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<br />Revised March, 1994 <br />4.05.6 Sedimentation Ponds <br />The location of all existing and planned sedimentation ponds are <br />presented on the Hydrology - South Map (Map 12) and the Hydrology <br />- North Map (Map 11). The design plans and specifications for the <br />sedimentation ponds are described in this section. The design <br />parameters and detailed sedimentation calculations for the <br />sedimentation ponds are shown in Exhibit 7, Hydrology Information. <br />The sedimentation pond embankment and emergency spillway design are <br />also shown in Exhibit 7, Hydrology Information. All sedimentation <br />ponds will be located as close as practical to the areas to be <br />disturbed. Other methods of sediment control will be located on <br />the reclaimed areas; these methods include the use of contour <br />furrowing, terraces, and chisel plowing. <br />This application contains calculations used to determine runoff <br />volumes and flow rates for the 10-year, 24-hour precipitation event <br />and subsequent sediment volumes. The precipitation data were <br />obtained from the NOA Atlas 2, Volume 3 for Colorado; soil types <br />were obtained from the Soil Conservation Service, and are shown on <br />the Soils - South Map (Map 6) and the Soils - North Map (Map 5). <br />Although the mining activities within each watershed of the permit <br />area will create changing hydrologic conditions, the designs are <br />based on worst case conditions. The worst case conditions are <br />usually those that exist during and just after topsoil operations <br />before any excavations are begun. <br />Sediment will be removed from all sedimentation ponds when the <br />volume of sediment accumulates to 60 percent of the designed <br />sediment storage volume, or to a volume that will not allow <br />effective treatment of the runoff and sediment that would result <br />from the 10-year, 24-hour precipitation event. Survey measurements <br />of sediment accumulations will be conducted bi-annually for <br />discharge points 003, 004, and 007 and any new sediment ponds and <br />every third year for 001, 002, 005, and 006. Ponds will be dredged <br />when water levels are lowest, and the least amount of precipitation <br />is expected. The removed sediment will be used as topsoil or <br />subsoil if it meets the suitability criteria discussed under <br />Section 2.04.9. The Division will be notified of this determina- <br />tion if the material is selected overburden material that can be <br />substituted for as a supplement to topsoil. <br />4.05-6 <br />