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April 19, 2002 2 003-2203 <br />To accommodate this peak flow, the temporary channel should be constructed as trapezoidal <br />channel with a minimum bottom width of 8 feet and a depth of 2 feet, with side-slopes of <br />2.5 horizontal (H): 1 vertical (V). The channel would still be graded 0.87 percent along the <br />Phase IVb and IVc boundary, with a 2 percent cross-slope along the bottom. Based on this <br />channel size, the peak flow can be safely passed while maintaining 0.8 feet of freeboard and <br />the flow velocity below 5 feet per second (i.e. no rip-rap required). The effective flow area of <br />the temporary channel is 14.5 square-feet. <br />The results of the hydraulic analysis also indicate that the temporary channel culvert diameter <br />must be increased from 2 feet to 3 feet to accommodate the peak flow. The culvert outfall <br />should be located within a small plunge pool to dissipate energy. The minimum dimensions <br />of the plunge pool are provided in the attachment. The pool should be lined with 9-inch <br />riprap to control erosion. <br />If you should have any questions or comments regarding this letter or the diversion channel <br />sizing, please contact me at (303) 980-0540. <br />Sincerely, <br />GOLDER ASSOCIATES INC. <br />John F. Lupo, Ph.D., P.E. <br />Sr. Project Manager <br />JL/bc <br />cc: Bob Nichols, Golder <br />Greg Eddy, Golder <br />1:\00'5203\0100\0032203.0100.02B19.DOC Golder Associates <br />