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• (DMG). The embankments were constructed over the last 20 years, from the Streeter <br />Pond in 1979 to the East Taylor Pond in 1995. <br />Following is a discussion of the geometry of the sedimentation pond <br />embankments, our opinions regarding construction procedures, and estimates of soil <br />conditions based on our visual observations and experience at the mine site, <br />Geometry <br />The crest width of the embankments varies from 15 to 23 feet. Crest lengths <br />range from about 100 to 1,000 feet. The interior and exterior embankment slopes are <br />typically either 3:1 or 2:1 (N:V). The 2:1 slope can be on the interior, with the 3:i <br />slope on the exterior, orvice-versa. The sum of the slope angles equals or exceeds <br />5 in accordance with the regulations. The design height of the embankments ranged <br />. from 6 to 16 feet. The design height was calculated per 30 CFR 77.216 as the <br />difference between the emergency spillway elevation and the elevation of the <br />upstream toe. Emergency spillways are 2 to 3 feet below the crest. Total <br />embankment heights (from crest to upstream toe) range from 9 to 19 feet. <br />The outlet for the ponds currently consists of a gated outlet valve. The <br />current height of long term water storage (from toe of embankment to gate valve) <br />ranges up to about 4 feet maximum. The riser modification will increase the height <br />of water up to 4.6 feet. The revised heights of long term water storage (from toe of <br />embankment to lowest pertoration in riser) range from 1 to 8 feet. Based upon the <br />height of water on the interior slope, we estimated steady state seepage will result <br />in one-third the height of water at the exterior slope (Fig, 1 and 2). The one-third <br />estimate was confirmed with a simplified flow net analysis. <br />The East Taylor embankment heights are based upon the height of <br />constructed embankment above the pre-existing natural ground surface. This <br />condition was discussed in a November 22, 1995 letter by Colowyo to DMG. <br />COLOWYO COAL COMPANY, LP <br />STA8ILITY EVALUATION - SEOIMENTA770M PONOS ~ <br />CTL/T 28,233 <br />