Laserfiche WebLink
<br />VERSiON 1.0 1/12/95 <br /> <br />B-C PROGRAM: Flood Hazard Risk <br /> <br />Flood <br />Recurrence <br />Intervals <br /> <br />Flood <br />Exceedance <br />Probabilities <br /> <br />Flood Hazard Risk: Technical Ap'pendix <br /> <br />Floods are a probabilistic natural phenomenon: it is impossible to <br />predict in what years floods will occur or how severe the floods will be. <br />Flood hazards are often expressed in terms of flood frequencies or <br />recurrence intervals, such as a 10-year flood or a 100-year flood. <br /> <br />A "1 OO-year" flood means that there is a 1 % chance per year of a flood <br />at the 100-year or higher flood elevation. A 10-year flood means that <br />there is a 10% chance of a flood of the 1 O-year or higher flood <br />elevation. In general, the annual probability of a flood of X-years is 1/X. <br />Thus, the annual probability of an 83-year flood is 1/83 or 0.012. <br /> <br />Flood recurrence intervals do not mean that floods occur exactly at <br />these intervals; rather they only express the probabilities of floods. <br />Thus, a given location may experience two 100-year floods in a short <br />time period 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 <br />to recurrence intervals. The recurrence interval in years of a flood <br />depth with Y annual probability is 1/Y. For example, the recurrence <br />interval for a flood with an annual probability of 0.01234 is 1/0.01234 or <br />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. <br />An exceedance probability means the probability of all floods greater <br />than or equal to some specified flood. Thus, the annual exceedance <br />probability for a 2-foot flood means the annual probability for all floods <br />greater than or equal to 2 feet. <br /> <br />7-7 <br />