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n <br />. -~ <br />Mr. William S. Tate -2- December 30, 1981 <br />1. Drainage Areas <br />It appears that the boundaries drawn for watershed areas 1 and 3 on <br />Exhibit H 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 H will need to be revised to accurately reflect <br />the boundaries of the natural watersheds. <br />2. Ditch Designs <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 of Greatest Slope, and <br />f. Existing 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, 24-hour storm (4.05.3(6)(b)). <br />3. Culvert Desi <br />The use of Manning's equation for culvert capacity estimation is inappro- <br />priate. Manning's equation is intended for use in open channels with <br />steady uniform flow. Amore appropriate method for sizing culverts <br />recommended by the Division is the Homographic method developed by hydraulic <br />laboratories. Enclosed please find a copy of Chart 5 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 all 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 />