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Section 816.46 (u) Continued. <br />cable State and Federal water quality requirements for the receiving <br />stream. When the sedimentation pond is removed, the affected land shall <br />be regraded and revegetated in accordance with Sections 816.100-816.106, <br />and 816.111-816.117, unless the pond has been approved by the regulatory <br />authority for retention as being compatible with the approved postmining <br />land use under Section 816.133. If the regulatory authority approves <br />retention, the sedimentation pond shall meet all the requirements for <br />permanent impoundments of Sections 816.49 and 816.56. <br />The plans contain all calculations used to determine runoff volumes <br />and flow rates for each precipitation event and sediment volumes. <br />The precipitation data were obtained from NOAA Atlas 2, Volume III <br />for Colorado; soil types were obtained from the Energy Mines No. <br />1 and 2 Soils (Map 29), and Eckman Park Soils (Map 30). The SCS <br />method was used for computing runoff volumes and rates; for <br />sediment yield, the method given in Section 816.56 (b) was used <br />for small ponds, generally those under 15-20 feet in height; <br />the Universal Soil Loss Equation, gully erosion rates and sediment <br />delivery rations as discussed in the plan were included in sediment <br />yield calculations for ponds over 15-20 feet in height. Although <br />mining activities within each watershed of the mine plan area will <br />create changing hydrologic conditions, the designs are based on <br />worst case conditions. The worst case conditions are usually those <br />that exist during and just after topsoil removal operations before <br />any excavations are begun. Sediment will be removed from all <br />sedimentation ponds when the volume of sediment accumulates to not <br />greater than 60 percent of the design sediment storage volume. <br />This design volume will be the volume of calculated sediment for a <br />three-year period. A staff gage installed in each pond will allow <br />twice-yearly measurements of sediment accumulation. Ponds will <br />usually be cleaned out once a year during the late summer months <br />when the water levels are lowest and the least amount of precipita- <br />tion is expected. The removed sediment will be used as topsoil if <br />it meets the topsoil suitability criteria discussed under Section <br />779.21. <br />Exhibit Bp 816-45 12/13/99 <br />