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<br />Table 2 -- Relationship Between Recurrence Interval, <br /> <br />Frequency, and Risk (in percent) <br /> <br /> Recurrence Probability in percent that a given flood <br />.1 will occur <br /> Probability <br /> Interval Per Year "N or more times" during a 50-year period <br /> In In <br /> Years Percent N=l N=2 N=3 N=4 N=5 <br /> 500 0.2 10 Nil Nil Nil Nil <br /> 200 0.5 22 3 Nil Nil Nil <br /> 100 1.0 39 9 1 Nil Nil <br /> 50 2.0 64 26 8 2 Nil <br /> 25 4.0 87 60 32 14 5 <br /> 10 10.0 99 97 89 75 57 <br />.. <br /> 5 20.0 100 100 100 99 98 <br />. <br /> Note: Nil less than 1% chance <br /> <br />Historical records of floods are an important source of information, but they <br /> <br />can be misleading. In most cases, the period of record is relatively short <br /> <br />(less than 100 years) and mayor may not include a severe flood. This limited <br />period may not accurately reflect the flooding potential for a community. <br />Floods that have occurred within the limited period of record provide a means <br /> <br />of communicating the relationship of flood magnitude and probability to the <br /> <br />community. <br /> <br />15 <br />