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~', <br /> <br />:even <br />reliminary Adequate Review <br />Crand Mesa Coal Co. - Red Canyon Mine <br />May 8, 1981 <br />9.03.1 (e) <br />3. Culverts: In many instances tlfe peak flow .shown for a culvert installation <br />does not coincide with the combined peak flows of the contributing ditches. <br />Culverts must be designed to safely handle the estimated peak flows from <br />the entire contributing area. <br />A11 of the culverts were found to have a head under the passage of the design <br />peak flow. GMCC must provide sufficient culverting to provide for no head <br />under passage of the design flow, or provide sufficient embankment protection <br />to prevent erosion. Estimates of design !read should be submitted. <br />9.05.6 Sedimentation Ponds: Estimates of peak flows, runoff storage volumes <br />and sediment yield are all acceptable as given for the mine site pond. how- <br />ever, a number of problems exist with regard to pond desiyn. <br />4. Decvatering and Principal Spillway: The usc~ nC manua7lq opcratrd dc- <br />watering devices is not desirable. Preferably, the applicant should provide <br />for automatically operated dewateriny devices and estimate detention time <br />for passage of the 10-year, 24-hour storm event. Otherwise, the applicant <br />must insure that the dewatering devices will be properly maintained. <br />Continued maintenance will be required during the time period following <br />cessation of operations, but prior to .bond release. <br />The principal spillway should have its crest at or above the 10-year, 24- <br />hour storage volume elevation (4.05.6(3)(a)). <br />Emergency Spillway: GMCC has used open channel hydraulics to design the <br />emergency spillways. This is inappropriate as the. re is not steady uniform <br />flow. A weir equation of the form, p = C1,R3~2 wou)d be mote appropriate. <br />Using this equation, the proposed spillway is .inodcquatc. GhtCC must provide <br />sufficient principal and emeryency spillways to safely pass the 25-year, <br />24-hour peak flow (9.05.6(3)(d)). <br />Zt is not apparent from information yiven whethr_r the requirements of <br />(9.06.8(d)) 1.0 ft. of freeboard under passage of the desiyn storm are met. <br />CMCC must provide sufficient information to insure that this requirement <br />is met. <br />5. Design Drawing: The design drawing provided by Gh1CC is inadequate. <br />Gh1CC must provide a design drawing irrcorporatin~7 tlrc following; <br />1. elevation of the design and cleanout sediment storage volumes, <br />2. elevation of the 10-year, 24-hour storm storage volume, <br />3. elevation of the crest of the principal and emergency spillway crests, <br />4. elevation of the dewatering device, <br />5. elevation of the water surface under passage of the 25-year, <br />24-hour storm event, and <br />6. the signaturr' and s".tl of a C'~•rt ifi•~d f'rnl•~;sienal invim~•v. <br />