, 24 -hour storm event is 6,613.1 feet. The SEDIMOT modeling and as- <br />built drawing indicate there is a minimum of 1.0 feet of freeboard in the <br />spillway system as 20 -year, 24 -hour storm event flows leave the pond. <br />Therefore, the pond as constructed and operated meets the intentions of Rule <br />4.05.9(2)(a)(i). <br />As noted above, the 25 -year, 24 -hour storm event peak stage is 6,613.1 feet, <br />with an peak inflow of 35.4 cfs (see Appendix 7 -6a). The peak discharge is <br />18.9 cfs with a peak velocity of 1.03 fps. The SEDIMOT modeling results <br />presented in Table 8 indicates that the spillway system can safely pass 25- <br />year, 24 -hour event peak discharge. Therefore, Pond 002 as constructed and <br />operated meets the intentions of Rule 4.05.9(2)(c)(ii). <br />TR -37 Revised 04/05 <br />