MISSISSIPPI RIVER FLOOD MAPS TV

So a 0.2 AEP flood has a 20% chance of occurring in any given year, and this corresponds to a 5-year recurrence-interval flood. The USGS and other agencies often refer to the percent chance of occurrence as an Annual Exceedance Probability or AEP.

What is an Annual Exceedance Probability?

Recurrence intervals and probabilities of occurrences But, just because it rained 10 inches in one day last year doesn't mean it can't rain 10 inches in one day again this year. In other words, over the course of 1 million years, these events would be expected to occur 10,000 times. Likewise, the term ' 100-year storm' is used to define a rainfall event that statistically has this same 1-percent chance of occurring. The term '100-year flood' is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year. This question points out the importance of proper terminology. (From the USGS CoreCast podcast and video series) How can we have two '100-year floods' in less than two years?īob Holmes, the National Flood Coordinator for the USGS, discusses this in a podcast. Under these conditions, essentially all of the rain that falls, whether on paved surfaces or on saturated soil, runs off and becomes streamflow. During these larger floods, the soil is saturated and does not have the capacity to absorb additional rainfall.

The effects of development (conversion of land from forested or agricultural uses to commercial, residential, or industrial uses) on peak flows is generally much greater for low-recurrence interval floods than for high-recurrence interval floods, such as 25- 50- or 100-year floods. Recurrence intervals for the annual peak streamflow at a given location change if there are significant changes in the flow patterns at that location, possibly caused by an impoundment or diversion of flow. Of course, the more years of historical data the better-a hydrologist will have more confidence on an analysis of a river with 30 years of record than one based on 10 years of record. Ten or more years of data are required to perform a frequency analysis for the determination of recurrence intervals. Rainfall recurrence intervals are based on both the magnitude and the duration of a rainfall event, whereas streamflow recurrence intervals are based solely on the magnitude of the annual peak flow. Thus, a peak flow of 15,000 ft 3/s at the site is said to have a 100-year recurrence interval. 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.

MISSISSIPPI RIVER FLOOD MAPS TV

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).

Mississippi river flood maps