What Determines A Flood Zone

Flooding poses many dangers.

Flood zone designation is a complex task that takes into account many aspects of an area, from its geology to flood frequency and additional hazard assessment.


A flood zone is defined as the land bordering a river, stream or other body of water that is subject to floods of equal frequency. It can be designated as part of the floodplain, which includes all of the land adjacent to a river (or coastline) that experiences occasional or frequent flooding.


The Federal Emergency Management Agency (FEMA) has designated four main kinds of flood zones: moderate to low risk, high risk, coastal zones and undetermined. Areas undergo a flood hazard analysis for risk assessment.


When FEMA or insurance companies designate flood zones, many factors are taken into consideration including drainage area, community protection by levees and dams, velocity hazards, potential high storm wave damage and storm surge.


The geology of the area is also essential in determining a flood zone. The permeability of the ground and topography of the region both affect how efficiently water moves through the system. The geologic record also can provide a history of flooding for a specific area, allowing for flood frequency calculations.


Flooding frequency is a major factor in determining a flood zone. If an area has a one percent chance of flooding in any given year, then it will experience a flood every one hundred years, on average. The higher the frequency, the greater chance for flooding. However, less-frequent floods are usually more devastating: FEMA-designated high-risk zones have a one percent or greater chance of flooding.

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The National Flood Insurance Program (NFIP) is in charge of mapping flood zones. They are available on a community’s Flood Hazard Boundary Map or Flood Insurance Rate Map (FIRM). A home mortgage should also provide flood zone information. To determine flood risk for a property, visit the NFIP’s online database of flood risk.