A levee is an engineered embankment, typically constructed from compacted earth, that runs parallel to a body of water such as a river or coastline. Built to a specific height and width, the structure is an artificial extension of the natural bank. Its purpose is to protect adjacent land, communities, and agricultural areas from water overflow during periods of high water.
Containment and Flood Control
The fundamental action of a levee is to increase the capacity of the natural water channel. By raising the banks of a river, the levee allows a greater volume of water to flow through the channel without spilling over into the surrounding floodplain. This containment excludes floodwater from a defined protected area.
Levees are engineered to manage water up to a specific height, which is determined by a calculation known as the “design flood.” This design flood represents the maximum water level the structure is intended to withstand based on a predicted flood event, such as a 100-year or 500-year flood.
The levee physically interacts with the water, constraining its movement and directing the flow downstream. While the structure manages the flow up to its design limit, concentrating the water within a narrower path can increase the speed and height of the water moving through the channel.
Essential Structural Components
An effective levee requires a carefully constructed internal anatomy to resist pressure and saturation from floodwaters. The base of the embankment is significantly wider than the crest to provide stability, often expressed through a shallow slope ratio (e.g., one vertical unit for every two to five horizontal units). The core is typically built from less permeable materials, such as compacted clay, which acts as a barrier to limit water movement.
The height of the levee includes a safety margin known as the “freeboard,” which is the vertical distance between the calculated design flood water level and the top of the embankment. Engineers often require this margin to be one to three feet above the expected flood height to account for waves, unforeseen surges, and measurement uncertainties. The exterior surfaces of the levee, particularly the waterside slope, are protected with vegetation, rip-rap, or concrete to resist erosion caused by water velocity and wave action.
Toe drains or seepage control features are often incorporated into the design, particularly in areas with permeable foundation soils, to manage water filtering through the embankment or foundation. Stability berms, constructed as wide earthen extensions at the base, may be added to increase the structure’s weight and lengthen the path water must travel to seep out, reducing internal water pressure.
How Levees Fail
Levees can cease to function effectively through three primary mechanisms. The most straightforward mode of failure is “overtopping,” which occurs when the flood water level exceeds the freeboard and flows over the levee crest. Once water flows over the top, it quickly erodes the unprotected landside slope of the earthen embankment, leading to a rapid breach.
A second common failure mechanism is known as “seepage” or “piping,” where water filters through the levee embankment or its foundation soil. As the water moves, it exerts pressure and begins to carry fine soil particles with it, gradually forming small, subsurface tunnels or “pipes.” This internal erosion weakens the structure from within and, if left unchecked, can cause a sudden collapse of a section of the levee wall.
The third mechanism is a structural breach, often triggered by the effects of piping, foundation instability, or mass movement of the embankment material. Prolonged saturation can significantly reduce the shear strength of the soil, making it susceptible to sliding. Foundation defects or high internal water pressure can cause a section of the levee to slump or give way entirely, releasing a torrent of water into the protected area.