![]() Stated another way, the flood probability analysis reveals that we are 90-percent sure that the river elevation will be between 20.6 and 22.85 on the Big Piney River at Big Piney for the 1-percent AEP flood. A streamflow of 36,600 cubic feet per second (ft 3/s) corresponds to a river level of 20.6 feet, whereas a streamflow of 56,400 ft 3/s corresponds to a river level of 22.85 feet. The uncertainty for the streamflow quantity of the 1-percent AEP flood for an example flood at the Big Piney River, Missouri can be translated into an uncertainty of the river level. Most policy makers and water managers often are more concerned with the height of the water in the river (river levels) than the streamflow quantity. The accuracy of the 1-percent AEP flood varies depending on the amount of data available, the accuracy of those data, land-use changes in the river drainage area, climate cycles, and how well the data fits the statistical probability distribution. How accurate are estimates of the 1-percent Annual Exceedance Probability (AEP) Flood (also known as the 100-year flood)? The "500-year flood" corresponds to an AEP of 0.2-percent, which means a flood of that size or greater has a 0.2-percent chance (or 1 in 500 chance) of occurring in a given year. More recently, people talk about larger floods, such as the "500-year flood," as tolerance for risk is reduced and increased protection from flooding is desired. For example, we can say there is a 1 in 100 chance that next year's flood will equal or exceed the 1-percent AEP flood. Examining all the annual peak streamflow values that occurred at a streamgage with time allows us to estimate the AEP for various flood magnitudes. Geological Survey (USGS) operates more than 7,500 streamgages nationwide (see map) that allow for assessment of the probability of floods. A streamgage is a location on a river where the height of the water and the quantity of flow (streamflow) are recorded. To determine these probabilities all the annual peak streamflow values measured at a streamgage are examined. If the probability of a particular flood magnitude being equaled or exceeded is known, then risk can be assessed. Scientists and engineers frequently use statistical probability (chance) to put a context to floods and their occurrence. Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, and it has an average recurrence interval of 100 years, it often is referred to as the "100-year flood". The 1-percent AEP flood was thought to be a fair balance between protecting the public and overly stringent regulation. In the 1960's, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. Likely lots of "100-year flooding" was happening. Large-scale flooding was occurring in December 2015. So what is a 100-year flood and how is it determined? Trying to place contextual framework around floods is where such terms as "100-year flood" came into being. Placing floods in context allows society to address such issues as the risk to life and property, and to study and understand the environmental benefits of floods. Because floods result from many different circumstances, not all floods are equal in magnitude, duration, or effect. Floods occur for many reasons, such as long-lasting rainfall over a broad area, locally intense thunderstorm- generated rainfall, or rapid melting of a large snow pack with or without accompanying rainfall. Ī flood is any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream. ![]() ![]() You can see Robert's full poster explaining the concept of the 100-year flood at. Karl Winters, USGS's National Flood Program Coordinator, offers up the following explanation of the 100-year flood that we can all understand. The USGS conducts research on the physical and statistical characteristics of flooding, estimating the probability of flooding at locations around the United States and attempting to understand how the frequency of flooding changes with urbanization, climate variability, and other factors The term " 100-year flood" is often used to describe a flood of great magnitude. Geological Survey (USGS), but to most of the population of the United States. Visit Media to see details.Īrea about 0.65 mile southwest of USGS streamflow-gaging station 08069500, West Fork of the San Jacinto River near Humble, Texas, August 30, 2017.Ĭredit: Steve Fitzgerald, Harris County Flood Control District.įlooding and high water are of major interest not only here at the U.S. ![]() Sources/Usage: Some content may have restrictions.
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