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<br />Guide For Approxima~e Zone A Areas <br /> <br />Developinq BFEs <br /> <br />Da~a Ex~raDola~ion <br /> <br />If a site is within 500 feet upstream of a stream reach for <br />which a 1DD-year flood profile has been computed by detailed <br />methods, and the floodplain and channel bottom slope <br />characteristics are relatively similar to the downstream <br />reaches, data extrapolation may be used to determine the BFE. <br />The stream in the vicinity of the site, however, must be free <br />of backwater effects from downstream hydraulic structures. <br />The procedure for usinq this method is outlined below. <br /> <br />step 1 - Determine the location of the site on the flood <br />profile for the detailed study stream <br /> <br />step 2 - Extrapolate the last segment of the 100-year flood <br />profile that has a constant water-surface slope to <br />the location of the site. The BFE at the site can <br />then be obtained directly from the profile <br /> <br />Fiqures 11-12 on the following pages depict situations (i.e., <br />properties "Y" and "Z"), in which the data extrapolation <br />method may and may not be used. Figures 13-14 depict a <br />situation in which the data extrapolation method may not be <br />used because the highway may have an effect on the 100-year <br />water-surface elevations. If the 100-year flood profile <br />changes just prior to the limit of detailed study, as shown in <br />Figure 15, the data extrapolation method should not be used. <br /> <br />f <br />I <br /> <br />V-7 <br />