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Last modified
1/26/2010 10:09:15 AM
Creation date
10/5/2006 4:19:31 AM
Metadata
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Template:
Floodplain Documents
County
Statewide
Basin
Statewide
Title
Floodway Determination Using Computer Program HEC-2: Document Number 5
Date
5/1/1974
Prepared For
US
Prepared By
US Army Corps of Engineers
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />and velocities should be considered as well as the change in water surface <br />elevations. <br />The increase in water surface elevation will frequently exceed the <br />incremental change used to adjust the conveyance of the section. In <br />cases where the canputed increase is larger than desired, the problem <br />may stem from excessive encroachment of a downstream section. In the <br />example, the water surface elevations at Sections .55 to .94 exceed <br />the one foot limit on the 10.4 run. Care must be taken to select down- <br />stream encroachment stations consistent with the desired upstream results. <br />Consideration should also be given to existing physical features and to <br />the reasonableness of the encroachment when selecting stations. Unusually <br />large depths and velocities at the encroachment line should be avoided. <br />Encroachment results should be plotted to aid analysis of results. The <br />following are considerations from reference 2. <br />a. The effect of existing development within the floodway should be <br />considered if it is sufficient to alter flow capacity or location of <br />the floodway limits. <br />b. The loss of valley storage by confinement of the flow to the <br />Designated Floodwav can be significant. If such losses accumulate to <br />where the flow increase exceeds 10 percent of the initial flow, it should <br />be accounted for in detennini n9 the floodway's areal requi rements. <br />c. Floodway delineations are not applicable in all cases, such as <br />for inland sheet flow areas or coastal areas. <br />d. In situations where encroachment into the Floodway Fringe area <br />causes excessive velocities to develop, particularly in those rare cases <br /> <br />12 <br />
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