BOX 1-1
One Percent Annual Chance Flood

One percent annual chance flood, base flood, and the 100-year flood, are terms commonly used to describe a hydrologic event that has a 100-year (average) recurrence interval, that is, a flood that has in any year a 1 in 100 chance of being equaled or exceeded. The National Flood Insurance Program (NFIP) refers to properties in the one percent annual chance floodplain as those in the Special Flood Hazard Area. An illustration showing the impact of a one percent annual chance flood on a natural floodplain with various levee heights is shown in Figure 1-1. Over the lifetime of a 30-year mortgage, this equates to at least a 26 percent chance that the property will be flooded if the property is in the one percent annual chance floodplain. Probability theory is used to derive the value 26 percent, where each of the 30 years is accounted for as having a one percent annual chance flood (USGS, 2010.)

Similarly, hydrologic events of any size are described using the same nomenclature, for example, the “50-year” or “500-year” describing events that have a 1 in 50 chance or 1 in 500 chance of occurring (2 percent annual chance flood and 0.2 percent annual chance flood, respectively). Levees are also classified in the same way; that is, a 100-year levee is of adequate height to withstand the one percent annual chance flood.

There is a degree of interchangeability in general use of these terms. However, the use of certain recurrence interval terms, such as the “100-year flood,” can cause confusion because it is interpreted as a flood that occurs once every 100 years (USGS, 2010).


FIGURE 1-1 Illustration of the one percent annual chance flood (Q100) and the impact on a natural floodplain with various levee heights.

is a “man-made structure, usually an earthen embankment, designed, and constructed in accordance with sound engineering practices to contain, control, or divert the flow of water so as to provide protection from temporary flooding.”5 Levees are generally designed to control water up to a given water elevation and, unlike dams, levees do not typically have spillways to reduce structural damages when water levels exceed the design criteria and overtop the structure (Box 1-2). A levee system is a “flood protection system which consists of a levee, or levees, and associated structures, such as closure and drainage devices, which are constructed and operated in accordance with sound engineering practices.”6 Under NFIP regulations, homes and commercial buildings located in the SFHA


5 See Code of Federal Regulations, Title 44, Section 59.1 (44 CFR §59.1).

6 See 44 CFR §59.1. An expanded definition of a levee system, adapted from the National Committee on Dam and Levee Safety, is as follows: A levee system comprises one or more components that collectively provide flood risk reduction to a defined area. Breach or malfunction of one component within a system constitutes breach or malfunction of the entire system. The levee system is inclusive of all components that are interconnected and necessary to ensure exclusion of floodwaters from the associated leveed area. The leveed area may also be considered as the associated separable floodplain or separable consequence area. Structures and features include levee and floodwall sections, closure structures, pumping stations, culverts, interior drainage works, and other structures and features, such as highway and railroad embankments that function as components of the system whether or not intentionally constructed as part of the system.

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