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Last modified
1/26/2010 10:09:15 AM
Creation date
10/5/2006 4:19:31 AM
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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 />side of a cross section. if possible. so that the re.aining conveyance <br />at the incrementally higher water surface elevation is the same as the <br />conveyance of the natural section at the natural water surface elevation. <br />There is one restriction to the encroachment process; the program will <br />not encroach into the channel as defined by the left and right bank <br />stations. If there is insufficient conveyance in one of the over- <br />banks. the program will automatically try to reduce the conveyance on <br />the other overbank to make up the deficit. Once the encroaclnent <br />staions (left and right) are set. the program cOqlutes the water sur- <br />face elevation fOr the encroached cross-section. A simple example <br />hand calculation (Appendix I) shows the steps involved, <br />A variation of the above procedure for determining the stations <br />of encroachment has been added to the program as Modification 58. <br />Instead of eliminating an equal percent of the increased conveyance <br />fro. each side of a cross section. this procedure distributes the <br />percent of conveyance reduction based on the distribution of conveyance <br />in the two overbanks. For example. if the left overbank of a cross <br />section had twice as much conveyance as the right overbank. this pro- <br />cedure would eliminate 67% of the increased conveyance from the left <br />overbank and 33% from the right. This procedure would reduce the <br />instances where the program completely eliminates a cross section <br />overbank because the overbank has less than half the increase in conveyance. <br />This revised procedure will work with Method 3 or 4 by using a negative <br />input value on the ET card (ET card input is presented on page 10). <br />f1owever. this procedure is not consistent with the current requirements <br /> <br />6 <br />
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