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<br />BETA VERSION 3/15/95 <br /> <br />B-C Program: Infrastructure <br /> <br />Expected <br />Annual Number <br />of Floods <br /> <br />Flood Elevation <br />vs. Flood Depth <br /> <br />To avoid confusion, the distinction between interval probabilities and <br />exceedance probabilities must be clearly made. The commonly used <br />term, "100-year flood," is actually an exceedance probability. In other <br />words, the 100-yearflood level with an annual probability of 0.01 means <br />all floods greater than or equal to this level. The interval probability of a <br />flood at exactly (within plus or minus 0.5 feet) the 1 OO-year flood level will <br />be smaller (sometimes much smaller) than the exceedance probability for <br />a 1 OO-year flood, because the exceedance probability includes ALL <br />floods greater than or equal to the 1 OO-year flood. <br /> <br />For completeness, the benefit-cost program tabulates both exceedance <br />probabilities and interval probabilities, although all calculations are done <br />using the interval probabilities. <br /> <br />The flood frequency data (i.e., 10,50,100, or 500 years) correspond to <br />exceedance probabilities (see Flood Recurrence Intervals section on <br />page 7-7). <br /> <br />This analysis gives the Annual Exceedance Probability for all floods, in <br />one-foot increments of depth. From the Annual Exceedance <br />Probabilities, calculated as described above, the Expected Annual <br />Number of Floods in a given one-foot increment are calculated from the <br />difference in exceedance probabilities of two flood depths. For example, <br />the expected annual number for a 2-foot flood (i.e., all floods between 1.5 <br />and 2.5 feet) at a given site (with a given Zero Flood Depth Elevation) <br />is calculated as the exceedance probability for a 1.5-foot flood minus the <br />exceedance probability for a 2.5-foot flood. <br /> <br />For a given flood (e.g., a 100-yearflood), the elevation of the flood water <br />surface varies with location along the stream as shown by the Flood <br />Profile (see Riverine Flood Use(s Guide, pages 7-2 to 7-4). <br />Furthermore, at a given location along the stream the flood depth <br />corresponding to a 1 OO-year flood varies depending on the Zero Flood <br />Depth Elevation of the building under evaluation. In the Benefit-Cost <br />Program, the Expected Annual Number of Floods are shown for each <br />flood depth from -2 to 18 feet for the building under evaluation. For a <br />different building with a different Zero Flood Depth Elevation, the <br />Expected Annual Number of Floods for each flood depth will be <br />different. Thus, for example, the depth of a 1 OO-year flood will differ for <br />buildings at different Zero Flood Depth Elevations. <br /> <br />8 <br />