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BETA VERSION 3/15/95 Exceedance Probability ofthe Expected Annual Number of Floods Flood Exceedance Probabilities Interval and Exceedance Probabilities B-C Program: Infrastructure YELLOW Blocks (Results). The program calculates the Exceedance Probability of the Expected Annual Number of the Interval probability Floods for floods between any two flood frequencies entered in the GREEN Blocks. Thus, in the example shown above, a 25-year flood frequency is calculated as the difference between the annual probability of a 25-year flood and the annual probability of a 75-year flood (e.g., 1/25 - 1/75, or 0.040 - 0.0133). Flood probabilities are often expressed as exceedance probabilities. An exceedance probability means the probability of all floods greater than or equal to some specified flood. Thus, the annual exceedance probability for a 25-year flood means the annual probability for all floods greater than or equal to the 25-year flood. To avoid confusion, the distinction between interval probabilities and exceedance probabilities must be clearly made. The commonly used tenn, "100-yearflood," is actually an exceedance probability. In other words, the 1 OO-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 1 OO-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. 4