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<br />Guide For Approxiaate Zone A Areas <br /> <br />Developing BFBs <br /> <br />Determining BFBs for Riverine flooding: <br /> <br />On each side of the stream in the vicinity of the site, <br />determine the ground elevation at which the superimposed Zone <br />A boundary lies by interpolating between two contour lines. <br />Add one-half of the map contour interval to the lower of the <br />two interpolated elevations. This is the approximate BFE for <br />the site (be sure to perform this method at each structure <br />location). <br /> <br />By adding one-half of the contour interval to the lowest <br />interpolated water-surface elevation, two things are achieved: <br />1) the final BFE is within one-half of the contour interval of <br />both interpolated water-surface elevations and, therefore, is <br />still within the acceptable tolerance of the topographic map <br />(generally regarded as r one-half of the map contour <br />interval); 2) it is a conservative estimate of the BFE. If <br />the BFE determined under this procedure seems too high, then <br />a detailed analysis may be performed to justify lowering it. <br /> <br />Example 1 <br /> <br />Using a county topographic map with a contour interval of <br />5 feet, the approximate Zone A boundary crosses contour <br />elevations on the left and right bank at 323 and 325 <br />feet, respectively, as shown in Figure 8, "Contour <br />Interpolation Method - Riverine Flooding Example 1." The <br />difference between these two water-surface elevations is <br />2 feet, which is less than one-half of the contour <br />interval or 2.5 feet. Therefore, this method is <br />acceptable for use on this portion of the stream. Add <br />323 feet (lowest interpolated water-surface elevation) <br />plus 2.5 feet (one-half of the contour interval), which <br />yields a BFE of 325.5. <br /> <br />V-3 <br />