A levee is an engineered embankment constructed parallel to a river, stream, or other body of water. Its primary function is to serve as a barrier, containing the water and preventing it from spreading onto the adjacent land during periods of high flow. This structural intervention protects communities and agricultural areas from floods.
How Levees Control Water Flow
The fundamental mechanism of a levee relies on increasing the effective capacity of the river channel. By building up the banks, the levee raises the height water must reach before it can overtop the structure and inundate the surrounding floodplain. This provides an elevated buffer against rising water levels caused by heavy rainfall or snowmelt.
During a high-water event, the levee confines the increased volume of water to a narrower pathway. This concentration of flow maintains the river’s path, protecting the lower-lying areas behind the embankment from flooding. The containment also prevents the lateral spreading of water, which can help maintain the navigable depth of the river channel.
The effectiveness of this control is directly related to the structural height of the levee compared to the expected flood stage. Engineers calculate a “design flood” level and ensure the structure is built significantly higher to maintain a safety margin, known as freeboard. Freeboard is an additional height allowance used to compensate for variables, acting as a buffer.
The Physical Structure of Levees
The typical artificial levee is constructed predominantly from locally available, compacted earthen materials, such as soil, sand, and clay. Clay is often preferred due to its low permeability, which minimizes the seepage of water through the embankment. The construction process involves layering and compacting these materials to achieve a dense and stable core.
A levee’s cross-section is designed with specific geometrical features to ensure stability against the lateral pressure of the water. The structure features a wide base, sloping sides (landside and waterside slopes), and a flat top known as the crest or crown. These slopes are generally gentle to prevent slumping, and they are often covered with vegetation or rock to prevent erosion.
The foundation is a particularly important component, as it must support the weight of the embankment and prevent water from seeping underneath the structure. Sometimes, a “cutoff wall” or trench is installed deep into the ground beneath the levee to block subsurface flow paths. Berms, which are extensions on one side, are sometimes added to stabilize the structure further by flattening the slope and increasing the weight of the toe.
Distinguishing Levees from Other Flood Barriers
While “levee” is often used interchangeably with “dike,” levees specifically refer to embankments along rivers, while dikes often describe barriers used for land reclamation or coastal defense. The defining characteristic of a levee is its alignment, running parallel to the river channel it protects.
Levees differ significantly from dams, which are massive structures built across a river channel to impede the flow of water and create a large reservoir upstream. Dams primarily manage water flow rates and storage, whereas levees only serve to contain the water during flood events. Dams typically have water on both sides, while levees are designed to protect the dry land on one side.
Floodwalls represent another type of barrier, typically found in urban environments where space is limited. Unlike the wide, earthen structure of a levee, a floodwall is a vertical barrier, often made of reinforced concrete, that offers a high degree of protection in a smaller footprint. Each structure serves a distinct purpose in hydraulic engineering.
Geographic Impact on River Systems
The installation of extensive levee systems fundamentally alters the natural geography of a river. By confining the flow, levees disconnect the river from its natural floodplain, which historically acted as a natural sponge, absorbing excess water and reducing flood peaks. This loss of floodplain storage can lead to higher water levels within the constrained channel.
Preventing the river from accessing its floodplain stops the natural process of sediment deposition that occurs during overbank flows. This sediment, rich in nutrients, normally contributes to the fertility of the surrounding land, but instead, the sediment load is carried further downstream. This process can cause silting up of river mouths or require dredging to maintain navigation.
By narrowing the river’s effective cross-section, levees increase the velocity of the water flowing through the channel. This increased speed and volume can exacerbate flood conditions immediately downstream, transferring the risk to other communities. The long-term consequence is an engineered system that requires continuous maintenance and adaptation to the changing flow dynamics it creates.