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The term 100-year flood indicates that the area has a one-percent chance of flooding in any given year, not that a flood will occur every 100 years. [ 2 ] Such maps are used in town planning , in the insurance industry, and by individuals who want to avoid moving into a home at risk of flooding or to know how to protect their property.
Theoretically a 100-year flood has a 1 percent chance (1/100 = 0.01 or 1 percent) of occurring in any given year and a 500-year flood has as a 0.2 percent chance (1/500 = 0.002 or 0.2 percent) of occurring in any given year. [12] However, these expected flood elevations actually occur more or less often than expected. [13]
Ten-year floods have a 10% chance of occurring in any given year (P e =0.10); 500-year have a 0.2% chance of occurring in any given year (P e =0.002); etc. The percent chance of an X-year flood occurring in a single year is 100/X. A similar analysis is commonly applied to coastal flooding or rainfall data.
The county is currently using 500-year flood maps to indicate areas where residents should be ready to evacuate. "We are using the 500-year flood plain maps as kind-of a guide.
Sellers and landlords are now required to disclose if a property has flooded or if it is situated in a FEMA Flood Hazard Zone Area.
For example, a 10-year flood has a 1/10 = 0.1 or 10% chance of being exceeded in any one year and a 50-year flood has a 0.02 or 2% chance of being exceeded in any one year. This does not mean that a 100-year flood will happen regularly every 100 years, or only once in 100 years. Despite the connotations of the name "return period".
The report added that “experts say the flood maps used by the national program are outdated, leaving many areas that should have flood insurance without it.”
A floodplain or flood plain or bottomlands [1] is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high discharge. [2] The soils usually consist of clays, silts, sands, and gravels deposited during floods. [3]