A levee is an elevated embankment, either natural or human-made, constructed parallel to a river or along coastlines. Its main function is to protect adjacent land from flooding, acting as a physical barrier. Levees are designed to contain water within a specific channel, safeguarding communities, agricultural areas, and infrastructure from the impacts of flooding. These structures are a key component of flood management strategies worldwide, providing security against water events.
Levee Structure and Materials
A typical artificial levee is an earthen embankment, built from compacted soil, clay, sand, and gravel. Its design features a wide base that tapers to a narrower top, or crown, which can serve as an access road for maintenance. The slopes of the levee, both on the waterside and the landside, are engineered for stability and to resist erosion. Often, the landside slope is gentler for stability and easier maintenance, while the waterside slope may be steeper.
Many levees incorporate an impervious core, typically made of compacted clay or other low-permeability materials, positioned centrally or near the waterside. This core significantly reduces water seepage through the levee body. The surrounding earth fill provides the mass and stability to withstand water pressure. These materials work together to create a robust structure.
Principle of Flood Containment
Levees contain floodwaters by physically blocking their lateral spread and counteracting hydrostatic pressure. The levee’s mass and broad base resist this outward force. Water can still move through or under the levee, a process known as seepage. Engineered features manage this.
Impervious cores, often made of clay, extend downward into the levee’s foundation to create a longer, more tortuous path for water. For water that seeps beneath the levee, especially through permeable foundation layers, toe drains and relief wells are installed on the landward side. Toe drains, typically gravel-filled trenches, collect shallow seepage and release it safely. Deeper relief wells are drilled to relieve uplift pressure from water accumulating beneath the levee’s foundation, preventing conditions like “sand boils” that can compromise stability. The levee’s height also includes a “freeboard,” an additional margin above the anticipated flood level, accounting for waves or unexpected water level increases.
Maintaining Levee Integrity
Maintaining a levee’s integrity involves regular inspections and proactive management. Visual inspections are conducted annually, with more rigorous periodic assessments typically occurring every five years. These inspections check for signs of damage, such as cracks, erosion, or animal burrows, that could compromise its structural soundness. Promptly addressing these issues prevents minor problems from escalating.
Vegetation management focuses on promoting healthy grass cover while controlling deep-rooted trees and shrubs. Grass helps to bind the soil and prevent surface erosion from rain and wind, whereas large tree roots can create pathways for water seepage or destabilize the embankment if the tree falls. Erosion control measures, such as riprap (large rocks) on slopes exposed to water flow or wave action, protect the levee surface. Continuous monitoring for indicators of instability, like slumping sections or the appearance of sand boils, allows for timely intervention and repair, preserving its protective ability.