A floodplain is a lowland adjacent to a river, lake or ocean. Floodplains are designated by the frequency of the flood that is large enough to cover them. For example, the 10-year floodplain will be covered by the 10-year flood and the 100-year floodplain by the 100-year flood.
Flood frequencies, such as the "100-year flood," are determined by plotting a graph of the size of all known floods for an area and determining how often floods of a particular size occur. Another way of expressing the flood frequency is the chance of occurrence in a given year, which is the percentage of the probability of flooding each year. For example, the 100-year flood has a 1% chance of occurring in any given year.
Dams, levees, channels, stormwater projects and other protective works are designed to provide protection against some specific level of flooding. The "level of protection" is selected based on cost, the desire of the community, potential damage, environmental impact, and other factors. Engineers can design and construct levees, dams and other measures providing a very high level of protection. Communities tend to choose lower levels of protection because of the initial financial cost rather than overall costs and benefits.
The National Flood Insurance Program has established a de facto minimum standard of protection against the 100-year flood. This is a relatively low level of protection. For example, there is a 26% chance that a levee or channel designed to contain the 100-year flood will be at that design capacity at least once over a 30 year period. All residents and businesses in areas vulnerable to flooding should have flood insurance.