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VERSION 1.0 1/12/95 B-C PROGRAM: Flood Hazard Risk Expected Annual Number of Floods To avoid confusion, the distinction between interval probabilities and exceedance probabilities must be clearly made. The commonly used term, "100-year flood," is actually an exceedance probability. In other words, the 100-year flood level with an annual probability of 0.01 means all floods greater than or equal to this level. The interval probability of a flood at exactly (within plus or minus 0.5 feet) the 1 OO-year flood level will be smaller (sometimes much smaller) than the exceedance probability for a 100-year flood, because the exceedance probability includes ALL floods greater than or equal to the 100-year flood. For completeness, the benefit-cost program tabulates both exceedance probabilities and interval probabilities, although all calculations are done using the interval probabilities., Graphs of flood probabilities (both exceedance and interval) may be viewed by clicking on the graph buttons at the end of the flood hazard screen in the Benefit-Cost Program. The Riverine Flood modeling uses an approach outlined by the U.S. Army Corps of Engineers for riverine flooding (Flood Proofing, How to Evaluate Your Options, 1993). The Expected Annual Number of Fioods at each flood depth are calculated from the flood frequency and flood elevation data entered by the user, along with the Zero Flood Depth Elevation of the building under evaluation. ;;: -- .~-.. ' "c FiR1I';); I ~\5;'~;;/;'i:' 'L_;/t,i~; . ,',',',' ,,'.,. ..:._.....:," . . ., ,~:..,.~'..'...,\.:,~.::::::~~\,;..;.: \?,.:..;~.'.;~r(i}~'~~~~;;Y~~:::i:::.~:; :' ..'.' The flood frequency data (Le., 10, 50, 100, or 500 years) correspond to exceedance probabilities (see Flood Recurrence Intervals section on page 7-7). The computer program does a regression analysis fit between the logarithm of exceedance probability and flood discharge to obtain a smooth curve relating exceedance probability and flood discharge. Then, flood elevations are read (by the program) from the "rating curve," which is the relationship between flood discharge and elevation. The regression analysis is done in this manner because the relationship between stream discharge and probabilities is smooth whereas the relationship between flood elevation and probabilities may be very, irregular because of variations in stream valley shape. Flood probabilities for floods below the 1 O-year flood elevation are determined using the standard A-1 to A-30 flood curves used previously on FIRMs. 7-8