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permanent pool elevation that is 0.3 feet above the invert elevation of the principal spillway and <br />results in constant flow from UCSS to LCSS. The LCSS design included the constant inflow from <br />• UCSS as well as in influx of perennial spoil spring inflow. Seneca Coal Company has a spoil spring <br />monitoring site in the near vicinity of the sumps; measurements for site SSSPG7 indicate a mean <br />discharge of 60 gpm. These inflows were incorporated into the LCSS design by setting the <br />permanent pool water level at an elevation above the spillway invert equal to the inflow volume. <br />The 25 -year, 24 -hour event demonstration was designed using a conservative emergency spillway <br />assumption — the sumps were 100 percent full to the emergency spillway invert elevation (e.g., <br />principal spillways were inoperable). <br />Precipitation and Storm Type: The precipitation amounts for the design storms were obtained from <br />Frequency Atlas of the Western United States — Volume 11 — Colorado (National Oceanic <br />Atmospheric Administration [NOAA], 1973). A standard Soil Conservation Service (SCS) Type II <br />storm was utilized for each analysis. <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 SEDCAD <br />demonstration are presented on Table 2.05.3 -E2 -1 in Exhibit 2.05.3 -E2. The "k" Factor values were <br />• based on approved soil survey information adapted from the Seneca II Mine Permit (Permit C -80- <br />003) and National Resources Conservation Service soil interpretations. The curve numbers and "k" <br />factors were then "weighted" for each sub - watershed as presented in the Hydrology Input Parameter <br />tables found at the back of Addendum 2.05.3- E2.E -1. <br />Sedimentoloav: To determine the sediment loading, the average annual "R" method was used. The <br />sediment storage volume needed is determined based on the ratio of the RUSLE annual R factor to <br />the calculated R storm value and the annual sediment yield to the storm sediment yield. The number <br />of years of sediment storage (Y) and the average annual R factor are input values. <br />The sediment storage analysis used an annual R value of 30. This is the historic value approved by <br />CDRMS for calculating sediment storage needs in the sediment ponds at the Seneca H Mine. <br />A sensitivity analysis was conducted by varying the number of storage years 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 the <br />model results indicated there was approximately 1.0 to 1.5 feet of storage room in the pond (the <br />elevation difference between calculated top of sediment storage and combination spillway invert <br />elevation). This elevation difference is within the range recommended by Warner et al (1998). <br />• Consequently, the design presents the maximum sediment storage volume while retaining capacity to <br />store the design storm runoff and maintain compliance with applicable rules and regulations. <br />2.05.3 -E2.E3 12/09 <br />