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<br />awareness and an operationally ready warning system. <br /> <br />The probability per year of flooding at a particular location varies with <br /> <br />flood magnitude. The larger the flood the less likely its occurrence. Since <br /> <br />flooding depends upon storm events that occur randomly in nature, there is no <br /> <br />way to predict when a flood will occur. It is possible by studying past <br />records to estimate the chances or likelihood that a flood of a given <br /> <br />magnitude will occur in a particular year or over a longer period of time. <br /> <br />. <br /> <br />The likelihood of experiencing a flood of a specified magnitude is expressed <br /> <br />by the percent chance of exceeding that magnitude in anyone-year period. <br /> <br />Thus, a l-percent-chance flood is a flood of a such magnitude that the <br /> <br />probability is one in lOa it will be exceeded in a given year. The recurrence <br />interval is often used to describe flood magnitude. Using the recurrence- <br />interval terminology, the 1-percent-chance flood is called a 100-year flood. <br />This terminology is misleading in that it suggests that one such event can be <br /> <br />expected in any given lOa-year period. Actually, a lOa-year flood can occur <br /> <br />at any time, and none or several can occur in a given lOa-year period. <br /> <br />~ <br /> <br />The risk of experiencing a flood of a given magnitude increases with the <br /> <br />length of the time period. For example, there is a 26-percent chance (the <br /> <br />odds are about one in four) that the 1-percent-chance flood will occur during <br /> <br />the 3a-year period of a normal mortgage. This increases to a 39-percent <br /> <br />chance (the odds are about two in five) in a 50-year period. Table 2 shows <br />the relationship between recurrence interval in years, probability per year in <br /> <br />percent, and probability per 50-year period in percent. <br /> <br />14 <br />