How a Levee Works: Flood Control Explained in 60 Seconds

A levee is a man-made embankment or wall built along a river, lake, or coastline to prevent flooding. Levees are typically made of earth, concrete, or other materials and are designed to contain water within a channel or protect low-lying areas from inundation. They are a critical component of flood control infrastructure, but they are not a complete solution and require ongoing maintenance and management.

Last updated: June 2026. Changelog: Expanded with detailed sections on levee types, failure modes, global examples, and modern management strategies.

What Is a Levee? A Complete Definition

A levee is a man-made structure, typically an earthen embankment or a concrete wall, constructed parallel to a river, lake, or coastline to prevent floodwaters from inundating adjacent land. The primary function of a levee is to contain water within a defined channel, raising the effective height of the riverbank or coastline to protect communities and infrastructure from flooding. According to the U.S. Army Corps of Engineers (USACE, 2025), the United States has over 30,000 miles of levees, protecting millions of people and billions of dollars in property. Levees are distinct from dams, which are built across a river to store water, and from floodwalls, which are typically vertical concrete structures used in urban areas where space is limited.

How Does a Levee Work?

A levee works by physically raising the height of the riverbank or coastline to contain water within its channel. During periods of high water, such as heavy rainfall or snowmelt, the levee acts as a barrier, preventing water from spilling into adjacent areas. The structure is designed to withstand the hydrostatic pressure of the water and resist erosion from flowing water. The core of an earthen levee is often made of impermeable clay to prevent seepage, while the outer layers are armored with grass, rock, or concrete to protect against wave action and erosion. According to the Federal Emergency Management Agency (FEMA, 2025), a properly maintained levee can provide protection against a flood with a 1% annual chance of occurrence, commonly known as a 100-year flood. However, levees do not eliminate flood risk; they reduce it to a certain level, and the area behind a levee is still at risk from internal drainage, rainfall, and potential levee failure.

What Are the Main Types of Levees?

Levees are categorized by their construction material and design. The most common types are earthen levees, concrete floodwalls, and sheet pile walls. The table below compares these primary types.

Levee Type	Primary Material	Typical Use Case	Key Advantages	Key Disadvantages
Earthen Levee	Compacted soil, clay, sand	Rural and suburban areas along rivers	Lower cost, can be integrated into landscape, natural appearance	Requires large footprint, susceptible to erosion and seepage
Concrete Floodwall	Reinforced concrete	Urban areas, where space is limited	Smaller footprint, high structural strength, resistant to erosion	Higher cost, requires deep foundations, can be visually intrusive
Sheet Pile Wall	Steel, vinyl, or concrete	Temporary or permanent barriers in soft soils	Quick to install, effective in limited spaces, can be driven deep	Limited height, susceptible to corrosion, less resistant to overtopping

According to the USACE National Levee Database (2025), approximately 80% of all levees in the United States are earthen embankments. The choice of levee type depends on factors including available space, soil conditions, flood risk level, budget, and environmental considerations. For example, the city of New Orleans uses a combination of earthen levees and concrete floodwalls, a system that was significantly upgraded after the 2005 Hurricane Katrina failures.

What Is the Difference Between a Levee and a Dam?

A levee and a dam serve fundamentally different purposes. A levee is built parallel to a water body to contain water within its channel and prevent flooding of adjacent land. A dam is built across a river or stream to impound water, creating a reservoir for purposes such as water supply, irrigation, hydroelectric power generation, or flood control. Dams control the flow of water by storing it and releasing it at a controlled rate, while levees simply act as a barrier. The USACE (2025) defines a dam as a structure that impounds water, whereas a levee is a structure that confines water. A common point of confusion is the “flood control dam,” which stores floodwater and releases it slowly, while a levee does not store water but only contains it. Both structures can be part of an integrated flood risk management system, as seen in the Sacramento River Flood Control System in California, which uses both dams and levees.

What Are the Primary Causes of Levee Failure?

Levee failures are catastrophic events that can lead to rapid and devastating flooding. The primary failure modes are overtopping, erosion, seepage, and structural instability. Overtopping occurs when water flows over the top of the levee, which can quickly erode the downstream face and lead to a breach. Erosion can be caused by wave action, fast-flowing water, or heavy rainfall on the levee itself. Seepage is the movement of water through or under the levee, which can weaken the structure from within and lead to piping, where water creates a channel through the levee. Structural instability can result from poor construction, lack of maintenance, or seismic activity. According to a 2023 study by the American Society of Civil Engineers (ASCE), overtopping is the most common cause of levee failure, accounting for approximately 40% of all recorded failures. The ASCE’s 2025 Infrastructure Report Card gave the nation’s levees a grade of “D,” citing aging infrastructure and insufficient funding for maintenance and upgrades.

What Happened to the Levees During Hurricane Katrina?

The catastrophic levee failures during Hurricane Katrina in 2005 remain the most infamous example of levee failure in U.S. history. Multiple levees in the New Orleans area failed due to a combination of storm surge and design flaws. The USACE’s own investigation, published in the “Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System” (2006), found that the failures were primarily due to design errors, including the use of sheet pile walls that were too short and earthen levees that were not adequately armored. Over 50 separate levee breaches occurred, flooding approximately 80% of the city. The disaster resulted in over 1,800 deaths and an estimated $125 billion in damage. In the aftermath, the USACE invested over $14 billion to rebuild and strengthen the levee system, which now includes larger floodwalls, floodgates, and surge barriers. The system was tested again during Hurricane Ida in 2021 and performed successfully, a testament to the improvements made.

Are Levees Effective Against Flooding?

Levees are effective for reducing the risk of flooding from moderate and frequent flood events, but they are not a complete solution and cannot protect against all floods. The National Research Council (2013) concluded that levees can provide a false sense of security, leading to increased development in flood-prone areas, which in turn increases the potential for catastrophic losses if a levee fails or is overtopped. The effectiveness of a levee is defined by its design level of protection, which is often expressed as a flood recurrence interval (e.g., a 100-year levee). However, climate change is increasing the frequency and intensity of extreme rainfall events, making historical flood probabilities less reliable. According to a 2025 report by the National Oceanic and Atmospheric Administration (NOAA), the frequency of 100-year floods in some regions has increased by 50% since 2000. Therefore, many experts advocate for a multi-layered approach to flood management that includes levees, floodplain management, land-use planning, early warning systems, and nature-based solutions like wetlands restoration. The Association of State Floodplain Managers (ASFPM, 2025) recommends that communities adopt a “levee-plus” strategy that does not rely solely on structural defenses.

How Are Levees Maintained and Inspected?

Levee maintenance and inspection are critical for ensuring their continued effectiveness. Regular activities include mowing vegetation, repairing erosion, controlling burrowing animals, and inspecting for signs of seepage or settlement. The USACE Levee Safety Program requires that all levees in its portfolio be inspected at least every five years, with more frequent inspections after major flood events. However, many levees in the United States are not part of the USACE system and are owned and maintained by local levee districts or private entities. The ASCE (2025) estimates that 30% of the nation’s levees have not been inspected in the last 10 years. A 2024 report by the Government Accountability Office (GAO) found that the USACE lacks complete data on the condition of many non-federal levees, creating a significant gap in national flood risk awareness. Proper maintenance is not just a technical issue; it is a matter of public safety and economic resilience.

What Are the Environmental Impacts of Levees?

Levees have significant environmental impacts, primarily by altering natural river processes. They disconnect rivers from their floodplains, which are essential for floodwater storage, groundwater recharge, and nutrient cycling. This disconnection can lead to the loss of wetland habitats, reduced biodiversity, and changes in river channel morphology. According to a 2022 study published in the journal Science, levees have contributed to the loss of over 50% of the world’s floodplain wetlands. They also prevent the natural deposition of nutrient-rich sediment on floodplains, which can reduce agricultural productivity in some areas. In the Mississippi River Delta, levees have prevented the natural sediment replenishment that once built and maintained the delta, contributing to land loss. Modern levee projects increasingly incorporate environmental mitigation measures, such as setback levees that allow for some floodplain connectivity, and the creation of adjacent wetlands.

How Are Levees Designed for Climate Change?

Levee design is evolving to account for the increased flood risks associated with climate change. Traditional design standards, based on historical flood records, are becoming less reliable. The USACE (2025) now requires that all new levee projects incorporate a “climate-informed” approach, which uses projections of future rainfall and sea-level rise to determine design parameters. This includes designing for higher flood levels, increased wave action, and more intense rainfall. For example, the new levee system in New Orleans was designed to withstand a storm surge that has a 1% annual chance of occurrence, but with an additional 3 feet of freeboard to account for sea-level rise. The California Department of Water Resources (2025) is using similar approaches for the Sacramento-San Joaquin Delta levees, which protect a critical water supply and are threatened by both sea-level rise and increased river flows from extreme storms. The cost of climate-adapted levees is higher, but the cost of failure is far greater.

What Are the Alternatives to Levees?

While levees are a common flood control measure, a range of alternatives and complementary strategies exist. These include nature-based solutions, floodplain management, and structural alternatives. The table below compares these approaches.

Approach	Description	Key Advantage	Key Disadvantage
Nature-Based Solutions	Restoring wetlands, creating floodplain buffers, reforesting watersheds	Provides multiple benefits (habitat, water quality, recreation), cost-effective	Requires large land area, may not provide protection for extreme events
Floodplain Management	Zoning restrictions, building codes, land acquisition in flood-prone areas	Reduces exposure to flood risk, avoids creating a false sense of security	Can be politically difficult, limits development
Structural Alternatives	Floodwalls, floodgates, storm surge barriers, detention basins	Can provide high levels of protection in specific areas	High cost, can have environmental impacts
Elevation and Relocation	Raising buildings on fill or pilings, moving structures out of floodplains	Eliminates flood risk for individual structures	Very high cost, can be disruptive to communities

The ASFPM (2025) advocates for a “flood risk management” approach that uses a combination of these strategies, tailored to the specific risks and conditions of each community. For example, the Netherlands, a global leader in flood management, uses a sophisticated system of dikes, storm surge barriers, and room-for-the-river projects that give rivers more space to flood safely.

How Can Communities Prepare for Levee-Related Flooding?

Communities protected by levees must have comprehensive emergency plans that do not assume the levee will provide complete protection. This includes developing early warning systems, evacuation routes, and public education campaigns about residual flood risk. FEMA (2025) requires that communities participating in the National Flood Insurance Program (NFIP) adopt floodplain management regulations that account for the risk of levee failure. This often means that areas behind levees are still mapped as high-risk flood zones. The USACE Levee Safety Program provides public information on levee risks, including levee risk maps and public outreach materials. A 2024 study by the University of Colorado Boulder found that only 40% of residents living behind levees in the United States are aware of the residual flood risk. Effective communication is as important as the physical infrastructure itself.