Flood Plain

A flood plain is a strip of relatively flat land bordering a river, stream, lake or coastline that becomes covered by water during a flood, when water levels rise beyond the capacity of the normal channel. Flood plains form naturally over long periods as rivers periodically overflow and deposit sediment across the surrounding land, which is also why flood plain soil is often unusually fertile and has historically attracted farming and settlement, despite the recurring flood risk.

Governments and insurers map flood plains to identify which properties face meaningful flood risk, dividing land into zones based on the estimated probability of flooding in a given year. In the United States, this mapping is carried out under the National Flood Insurance Program, and the resulting flood maps determine where flood insurance is mandatory for properties carrying a mortgage and how flood insurance premiums are priced.

A flood plain is the land a river effectively "borrows" during periods of high water. Most of the time that land looks and functions like any other flat field or neighborhood; during a major storm or spring snowmelt, the river reclaims part of it temporarily, sometimes for days, before receding again. Building on a flood plain does not guarantee a flood will happen in any given year, but it does mean the land sits within the area a river is statistically likely to cover at some point.

• A 100-year flood plain is land estimated to have roughly a 1 percent chance of flooding in any given year, the most common threshold used to define high-risk zones for insurance and building regulation purposes.
• A 500-year flood plain covers land with a lower, but still meaningful, roughly 0.2 percent annual chance of flooding.
• These labels describe a statistical likelihood, not a strict interval; a property in a "100-year" flood plain can flood more than once in a shorter period if conditions are severe enough.

Being located within a mapped flood plain can affect a property's resale value and the cost of ownership well beyond the insurance premium itself, since mandatory flood insurance, stricter building rules, and buyer awareness of flood history can all make a flood plain property less attractive, all else equal, than a comparable property outside the mapped area. Sellers and buyers increasingly check official flood maps, alongside a property's prior flood claim history where available, as a standard part of due diligence before a purchase, much as they would check a title or lien status on the property.

Many local governments restrict or add special conditions to construction within mapped flood plains, such as requiring elevated foundations, flood-resistant materials or restrictions on basements, to reduce future damage and the strain major floods place on emergency services, banks holding mortgages in the area, and insurance programs. Development pressure in growing cities has nonetheless pushed significant construction into flood plain areas over recent decades, which several flood researchers point to as a factor in rising flood insurance claims even without any change in actual rainfall patterns.

A property's position within a mapped flood plain is one of the single biggest factors in whether flood insurance is required and how much it costs, since insurers and government flood insurance programs base premiums heavily on modeled flood probability at a specific location rather than on a flat nationwide rate. As flood maps are updated to reflect changing rainfall patterns, urban development that changes drainage, and new engineering studies, properties can be reclassified into a higher or lower risk zone even without any physical change to the land itself, which can meaningfully change the insurance cost a homeowner faces from one renewal to the next. A buyer evaluating a property's long-term affordability is generally advised to factor in the possibility of a future flood zone reclassification alongside more familiar costs such as interest rates and property taxes, rather than assuming the current flood zone and premium will remain unchanged for the life of a loan.

• Federal Emergency Management Agency — Flood Maps