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<br />I
<br />
<br />I
<br />
<br />Therefore, no floodway data was shown, For all cross-sections the floodway
<br />
<br />is assumed to be identical to the lOO-year flood plain. Floodway data
<br />
<br />within those locations where future backwater conditions have been
<br />
<br />
<br />evaluated would not be meaningful since the proposed backwater has
<br />
<br />already been set at its maximum allowable elevation and dictates
<br />
<br />
<br />required right-of-way drainage and flood control purposes.
<br />
<br />I
<br />
<br />I
<br />
<br />I
<br />
<br />C, FLOOD VELOCITIES AND EROSION POTENTIAL
<br />
<br />I
<br />
<br />Average flow velocities are presented on the Flood Hazard Area Delineation
<br />
<br />drawings 1-6 for specific eross-sections which have been detailed, The
<br />
<br />range of average flow velocities for the lOO-year flood event is tabulated
<br />
<br />below for the reach, The mean velocity is representative for all the
<br />
<br />cross-sections analyzed within the reach,
<br />
<br />I
<br />
<br />I
<br />
<br />Velocity
<br />Range
<br />1 to 13 fps
<br />
<br />Mean
<br />Velocitv
<br />6,76 fps
<br />
<br />I
<br />
<br />Reach
<br />1
<br />
<br />I
<br />
<br />Water traveling at a rate greater than six to eight feet per second (fps)
<br />
<br />
<br />can cause erosion of stream banks and is capable of transporting silt,
<br />
<br />cobbles and large rocks, Water travelling at 4 fps or greater will
<br />
<br />transport colloidal silts and can erode embankment and/or bridge abutment
<br />
<br />fills, Water flowing at less than 2 fps will deposit debris and silt and
<br />
<br />
<br />will tend to produce clear water at least near the water surface. It is
<br />
<br />
<br />concluded that relatively high velocities can be expected in the Spring
<br />
<br />
<br />Gulch drainage basin resulting in an enlarged natural channel capable of
<br />
<br />
<br />conveying the lOO-year flood event in most cases,
<br />
<br />I
<br />
<br />I
<br />
<br />I
<br />
<br />I
<br />
<br />I
<br />
<br />This does not mean that the existing channel is now stable; erosion can and
<br />
<br />
<br />will continue to occur and special precautions should be taken to assure
<br />
<br />
<br />that structures, utilities, and other development facilities close to the
<br />
<br />
<br />flood plain are protected from the erosive effects of high velocity flow.
<br />
<br />
<br />As part of development planning, grade checks should be considered as a
<br />
<br />
<br />means of controlling bank sloughing and stream bed degradation,
<br />
<br />I
<br />
<br />I
<br />
<br />I
<br />
<br />I
<br />
<br />VI. REFERENCES
<br />
<br />A.
<br />
<br />"Urban Storm Drainage Criteria Manual", Urban Drainage and Flood
<br />
<br />Control District, March, 1969, Revised May 1975, Revised May, 1984,
<br />
<br />B,
<br />
<br />Colorado Urban Hvdrop.:raoh Procedure Comnuter Proe:ram, User Manual
<br />
<br />CUHPE/PC, Urban Drainage and Flood Control District, January, 1985,
<br />
<br />C,
<br />
<br />PreciDitation - Freauencv Atlas of the Western United States, Vol,
<br />
<br />
<br />III, Colorado, National Oceanic and Atmospheric Administration, 1973,
<br />
<br />D,
<br />
<br />Storm Water Management Model User's Manual. Version II,
<br />
<br />U,S,
<br />
<br />Environmental Protection Agency, EPA-670/2-75-017, March 1975, See
<br />
<br />
<br />also Revisions to Runoff Block of Storm Water Management Model",
<br />
<br />
<br />Missouri River Division, U.S. Army Corps of Engineers. Revised for
<br />
<br />UDFCD by Boyle Engineering, - March 1982,
<br />
<br />E.
<br />
<br />"HEC-2 Water Surface Profiles Users Manual", September 1982, by
<br />
<br />
<br />Hydrologic Engineering Center, U,S, Army Corps of Engineers, Davis
<br />
<br />California.
<br />
<br />F.
<br />
<br />"STORM DRAINAGE DESIGN AND TECHNICAL CRITERIA MANUAL", Douglas County,
<br />January, 1986,
<br />
<br />8
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