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I I I I I I I I I I I I I I I I I I I of average flow velocities for the 100-year flood event is tabulated below for each of the various reaches of Dad Clark Gulch. The mean velocity for all the cross sections analyzed within each reach is also tabulated. Velocity r.1ean Reach Range .'Ie_loc_i_tr -- I 1 to 14 fps 8.8 fps 2 5 to 9 fps 7.2 f ps 3 2 to 11 fps 8.5 fps 4 1 to 9 fps 6.3 fps 5 4 to 10 fps 7.6 fps 6 6 to 12 fps ]0.6 fps Water travelling at a rate greater than six to eight feet per second (fps) can cause erosion of stream banks and is capable of transporting silt, cobbles and large rocks. Water travelling at four to six fps or gt'eater will transport colloidal silts and can et'ode embankment and/or bridge abutment fills. ,later flowing at less than two fps will deposit debris and silt and will tend to produce clear water at least near the water surface. It is concluded that the relatively high velocities that can be seen in the Dad Clark Gulch drainage basin, combined with erodible soils, have resulted in an enlarged natural channel which is in most cases of adequate size to carry the 100-year flood event. This does not mean that the existing channel is now stable; erosion can and will continue to occur and special precautions should be taken to assure that structures, utilities, and other development facil ities close to the flood plain are protected from the erosion effects of high velocity flow. VI. REFERI'NCES A. "Lee Gulch, Little Creek, Major Drainageway Planning Report," KKBNA, Inc., Consulting Engineers, October ]977. B. "Precipitation - III, Colorado," ]973. Frequency Atlas of the \,estern United States, Volume National Oceanic and Atmospheric Mrninistration, C. "Urban Storm Drainage Criteria Manual," Urban Drainage & Flood Control District, March 1969, Revised ~1ay 1975. D. "Co 1 orado Urb an l1ydrogr aph Pt'oced ure Computer Pt'ogram," User Man ua 1 , Urban Dt'ainage & Flood Control District, Decel;lber 1977. E. "HEC-2 Water Surface Profiles Users ~1anual ," November 1976, by Hydrologic Engineering Center, U.S. Army Corps of Engineers, Davis, Cal ifornia. F. "Course Manual, Water Surface Profile Computation Using Computer Program I1EC-2," University Extension, University of Cal Hornia, Davis, Cal ifornia. G. "Floodway Determination Using Computer Program HEC-2, Training Docu- ment No.5," by Hydrologic Engineering Center, U.S. Army Corps of Engineers, Davis, Cal ifornia. H. "Design Flood Inflow and Spillway Requirements for Dad Clark Gulch Dam," by W. W. Wheeler and Associates, Consulting Engineers, November ]961. 12