![]() Likewise, using a frequency analysis (Interagency Advisory Committee on Water Data, 1982) there is a 1 in 100 chance that a streamflow of 15,000 cubic feet per second (ft 3/s) will occur during any year at a certain streamflow-measurement site. Thus, a rainfall total of 6.60 inches in a consecutive 24-hour period is said to have a 50-year recurrence interval. For example, assume there is a 1 in 50 chance that 6.60 inches of rain will fall in a certain area in a 24-hour period during any given year. The recurrence interval is based on the probability that the given event will be equaled or exceeded in any given year. Statistical techniques, through a process called frequency analysis, are used to estimate the probability of the occurrence of a given precipitation event. "100-year floods can happen 2 years in a row" In other words, a flood of that magnitude has a 1 percent chance of happening in any year. What this means is described in detail below, but a short explanation is that, according to historical data about rainfall and stream stage, the probability of Soandso River reaching a stage of 20 feet is once in 100 years. Instead of the term "100-year flood" a hydrologist would rather describe this extreme hydrologic event as a flood having a 100-year recurrence interval. If flood conditions occurred because of the rain then you might have heard the radio or TV weatherman say something like "This storm has resulted in a 100-year flood on Soandso River, which crested at a stage of 20 feet." Obviously, this means that the river reached a peak stage (height) that happens only once every 100 years, right? A hydrologist would answer "Well, not exactly." Hydrologists don't like to hear a term like "100-year flood" because, scientifically, it is a misinterpretation of terminology that leads to a misconception of what a 100-year flood really is. Possibly you can remember when a really big rain, be it from a hurricane or a large frontal system, hit your town. Floods: Recurrence intervals and 100-year floods (USGS).
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