The "100-Year Flood"

Abstract

Photo by Geff Hinds, Tacoma News Tribune Flood designations are based on statistical averages, not on the number of years between big floods. The estimates are only as good as the available data. Flood designations are updated as more data are collected or when the conditions change in a river basin. The "100-Year Flood" BIG FLOODS COULD HAPPEN AGAIN IN WASHINGTON DURING ANY YEAR Rivers across the Nation seem to be rising to record flood levels almost every year. In Washington, more than one 100-year flood has happened on a few rivers in just the past several years. How can 100-year floods happen so often? WHY DON'T THESE FLOODS HAPPEN EVERY 100 YEARS?

Development of regression equations for the estimation of flood flows at ungaged streams in Pennsylvania

“…Respectively, these AEPs correspond to flood flows occurring, on average, once every 2, 5, 10, 20, 50, 100, 200, and 500 years. These flood flows are estimates based on statistical probabilities or frequencies, and the recurrence intervals associated with each flood flow refer to the average number of years between the floods (Dinicola, 1996). For example, the 100-year flood has a 1 in 100 chance (or 1 percent probability) that a flood of this magnitude will occur in any given year (table 1).…”

Methods for estimating magnitude and frequency of floods in Arizona, developed with unregulated and rural peak-flow data through water year 2010

“…A flood frequency analysis determines the magnitude of flow that can be expected with a given probability in any given year. Early USGS flood frequency publications describe flood frequency using the T-year recurrence interval (Dinicola, 1996) rather than as a probability of occurrence. Since the mid-2000's federal agencies that work with flood data have suggested moving away from this terminology because it confuses the public.…”

Development of a methodology for the estimation of the dynamic evolution of the flood - related hydrological behavior of periurban catchments under post - fire conditions

“…Return Period or else called Recurrence Interval (Dinicola, 1996) refers to the average time period (in years) between floods of certain extent. High, intermediate or low return periods correspond to events of low probability (extreme events), medium probability or high probability of occurrence, respectively.…”