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recurrence intervals
Posted by kaslop [Email] (more from kaslop) on Tue, 9 Aug 2005 13:23:00
In Reply to: Re: 500 year flood, BobS, Tue, 9 Aug 2005 10:24:56
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Yes, a 500y flood is higher than a 100y flood. How much higher depends on the particular river.
These numbers are probabilistic estimates of the recurrence interval for a flood of a given height. For example, a 500y flood is one that occurs, on average, once every 500 y. The smaller 100y flood occurs on average once every 100y.
Things to keep in mind about these flood recurrence intervals:
1) because recurrence intervals are probabilistic, a 500y flood _could_ occur 2 years in a row. It is unlikely, but it could happen.
2) the estimates of recurrence intervals are based on sparse data. This is especially the case in North America where we have <100y of historical data on river levels in most cases. There are geological methods for reconstructing the history of past floods, but these are not precise and have gaps. Therefore, a municipality (or insurance company) may consider an area to be outside the 500y flood level, but often this is only an educated guess.
3) When climate changes significantly, or engineers change river beds, old estimates of recurrence interval are next to useless and all bets are off.
4) Changes like urbanization, intensive agriculture, and the construction of 'flood control measures' like dikes tend to make 500y floods into 100y floods in areas downstream of the change.
You asked :-) !
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