Do I live behind a levee?
If you live near a major river or other body of water, there is a good chance that a levee may be nearby. Levees have been built in all 50 states, and can be found in 22% of counties nationwide. Currently there is no national inventory of levees, so a single central search is impossible.
Here are some ways you can find out if you live behind a levee:
- Contact a local government agency, such as your public works department, flood control district, water commission or levee board to find out about whether levees are nearby and if your home depends on them to reduce the risk of flooding.
- Check www.floodsmart.gov and take the "One Step Flood Risk Profile" quiz. In it, you enter your address to learn if you live in an area at risk of flooding. If so, nearby levees may appear on the Flood Insurance Rate Map created by the U.S. Federal Emergency Management Agency (FEMA) to show the flood risk in your area. Follow the links on the site to FEMA's Map Service Center to view or download the map for your area. While this is helpful information, it should be kept in mind that levees are not always depicted on flood maps.
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What is a levee?
Levees are designed to a certain level of flood event. We have learned that while communities can employ model planning and engineering, Mother Nature can — and eventually will — always throw a curveball.
What is a levee? If your property or community is facing imminent flooding, you will look to any of the following levee or levee-like structures to reduce your risk of flooding.
- A pile of sandbags or dirt forming a wall to ward off approaching floodwaters.
- A highway or railroad embankment running parallel to a river that is integral to the performance of a flood damage reduction system.
- A structure built to reduce the risk and impact of flooding, meeting specific engineering design, construction and operations and maintenance criteria.
- A temporary emergency measure. A permanent engineered structure. A temporary emergency measure that was never removed.
- All of the above.
Levees are not subject to consistent standards for design, construction, operations, and maintenance, and you cannot easily tell how a levee will perform in a flood by observation alone. While levees under the authority of the Corps of Engineers are regularly inspected and assessed for their level of protection, this may represent only a small fraction — an estimated 15% of the total levees in the United States. The rest are literally an unknown — we don't know where they are, what condition they are in, or how they might perform in a flood.
Levees function as part of a system — what is happening upstream and downstream matters. A levee in a rural agricultural area may be designed to overtop in order to release floodwaters before it reaches levees protecting a larger population where flood damages may be greater. Even the unexpected breach or levee failure in one part of the system may reduce the pressure and save a levee in the system from failing in another section.
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When it floods behind a levee
Floodwater behind a levee can generally be attributed to water overtopping the levee, the levee breaching and letting floodwaters rush through, or the failure of internal drainage or pumping systems that are meant to keep water on the water-side of the levee.
In overtopping, the level of the flood water is higher than the height of the levee. Levee overtopping can cause significant damage, but there is usually sufficient time to prepare for the flooding, including evacuation if advised. During a flood event, the top of the levee may be raised temporarily by sandbags to prevent overtopping.
In a breach, a section of the levee collapses, breaks, or is washed away, allowing water to flow through the levee. An unexpected breach can cause rapid flooding and put greater numbers of people at risk who have been caught unprepared. In a flood, special attention may be paid to known weak spots or problem areas in the levee, and residents advised of the high risk of a breach in the levee by local emergency management personnel.
Flooding when internal drainage systems fail is generally not severe because the bulk of the water remains on the water side of the levee, but damages to property can occur.
For those who live or work behind a levee, the amount of time you have to prepare can make a significant difference. Studies have indicated that as little as 1 hour of notice can result in a ten-percent reduction in flood damages and lives saved. Saving property with less than 18 hours lead time is generally restricted to moving highly valued property, such as automobiles and major appliances, out of harm's way. When lead times are longer than 18 hours, floodplain residents can flood proof and flood fight (construct temporary levees, place sand bags, etc.) and evacuate well ahead of the flood.
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Understanding levee failures
"Levee failure" implies that something about the levee failed to prevent flooding on the land side of the levee. If you ask an engineer, they will tell you there are many reasons for a levee failing — and then they will probably go into some detail about how or why the levee did not hold back the floodwaters.
In overtopping, the level of the flood water is simply higher than the height of the levee, and water flows over the top. Up to date surveys of the height of the levee relative to its surroundings and awareness of any low areas at the top of the levee are important in reducing unexpected overtopping. During a flood event, the top of the levee may be raised temporarily by sandbags to prevent overtopping.
If a levee is overtopped and the land side of the levee is not armored or reinforced, the waters can undercut the levee and cause it to collapse or breach. Examples of levee armoring include specific grasses, pavers or bricks, and in cases of emergency, plastic sheeting.
Water flowing through or under a levee will both weaken the levee and cause flooding on the land side. Water can flow through animal tunnels, along channels in the soil left by root systems, or through poorly compacted or sandy soils. Infiltration of water under the soils supporting the levee can cause "liquefaction" and collapse of the levee itself. "Sand boils" on the land side of the levee are an indication of water seepage.
The force of water on the levee can cause the levee to fail by eroding the water side of the levee through wave action or scouring. In cases of extreme pressure on the levee, the water can actually push the levee from its original position, causing it to fail.
A levee may be breached by an object hitting it or when an object on the levee, such as a tree or building, falls and pulls part of the levee out with it, destabilizing the structure.
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