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Peabody Sage Creek Mine — Sump Design Report <br />• 10 -year, 24 -hour event: 1.7 inches <br />• 25 -year, 24 -hour event: 2.1 inches <br />SCS Curve Number and "k" Factor: The approved curve number (CN) values used for the <br />SEDCAD demonstrations are presented on Table 2.05.3 -E2 -1 in Exhibit 2.05.3 -E2. The <br />"k" Factor values were based on approved soil survey information adapted from the Seneca <br />II Mine Permit (C -1980 -005) and National Resources Conservation Service soil <br />interpretations. The curve numbers and "k" factors were then "weighted" for each sub - <br />watershed. <br />Sedimentology: To determine the sediment loading, the average annual "R" method was <br />used. The sediment storage volume is determined based on the ratio of the RUSLE annual R <br />factor to the calculated R storm value and the annual sediment yield to the storm sediment <br />yield. The number of years of sediment storage (Y) and the average annual R factor are <br />input values. The sediment storage analysis used an annual R value of 30. This is a historic <br />value approved by CDRMS for calculating sediment storage needs in the sediment ponds at <br />the Seneca II Mine. <br />A sensitivity analysis was conducted by varying the number of storage years which allowed <br />the design to maximize storage capacity and maintain compliance with applicable rules and <br />regulations. The number of storage years was increased in the sedimentology sub - routine <br />until the model results indicated there was approximately 1.0 to 1.5 feet of storage room in <br />the pond (the elevation difference between the calculated top of sediment storage and <br />combination spillway invert elevation). This elevation difference is within the range <br />recommended by Warner et al, (1998). Consequently, the design presents the maximum <br />sediment storage volume while retaining capacity to store the design storm runoff and <br />maintain compliance with applicable rules and regulations. <br />Storage Capacity Results of the design SEDCAD demonstrations show that the sump <br />configurations allow for adequate retention time to control the design storm runoff volume <br />from the 10 -year, 24 -hour storm event, capturing most of the sediment load and minimizing <br />settleable solids concentrations in the discharge. Furthermore, there will be no discharge <br />through either principal spillway as a result of the 10 -year, 24 -hour event (Rule <br />4.05.9(2)(b)). In addition, there will be adequate freeboard between the water surface <br />elevation and the top of embankment during the 25 -year, 24 -hour event in accordance with <br />Rule 4.05.9(7)(d). Since the amount of sedimentation from the coal stockpiles is unknown, <br />the ponds will be over - excavated to allow sediment containment. Cleanout requirements <br />will be determined annually. <br />The following summarizes the storage capacity for the 10 -year, 24 -hour event SEDCAD <br />demonstration. <br />Total pond storage capacity (at invert of principal) <br />Predicted 10 -year, 24 -hour runoff volume <br />Principal spillway invert elevation (ft amsl) <br />Emergency spillway invert elevation (ft amsl) <br />Predicted peak water elevation (ft amsl) <br />Upper Sump <br />Lower Sump <br />6.35 ac -ft <br />6.42 ac -ft <br />3.67 ac -ft <br />6.39 ac -ft <br />6772.5 ft <br />6653.8 ft <br />6773.5 ft <br />6655.0 ft <br />6772.8 ft <br />6654.5 ft <br />TRO 1 -10 2.05.3 -E2.E3 2/11 <br />