. Floods:
The flood discharge exceeded in any year with a 1% probability is the 100 year flood. In general, if a flood discharge is exceeded with a probability p in any year then that flood has a recurrence interval or return period of 1/p years.
An annual flood series can be used to estimate the recurrence interval of floods the size of which have occurred historically. The annual flood series is a list of the largest flood that occurred in each year within the historical period. The annual flood series is sorted by magnitude so that one can refer to the ith largest. The ith largest flood has a return period of (n+1)/i where n is the number of years in the series.
A number of frequency-size distributions such as the lognormal, log-Gumbel, and log-Pearson distributions of different types are used in estimating flood recurrence intervals. They differ mainly in the relative frequency of very large events.
Paleoflood estimates can be very helpful in extending the historical flood estimates to higher discharges (Figure 1).
Figure 1
The fact that large paleofloods are often determined to be more frequent than one would expect from a distribution fit to historical flood series data is called the Noah effect. The fact that floods can often cluster over a range of time scales is known as the Joseph effect.
Images from the 1993 Mississippi flood:
> 50 dead
> $10 billion in damages.
.
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