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1 I I I I I I I I I I I I I I I I I I ",~eningful floodway data could be tabulated in Tables 3 and 4 (Appendix). "' have therefore chosen to show floodway d at a on 1 y at those few cross :~:tions where encroachments into the overbank areas could have an effect :. the water surface elevation_ Floodway data for all other cross sections ,"e assumed to be identical to the lOO-year flood pl ain data. Floodway :=ta within those subreaches where backwater conditions have been evaluated .~uld not be meaningful since the backwater being considered has already :~en set at its maximum allDwable elevation. 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 facilities close to the flood plain are protected from the erosive effects of high velocity flow. VI. REFERENCES FLOOD VELOCITIES AND EROSION POTENTIAL ~/erage flow velocities are given on the Flood Hazard Area Del ineation ccawings for specific cross sections which have been detailed. The range c~ average flow velocities for the 100-year flood event is tabulated below ~or each of the various reaches. The mean velocity for all the cross sections analyzed within each reach is also tabulated. A. "Precipitation - III, Colorado," 1973. Frequency Atlas of National Oceanic the Western United States, Volume and Atmospheric Administration, Ve 1 oc ity Mean Reach Range Velocity 1 1 to 16 fps 7.73 fps 2 6 to 12 fps 9.83 fps 3 2 to 12 fps 6.49 fps 4 1 to 15 fps 9.83 fps B. "Urban Storm Drainage Criteria Manual," Urban Drainage & Flood CDntrol District, March 1969, Revised May 1975. C. "Colorado Urban Hydrograph Procedure Computer Program," User Manual, Urban Drainage & Flood Control District, December 1977. Water travell ing at a rate greater than six to eight feet per second (fps) can cause erosion of stream banks and is capable of transporting sil t, cobbles and large rocks. Water travelling at four fps or greater will transport colloidal silts and can erode embankment and/or bridge abutment fills. Water flowing at less than two fps will deposit debris and silt and wi 11 tend to prod uce clear water at 1 east near the water surf ace. It is concluded that the relatively high velocities that can be seen in the Marcy Gulch drainage basin, combined with erodible soils, have resulted in an enlarged natural channel which is in most cases Df adequate size to carry the 100-year flood event. D. "HEC-2 Water Surface Profi 1 es Users Manual," January 1981, by Hydro- logic Engineering Center, U.S. Army Corps of Engineers, Davis, Cal iforni a. 12