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<br />BETA VERSION 3/15/95 <br /> <br />B-C Program: Infrastructure <br /> <br />Flood Recurrence <br />Intervals <br /> <br />Flood <br />Exceedance <br />Probabilities <br /> <br />Flood Hazard Risk: Technical Aependix <br /> <br />Floods are a probabilistic natural phenomenon: it is impossible to predict <br />in what years floods will occur or how severe the floods will be. Flood <br />hazards are often expressed in terms of flood frequencies or recurrence <br />intervals, such as a 1 O-year flood or a 1 OO-year flood. <br /> <br />A "1 OO-year" flood means that there is a 1 % chance per year of a flood at <br />the 100-year or higher flood elevation. A 1 O-year flood means that there <br />is a 10% chance of a flood of the 1 O-year or higher flood elevation. In <br />general, the annual probability of a flood of X-years is 1/X. Thus, the <br />annual probability of an 83-yearflood is 1/83 or 0.012. <br /> <br />Flood recurrence intervals do not mean that floods occur exactly at these <br />intervals; rather they only express the probabilities of floods. Thus, a <br />given location may experience two 1 OO-year floods in a short time period <br />or go several decades without experiencing a 1 O-year flood. <br /> <br />Flood recurrence intervals <in years) and annual flood probabilities <br />contain exactly the same probabilistic information. The previous <br />paragraphs explained how to convert recurrence intervals in years into <br />annual probabilities. Conversely, annual probabilities can be converted to <br />recurrence intervals. The recurrence interval in years of a flood depth <br />with Y annual probability is1/Y. For example, the recurrence interval for <br />a flood with an annual probability of 0.01234 is 1/0.01234 or 81 years. <br /> <br />In the benefit-cost program, flood probabilities are expressed in terms of <br />annual probabilities. If desired, these probabilities can be converted to <br />recurrence intervals by the procedure discussed above. <br /> <br />The Expected Annual Number of Floods for each flood depth <br />correspond closely to Annual Probabilities of floods. Such probabilities <br />are interval probabilities; that is, they express the probabilities for each <br />flood depth. For example, in the Benefit-Cost Program, the annual <br />probability of a 2-foot flood is considered to be the annual probability for <br />all floods between 1.5 and 2.5 feet of depth at that site. <br /> <br />Flood probabilities are often expressed as exceedance probabilities. An <br />exceedance probability means the probability of all floods greater than or <br />equal to some specified flood. Thus, the annual exceedance probability <br />for a 2-foot flood means the annual probability for all floods greater than <br />or equal to 2 feet. <br /> <br />7 <br />