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REFUSE P/LE EXPANSION <br />meet the discharge requirements of the CDPS permit <br />administrated by the Colorado Department of Public <br />~ealth and Environment (CDPHE). Because of <br />location constraints, the sedimentation pond was <br />designed to rely on storage capacity to accommodate <br />runofT discharge from a Type li 25-yr, 24-hr storm <br />event as allowed in the Regulations of the CMLRB <br />for Coal Mining Section 4.05.6(6): <br />"The Division may approve a sedimentation <br />pond that relies primarily on storage to <br />control the runoff from the design event <br />specified in 4.05.6(5) `the 25-yr, 24-hr <br />precipitation event,' when it is demonstrated <br />by the operator and certified by a qualified <br />registered professional engineer that the <br />sedimentation pond will safely control the <br />design precipitation event, the water from <br />which shall be safely removed in accordance <br />with current, prudent, engineering practices. <br />Such a sedimentation pond shall be located <br />where failure would not be expected to cause <br />loss of life or serious propertydamage...." <br />The RPE sediment pond has been designed to meet <br />``~these criteria because the location of Highway 133 <br />~'$nhibiu the construction of a spillway with capacity <br />to carry a 25-yr, 24-hr precipitationevent. <br />The pond, shown in Figure L-2, is located in a <br />depression a[ the base of the RPE and has the <br />following features: <br /> <br />• 2H:1 V side slopes, <br />• primary (cell A) and secondary (cell B) <br />settling ponds, <br />• HDPE or PVC liner with 1-ft of protective <br />fill on cell floor, <br />• access ramps to allow loader entry for <br />cleanout, <br />• capacities of 5.44 & 3.65 acre-ft in cells A & <br />B respectively, while maintaining a I-ft <br />freeboard, <br />• outlet control tower located at southwest <br />comer of cell B. <br />• outlet discharge capaciq~ of 45 cfs through <br />existing 30-inch diameterCMP culvert, <br />• culvert outlet is 220-fr north of pond, on <br />north side of Highway 133. <br />• receives runoff water through inlet control at <br />southeast comer of cell A, <br />• receives RPE subdrain flow through inlet at <br />southwest corner of cell B. <br />The sedimentation pond will also comply with the <br />Section 4.05.5(3)(a) of [he Regulations of the <br />CMLRB for Coal Mining which states "the pond <br />shall provide adequate capacity to contain or treat the <br />runoff or inflow entering the pond as a result of 10-yr, <br />24-hr precipitation event..." and Section 4.05.5(3)(b) <br />"provide for an adequate sediment storage volume <br />and periodic sedimentremoval...." <br />Table 15 summarizes the pond capacity and <br />capability to contain the stated precipitation event. <br />The factor of safety computed assumes aworst-case <br />scenario where an annual load of sediment exists in <br />the pond and a storm event occurs, bringing the <br />maximum estimate of runoff and sediment load <br />possible from the RPE during its life, during the <br />Phase 4 -fill. The pond can safely contain a 100-yr, <br />24-hr precipitation event. The pond can hold a <br />maximum sediment volume of 4.82 acre-feet (S-year <br />capacity) and still contain the 24-hour 100-year storm <br />event. <br />Because the pond is designed to contain the 100-yr, <br />24-hr precipitation event, there is adequate capacity <br />to permit sediment settlement before discharge. <br />The sediment pond is designed to allow primary <br />sediment settling in cell A, and secondary settling in <br />cell B. For cleanout and service, each cell can <br />operate as a stand-alone module, with independent <br />inlet and outlet control. Both cells drain into the <br />same outlet control structure, which flows into the <br />existing culvert leading under Highway ]33. The <br />pond will be lined with HDPE/PVC (or clay) and a 1- <br />ft cover of protective fill will be placed on top of the <br />cell floors to allow loader access for cleanout. <br />The pond embankment will be composed ofclean fill <br />and will comply with criteria stated in Section 4.11.5 <br />of the Regula[ionsof the CMLRB for Coal Mining. <br />6.7 Culvert Design For Haul Road <br />Crossing At Sylvester Gulch <br />A corrugated metal pipe (CMP) culvert was sized for <br />the RPE haul road crossing at Sylvester Gulch. The <br />culvert was sized to surpass the CMLRB Sections <br />4.03. I (2) and 4.03.1(4) wh ich states "culverts with an <br />Harding Lawson Associates 74 <br />