Levees, Dams, and Flood Zones

A levee is a man-made structure or engineered fill designed to prevent floodwater from reaching the area behind it. Levees are built along rivers, channels, and coastlines. On FEMA flood maps, the flood zone designation for the area behind a levee depends entirely on whether the levee meets FEMA's accreditation standards.

FEMA does not own, maintain, or set construction standards for levees. The agency only determines whether a levee meets the criteria to be recognized on the flood map. Levee maintenance and certification are the responsibility of the levee owner (typically a local levee district, municipality, or the US Army Corps of Engineers).

To be recognized by FEMA on a flood map, a levee must meet all of the following accreditation requirements under 44 CFR 65.10:

• Freeboard: The levee must provide at least 3 feet of freeboard above the 1% annual chance flood elevation (the base flood). Freeboard is the additional height above the BFE that provides a safety margin.
• Closure devices: All openings in the levee (such as gates, pumping stations, and roadway crossings) must have certified closure devices that can be operated in time to prevent floodwater from passing through.
• Embankment stability: The levee embankments and foundations must be certified by a professional engineer as resistant to erosion, seepage, settlement, and structural instability under flood conditions.
• Settlement: The levee must be certified against future loss of freeboard due to settling of the embankment or foundation materials.
• Operation and maintenance plan: The levee must have an active operation and maintenance plan, and the responsible entity must demonstrate the capacity to carry it out.

Zone AR is a special SFHA designation for areas that were previously protected by a levee or flood control system that has been decertified and is actively being restored. Zone AR acknowledges that the area is temporarily at higher risk while the restoration project is underway.

Once the levee is restored and re-accredited, FEMA remaps the area behind it, typically moving it back out of the SFHA. During the AR designation period, flood insurance is required for federally-backed mortgages, but specific rating provisions apply.

Dams are built across watercourses to impound water for flood control, water supply, hydropower, recreation, or other purposes. The Flood Insurance Study (FIS) for your community documents any dams that affect flood flows in the study area, including their type, dimensions, purpose, and historical performance.

Dams can reduce downstream flood flows by retaining and slowly releasing floodwater. If a dam provides recognized flood protection, the downstream area may be mapped in a lower-risk zone. However, dams also introduce a risk that is not shown on FEMA flood maps: dam failure.

Even when a levee is accredited or a dam provides recognized flood protection, residual flood risk remains. No flood control structure provides absolute protection.

• Overtopping: Floods larger than the design event can overtop a levee or exceed a dam's capacity. An accredited levee provides 3 feet of freeboard above the base flood, but a more extreme event could exceed that margin.
• Structural failure: Levees and dams can fail due to erosion, seepage, foundation problems, or lack of maintenance. Failure can happen during a flood or even under normal conditions.
• Interior drainage: Even when a levee holds, rainfall on the protected side can cause interior flooding if the pumping or drainage systems cannot keep up.
• False sense of security: Properties behind levees often have lower rates of flood insurance, which means residents face greater financial exposure if the levee fails or is overtopped.

FEMA and the US Army Corps of Engineers recommend that property owners behind levees maintain flood insurance even when it is not required. The cost of a Zone X policy is relatively low, and it provides financial protection against events that exceed the levee's design capacity.