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Doc Date:12/11/20O1 III IIIIIIIIIIIII III <br />'~ J/~ ~ 4.6 HYDROLOGIC BALANCE CONTROL PLAN <br />4.6.1 SEDIMENTATION CONTROL SUMMARY <br />The permit area is bounded on the west by Trout Creek and on the east by <br />a mountain top ridge. The east to west slope varies from 96 to 218. Drainage <br />is either by sheet flow or by ephemeral streams to Trout Creek. Due to the <br />low precipitation and small drainage area, no major drainage channels exist in <br />the area. <br />Runoff and sediment control in the Moffat Area will be performed by using <br />a single sedimentation pond in conjunction with diversion ditches north and <br />south of the pond. The northern ditch will be constructed over time beginning <br />in year 5 and extending northward as mining progresses in that direction. The <br />sedimentation pond and the southern ditch will be constructed before mining <br />begins. <br />The Sediment pond was designed to contain approximately three years of <br />sediment storage and treat the runoff from a 10-year, 24-hour storm event. <br />Runoff volume and sediment pond performance were determined by computer <br />simulation using the SEDIMOT II computer program developed by the University <br />of Kentucky, Lexington, Kentucky. Diversion ditches were designed to safely <br />pass the peak flow from a 10-year, 24-hour storm event. Two small areas that <br />will be disturbed drain away from the sedimentation pond. Drainage from these <br />areas will be controlled using contour furrows, and where necessary, straw <br />bale filters. <br />All sediment control structures on the West Ridge Area are in place. <br />Many of these structures were built prior to the permanent program standards. <br />A review of the various structures designs and construction details are <br />provided latter in this section. <br />9.6.1.1 Sediment Yield Calculation <br />Sediment yield for the Moffat pond was determined using the SEDIMOT II <br />subroutine MUSLE computer program. The sediment volume for storage capacity <br />was determined through a time-step process of sediment yields from the water <br />shed over the mining years. From this process it was determined that Year 5 <br />4.6-1 <br />November 1,1989 <br />