Monday, March 22, 2010

How Can We Have Four “100-Year” Floods In 15 Years?

By Jim Northrop, GSWA Member

In Bound Brook, New Jersey, there was a flood in 1996, and in 1999 Hurricane Floyd brought 17 feet of water to downtown Bound Brook. In 2007, there was another flood, followed recently by heavy rainfall which caused more downtown flooding. Some people call these “100-year floods,” but they all occurred over a mere 15 years. How can we have several “100-year” floods in the short span of 15 years?

Some people mistakenly believe that a “100-year” storm or “100-year” flood happens only once every 100 years. The term “100-year” flood is shorthand for a flood that statistically has a 1% (1/100) chance of occurring in any given year. Likewise, the term “100-year” storm is used to define a rainfall event that statistically has this same 1% chance of occurring this year. For a hydrologist studying water flows, “100-year” means an extreme hydrologic event having a 100-year recurrence interval.

Based on historical data about rainfall and stream stage at a given location, a process of frequency analysis is used to determine the probability that an extreme hydrologic event (such as a river cresting at a flood stage of 20 feet) will be equaled or exceeded in any given year.

Ten or more years of data are required to perform a frequency analysis for the determination of recurrence intervals. Of course, the more years of historical data the better -- a hydrologist will have more confidence for an analysis of a river with 30 years of record than one based on 10 years of record.

Thus, the terms “100-year” storm or “100-year” flood, are used merely to simplify the definition of a storm or flood that statistically has a 1% chance of occurring in any given year. It is not the promise of a 100-year interval.

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