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SEP-24-02 09e55AM FROM-Rend Miii 2nd Flnor <br />9T0 T25 0035 T-1T2 P.004 F-459 <br />W. lU. Wh~~ler end (~ssodotles, Inc. <br />UJater Resources Engineers <br />September 12, 2002 <br />Mr. Erich Bower <br />Henderson MiH <br />19302 County Road 3 <br />Parshall, CO 80468 <br />Dear Erich: <br />Re: #1333.0 Henderson Topsoil Haul <br />Flood Bypass Culverts <br />As requested, we have evaluated the culvert size requirements for the two <br />crossings of the Lower Flood Bypass Canal at the Henderson Mil(. The crossings <br />are required as part of the topsoil hauling and stockpiling project that will bey done <br />during the 2003 construction season. The canal crossings would be located along <br />the topsoil haul routes as shown on the attached Plate I. It is anticipated that the <br />crossings would consist of corrugated metal pipe (CMP) culverts set in the invert of <br />the existing canal and baekfilled. <br />Tha existing flood bypass canal was originally designed for the probable <br />maximum snowmelt flood in accordance with the hydrologic data summarized in the <br />report "Rainflood and Snowmen Flood Studies for the Henderson Mill Site" dated <br />October 1982. Table A-3 of that report indicates the peak snowmelt flow values <br />for various reaches of the bypass canal and various return periods. A copy of the <br />pottion of the report relating to the bypass canal is attached. The crossings <br />discussed herein are located in the "F-End" ditch section. <br />Due to the mild slope of the existing canal, the culvert size that would be <br />required to pass the probable maximum snowmelt flow of 98 c}s would be over 84 <br />inches in diameter. However, the depth of the existing canal is inadequate to <br />accommodate a culvert of this size. What is proposed herein is that a more <br />reasonable culvert size of 60-inch diameter be used at the crossings. We <br />3700 South Inca Street Englewood, Colorado 80110 (303) 761-4130 Ff~X (303) 761-QSOQ <br />