ABSTRACT

Flood control engineers speak of 20-year floods, 50-year floods, and 100year floods. By a 20-year flood they mean the worst flood that can be expected during a given 20-year period. Similarly, a 100-year flood is the worst flood to be expected in a 100-yearperiod. Generally a l00-year flood is worse than a 20-year flood. The average flood in a short period of time is likely to be about the same size as the average flood during a longer period, but this is not true of the worst flood.