A levee is an elevated embankment constructed parallel to a river or coastline, designed to reduce the risk of flooding. These structures provide a barrier that contains rising water levels and protects adjacent low-lying areas. Levees safeguard human settlements, infrastructure, and agricultural lands from inundation during periods of high flow or storm surge.
Defining the Levee and Its Core Purpose
A levee is a ridge of compacted material used to control or divert the flow of water. This structure is distinct from a dam, which is built across a body of water to create a reservoir. Levees run alongside a water channel, typically having water on only one side, acting as a linear shield against overflow.
Levees are categorized as natural or artificial. Natural levees form over centuries as floodwaters deposit sediment and silt along riverbanks, creating slightly raised ridges. Artificial levees are man-made structures engineered to a specific height and strength to protect areas from a predetermined flood event. The ultimate goal of an artificial levee system is to contain seasonal high water, protecting developed land that would otherwise be part of the active floodplain.
Structural Components and Construction
Artificial levees are constructed from compacted earthen materials, including clay, sand, and gravel, though some modern systems incorporate concrete or steel floodwalls. The structure is designed with a trapezoidal cross-section to distribute hydraulic pressure evenly and ensure stability. This shape includes a wide base, gently sloped sides (or “banks”), and a flat top surface known as the crown.
The slopes of the levee, often designed at a ratio like one vertical unit to three horizontal units, are crucial for stability and to resist erosion from wave action and currents. To prevent water from seeping through the embankment, an impervious core or cut-off wall is integrated into the design. These internal barriers limit saturation and maintain the structural integrity of the levee body.
Mechanism of Flood Control
The mechanism by which a levee controls flooding is by raising the effective height of the riverbank, increasing the water-holding capacity of the channel. By confining the water to a narrower, deeper path, the levee prevents the river from spreading onto the floodplain during high-water events. Engineers design the levee height to include a safety margin, known as “freeboard,” which is an extra vertical distance above the calculated peak flood level.
While levees protect the land directly behind them, they also introduce specific hydraulic consequences to the river system. By restricting the channel, the levee can increase the velocity and height of the water flow. This effect can transfer the flood risk downstream or cause a shift in the location where water eventually inundates the surrounding landscape.
Recognizing Levee Failure and Necessary Maintenance
Levees can fail through several different mechanisms. Overtopping occurs when the water level exceeds the height of the levee crown, causing water to flow over the top. This flow rapidly erodes the landside slope, leading to a breach and collapse of the embankment.
A major failure mode is seepage or piping, which involves water leaking through the levee body or its foundation. This seepage can wash away fine soil particles, forming small, internal channels that grow into larger subsurface tunnels, eventually weakening the structure until it collapses. Surface erosion of the slopes, caused by strong currents, waves, or inadequate protective cover, can also undermine the levee’s stability.
Maintaining a levee system involves continuous management and monitoring to prevent these failures. Regular inspections look for signs of animal burrows, which provide direct paths for piping to begin, and check for cracks or surface slumping. Vegetation management ensures that shallow-rooted grasses stabilize the surface while preventing large-rooted plants from compromising the internal structure.