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<br />Culvert Designs: <br />Estimated culvert capacities were obtained by use of Morning's equation. <br />The use of Morning's equation for culvert capacity estimation is <br />inappropriate. Morning's equation is intended for use in open channels <br />with steady uniform flow. The best method for culvert sizing of which <br />this Division is aware is Homographic methods 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 <br />Hydraulic Charts for the Selection of Highway Culverts, 1965. This chart <br />assumes inlet control which is generally the case in highway culvert <br />crossings. BRCC must provide the following for all culvert designs: <br />1. Drainage Area <br />2. Design Return Period Storm and Peak Flow <br />3. Culvert Diameter, and <br />4. Available Headwater <br />Sediment Ponds <br />The method employed by Ute Engineering for pond designs is a simplified <br />triangular hydrograph methodology. The calculations appear to be fairly <br />well done. Unfortunately the designers failed to make use of some of <br />the information made available by the process. The designers obtained <br />a value for qpo (peak overflow) which would be required to provide a <br />settling time of 24 hours. This information was ignored during dewatering <br />device design. The proposed dewatering devices have much higher peak <br />outflows and provide much shorter detention times. In order to provide <br />the 24 hour detention times desired the following procedure should be <br />followed. <br />1. Determine the maximum head on the orifice for the dewatering <br />device during storage of the 10 year-24 hour st6rm, <br />2. assume that the control will be orifice A=CA(2gH)D'S C 0.6 and, <br />3. solve for area A. <br />This approach is somewhat simplistic bvt will provide designs for dewatering <br />orifice sizes which are appropriate for pond detention time requirements. <br />These orifices will undoubtedly be less than 2 inches in diameter. The <br />applicant has expressed concern that orifices Less than 2 inches in <br />diameter may tend to clog. These concerns are mitigated by the fact <br />that all ponds have primary as well as emergency spillways. <br />Emergency Spillways <br />The method used for design of emergency spillways at the site was an <br />application of Morning's equation. This is unacceptable for reasons <br />stated previously. As the emergency spillways are CMP the applicant <br />may use the chart for culvert sizing to verify the adequacy of the <br />proposed spillways. From work done in this office the outlet systems <br /> <br />