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<br />of the Flood Insurance Adminstration (FIA) for developing floodways. <br /> <br />~ <br />. <br /> <br />Current FIA requirements are for equal conveyance reduction. <br /> <br /> <br />Because the program considers only the cross section conveyance <br /> <br /> <br />in its determination of the encroachment stations, a water surface <br /> <br />profile computed for the encroached floodway will not have elevations <br /> <br /> <br />exactly equal to the desired value (natural elevation plus specified <br /> <br /> <br />incremental amount). One reason for variance from desired elevations <br /> <br /> <br />is that a redistribution of flow and velocity occurs whenever a cross <br /> <br /> <br />section is modified. Changes at the current cross section plus the <br /> <br /> <br />backwater effects from downstream encroachments can produce a result <br /> <br /> <br />quite different than the specified incremental change in water surface <br /> <br /> <br />elevation. For low velocity streams of fairly uniform shape, the <br /> <br /> <br />results are usually very close to the desired results. <br /> <br /> <br />The specified incremental change in water surface is usually <br /> <br />considered a maximum desired; however, when running the encroachment <br /> <br />o <br /> <br />profiles, other specified values can also be used. For example, with <br /> <br /> <br />a desired one-foot maximum change in water surface elevation, a series <br /> <br /> <br />of encroachment profiles could be run with 0.5 foot, 0.8 foot, and 1 <br /> <br /> <br />foot incremental increases. Analysis of these profiles would pro- <br /> <br /> <br />vide a means for selecting the limits of encroachment. <br /> <br /> <br />The stations of encroachment should be plotted in plan view <br /> <br /> <br />and also on cross section plots to further evaluate the results. <br /> <br /> <br />The HEC-2 printer plot routine can provide a set of cross sections <br /> <br /> <br />suitable for the cross section review. The visual presentation of <br /> <br /> <br />the data often facilitates selection of encroachment stations. If <br /> <br />7 <br />