| FEMA P-259, Table 3-6, provides the probability of one or more occurrences of a flood of a given recurrence interval (magnitude) within a specified number of years.
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Length of Period (Years): 1 | 10% | 4% | 2% | 1% | 0.2% | Length of Period (Years): 10 | 65% | 34% | 18% | 10% | 2% | Length of Period (Years): 20 | 88% | 56% | 33% | 18% | 2% | Length of Period (Years): 25 | 93% | 64% | 40% | 22% | 5% | Length of Period (Years): 30 | 96% | 71% | 45% | 26% | 6% | Length of Period (Years): 50 | 99+% | 87% | 64% | 39% | 10% | Length of Period (Years): 100 | 99.99+% | 98% | 87% | 63% | 18% |
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| The table values represent the probabilities, expressed in percentages, of one or more occurrences of a flood of given magnitude or larger within a specified number of years. Probability (P) may be calculated for any given Length of Period (n) and Recurrence Interval (RI) using the following equation: P=[1 minus (1 minus 1 over RI)] x 100%, where RI and n are in years. |