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• McClave Canyon Sediment Pond <br />Settleable Solids Efficciency <br />The Division found that Grand Valley had failed to submit the settleable solids efficiency <br />calculations in the initial application for TR-09. Grand Valley had relied on previous designs for <br />the sediment pond which used 0.1 acre feet of sediment per yeaz for the designs. The volume of <br />the pond was based upon the original sizing which used 0.52 acre feet of sediment and 0.58 acre <br />feet for run-off. Anew calculation of sediment yield is provided on Table 4.4-8. This calculation <br />also shows that soil loss will be about 0.1 acre feet per year. The existing pond provides 5 years <br />of sediment storage. The actual storage capacity is extended by the precleaner that Grand Valley <br />has installed in the inflow ditch. During "normal" operations, little sediment actually reaches the <br />pond because the precleaner is maintained regulazly. <br />Calculations for settleable solids require inflow sediment analysis which are not available. <br />The analysis would also be complicated by the effect of precleaner. After discussions with the <br />Division it was decided that an empirical proof of the efficiency of the pond would be appropriate. <br />During 1997, a series of storms in September put down 5.25 inches of rain in 5 days. <br />The most rain received was on the fourth day when there was 2.5 inches. This was a series of <br />storms that were generated by the remnants of the Pacific hurricane "Linda". The 24 hour period <br />when there was 2.5 inches of rain was followed by another 0.8 inches of rain in the next three <br />hours. The elevation of the water in the pond was approaching the emergency spillway and <br />would have dischazged within a short time unless the discharge on the primary spillway was <br />opened. A water quality sample was obtained shortly after the spillway was opened. Laboratory <br />analysis of the sample indicated a settleable solids level of 0.1 m1Nhr. Even though this storm was <br />almost as large as the 100 year event, the settleable solids concentration was well within the <br />allowable limit of 0.5 mWhr. Grand Valley believes that this measurement of actual performance <br />of the pond is as good, if not better, way of determining that the pond will perform properly. <br /> <br />MC Voi QI Page M- 10 ]0 - 1R - 98 <br />