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Mr. William S, Tate -2- December 30, 1981 <br />1. Drainage Areas <br />It appears that the boundaries drawn for watershed areas I and 3 on <br />Exhibit X do not correspond with the complete watershed boundaries. The <br />applicant must explain why the boundaries as shown do not conform with <br />the natural watershed areas. If there are diversion systems in these <br />watersheds, then complete designs, including contributing areas, peak <br />flows and complete ditch geometries must be submitted to the Division for <br />review, Otherwise, Exhibit X will need to be revised to accurately reflect <br />the boundaries of the natural watersheds. <br />2. Ditch Desi qns <br />The method used for ditch design is not described in sufficient detail, <br />The designs appear to have been generated by use of some variation on the <br />Chezy equation. The values of the Chezy coefficient and ditch slope <br />measurement are not given. In order to adequately review the proposed <br />ditch system for the Blue Ribbon mine, the applicant must supply the <br />following information to the Division: <br />a. Drainage Areas, <br />b. Design Return Period Storm and Peak Flow, <br />c. Ditch Geometries, <br />d. Estimated Depth of Flow in Reach of Least Slope, <br />e. Estimated Velocity in Reach o.` Greatest Slope, and <br />f. £xistiny and Proposed Ditch Lining <br />Blue Ribbon Coal Company is reminded of the requirement for 0.3 feet of <br />freeboard during passage of the 10-year, 29-hour storm (9.05.3(6)(b)). <br />3. Culvert Designs <br />The use of Manning's equation for culvert capacity estimation is inappro- <br />priate, Manning's equation is intended for vse in open channels with <br />steady uniform f1 ow. Amore appropriate method for sizing culverts <br />recommended by the Division is the nom graphic method developed by hydraulic <br />laboratories. Enclosed please find a copy of Chart S from the U. S. <br />Department of Transportation Federal Highway Department Publication, Hydraulic <br />Charts for the Selection of Highway Culverts, 1965. This chart assumes <br />inlet control which is generally the case in highway culvert crossings. <br />B.R. C. C. must provide the following for a1I culvert designs: <br />a, Drainage Area, <br />b. Design Return Period Storm and Peak Flow, <br />c. Culvert Diameter, and <br />d. Available Headwater <br />