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EUGENE M, SHEARER <br />5720 SOUTH .JASMINE STREET <br />ENGLEW OO D, COLORADO 80110 <br />303-771-2371 <br />3 October 91 <br />Mr. Bryan Archer <br />Sun Coal Company <br />10200 lJest 44th Avenue, Suite 120 REF: Yampa River Sedimentation Pond <br />Wheat Ridge, Colorado 80033 Emergency Dewatering Device <br />Dear Bryan: <br />Pursuant to your recent request, an Emergency Dewatering Device for the <br />Yampa River Sedimentation Pond has been designed as per Regulations of the <br />Colorado Mined Land Reclamation Board, Section 4.05.6. Various <br />measurements and data are selected from previous work and the accompanying <br />drawing titled, "As Functioning, Yampa River Sedimentation Pond", dated 26 <br />June 90. <br />It is recommended that the emergency spillway be located on the north <br />sedimentation pond embankment, in a position that optimizes flow from the <br />spillway, northward towards the Yampa River. Although the north <br />embankment width varies from 10 to 15 feet, the accompanying emergency <br />spillway drawing addresses a 10 foot embankment width. The drawing design <br />is readily applicable to embankment widths in excess of 10 feet. A <br />trough-shape, riprapped spillway design is suggested in preference to a <br />flat-bottom, riprapped spillway to better control erosion of the spillway <br />slopes as well as the embankment. <br />Angular riprap, 0.3' to 0.5' in size should be compacted into a matrix of <br />0.05' gravel. Rounded, stream-worn rocks are not recommended riprap for <br />this type of spillway design. <br />The primary dewatering device, a 1.5 foot pipe with a cross section area <br />of 1.77 square feet, is capable of delivering 5.5 cfs at no head. Crass <br />section A-A' shows that at an apex water depth of 1.0 foot, the cross <br />section area of water flow in the trough will be 2.0 square feet. Thus, <br />the emergency dewatering device has a slightly increased flow capacity <br />compared to the pipe and the 100-year, 24-hour event water level is 1.0' <br />below the embankment crest, as per Section 4.05.6 (11)(e). <br />The Yampa River Sedimentation Pond has considerable storage capacity in <br />excess of requirements for a 100-year, 24-hour precipitation event <br />(reference report and drawing submitted 26 June 90); the primary <br />dewatering device is capable of conveying 43 percent more flow than <br />anticipated from the 100-year, 24-hour event; and the emergency spillway <br />