Laserfiche WebLink
A sensitivity analysis was conducted by varying the number of storage years (Y) which allowed the <br />design to maximize storage capacity and maintain compliance with the applicable rules and <br />regulations. The number of storage years was increased in the sedimentology sub - routine until there <br />was approximately 1.5 to 2 feet of elevation difference between the top of the top of sediment <br />storage and principal spillway elevation. This elevation difference is within the range recommended <br />by Warner et al (1998). The SEDCAD4 model results presented herewith reflect a (Y) value that <br />allows for the maximum sediment storage while retaining capacity to store the design storm runoff <br />and maintain compliance with applicable rules and regulations. <br />The 25 -year, 24 -hour event demonstration also utilized a user input sediment storage volume. The <br />sedimentology subroutine calculates the R storm value as a function of the design storm rainfall <br />amount and distribution. Consequently, the 25 -year, 24 -hour storm event "over- estimates" the <br />"design" sediment storage requirements. To eliminate this bias a user input sediment storage value, <br />equal to the volume calculated by the 10 -year, 24 -hour storm event, was used as the calculated <br />sediment storage requirement for the 25 -year, 24 -hour event. <br />Storage Capacity: Results of the design SEDCAD demonstration show a designed pond capacity of <br />19.7 acre -feet. The top of sediment elevation (6,774.9 feet) corresponds to 18.5 acre -feet of <br />available storage, which equates to 125 years of sediment storage. As noted above, PECOCO is a <br />pre- treatment structure originally constructed to assure that Pond 002 met the intentions of Rule <br />4.05.6(3)(a). Consequently, the settleable solids concentration water quality standard is not <br />applicable. <br />The following summarizes the storage capacity for the 10 -year, 24 -hour event SEDCAD <br />demonstration (see Addendum 2.05.3- E2.D -1). <br />PECOCO <br />Total pond storage capacity (at combination spillway): <br />20.7 ac -ft <br />Predicted 10 -year, 24 -hour event runoff volume: <br />3.2 ac -ft <br />Combination spillway invert elevation (ft msl): <br />6,776.6 ft <br />Permanent pool elevation (ft msl): <br />6,776.8 ft <br />Predicted peak water elevation (ft msl): <br />6,776.8 ft <br />Predicted annual sediment loading: <br />0.2 ac -ft <br />Predicted maximum sediment storage elevation (ft msl): <br />6,774.9 ft <br />Predicted sediment storage volume: <br />18.5 ac -ft <br />Predicted number of storage years: <br />125 years <br />Spillways: SCCC has re- designed the spillway configuration for the Peabody Sage Creek Mine <br />operations. Construction of the Overburden Stockpile interferes with the existing outflow ditch that <br />carried principal spillway discharge. To minimize water flowing from PECOCO towards the <br />Overburden Stockpile, the 24 -inch corrugated metal pipe constructed by SCC for the 1990 remedial <br />plan (as documented in the TR -37 as -built report) has been removed. The emergency spillway has <br />been reconfigured as an trapezoidal open channel combination spillway. The channel is 10 feet in <br />2.05.3 -E2.D4 12/09 <br />