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DISCUSSION <br />1NTRODUC'.']ON <br />The following plan is for the design, construction, and maintenance of the New Elk <br />Refuse Dispc>sal Area Sedimentation Pond. This pond has been designed to handle runoff <br />and sediment from an 85.1 acre drainage area of which 61.4 acres will ultimately be coal <br />refuse. The pond is located at N 180,200E 2,156,500 Colorado State Piane coordinates <br />south zone, rind has an embankment height of 31 feet from the upstream toe to the dam <br />crest. The p~~nd will be a modified homogeneous type dam with a center keyway and a <br />rockfill toe. Total earth volume for the embankment is approximately 10,000 cubic yards. <br />Soil borings and testing were done by Lincoln-Devore, with results of the testing enclosed. <br />DESIGN <br />Pond sizing was done according to Colorado Mined Land Reclamation Division <br /> <br />(CMLRD) reF;ulations combining the runoff fora !0 year - 24 hour rainfall event and 60 <br />percent of tf~e 3 year sediment yield. The total storage volume required is 15.14 acre- •, <br />feet. Area capacity curves are shown on Figure 4 with pertinent elevations shown on the <br />stage-volume curve. The required storage volume (15.L4 Acre-Feet) represents an <br />elevation of 7451.75 feet. This will be the elevation of the top of the principal spillway <br />riser with the bottom of the emergency spillway one foot higher (elevation 7,452.75'). <br />Emergency spillway design was done using MSHA guidelines.(!) The mine is located <br />in a rural area and the pond is less than 1,000 Acre-Feet and under 40 feet in height. It is <br />above a state highway, however, which makes the hazard potential moderate. One-half <br />the probable maximum precipitation fora 6 hour duration was used as the design flood. <br />The New Elk Mine is located 2.08 miles east of the lOSo meridan which puts the PMP at <br />23.50 inches of rainfall(2), The peak runoff for this area during a y: PMP event will be <br />1292.50 CFS as shown on Calculation Sheet No. 4. A trapezoidal emergencv spillway <br />ditch with an 8 foot bottom width and 2H:1V side slopes will contain this volume at a <br />flowline of 3..1' or 7,555.85' elevation. <br />The frea~board requirement for the pond was calculated by adding the wave height ' <br />run-up plus the embankment settlement height. The wave height was determined by using <br />