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Storm water flood events

What is a "n" year flood event? by rowingengineer
Posted: 23 Jul 09

Flood events are defined by the probability that a certain amount of water is possible any one year. For example, the infamous "100-year flood" is in fact the level of water with a 1-percent chance (1 in 100) occurring any one year. The amount of water actually varies from river to river. In fact, that amount can vary along a river. The use of "n year flood" is technical jargon that has caused endless problems with the public. The term does not mean that a flood occurs every n years, but that it has a chance of 1/n of occurring any one year. Water volume increases as the probability decreases. The table below shows how this might affect you:

Flood Probabilities for any one year  
"Year" Probability Percent Flows
500 0.002 00.2% Extreme
100 0.010 01.0% Heavy to extreme
50 0.020 02.0% Moderate
25 0.040 04.0% Light to moderate
10 0.100 10.0% Light
5 0.200 20.0% Mild

 

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