Floods & Recurrence Intervals

Flood – when a stream overflows its channel.

Flood size: measured by maximum discharge, or by stage (elevation of water surface). This varies from year to year.

By analyzing the frequency of floods of various sizes, a recurrence interval can be developed for a river at a particular locality.

Recurrence Interval: usually measured in years. It is the average interval between floods of a particular size. Therefore, on average, a 100-year flood will occur at regular intervals of 100 years. This means that a 100-year flood size could not occur two years in a row! It means that the chance of a 100-year flood occurring in any year is 1 in 100.

A Flood Frequency Curve plots discharge, or in some cases stage, against recurrence interval.

Recurrence intervals constructed from data obtained by the gaging  stations are used by planners in “floodplain zoning”. 

By understanding the size of floods in a given period of time, the planners can estimate those parts of the floodplain that are likely to be flooded, say, every 50 or 100 years. Zoning of parks, recreation areas, homes, businesses, hospitals, etc., can then be made accordingly.