A levee is a raised landform found alongside bodies of water such as rivers, lakes, or coastlines. These structures play a role in influencing the movement and containment of water. They contribute to managing water levels, which affects both natural environments and human settlements.
Defining Levees
A levee refers to an elevated ridge or embankment positioned adjacent to a waterway. These features serve as barriers that influence water flow and prevent inundation of nearby areas. The term itself originates from the French word “levée,” meaning “to raise.” Levees can be naturally occurring formations, shaped by geological processes over long periods, or they can be human-made structures constructed for specific purposes. This fundamental classification highlights the two primary ways these elevated banks come into existence.
Levees are essentially walls that prevent water from overflowing its typical course. They are usually earthen and often run parallel to a river’s path within its floodplain, or along low-lying coastal regions. While both natural and artificial levees share the common role of elevating the bank of a water body, their formation processes, materials, and overall scale can differ significantly.
Natural Levees: Formation and Characteristics
Natural levees develop through a gradual process of sediment deposition during flood events. When a river overflows its banks, the water spreads out onto the floodplain, causing a rapid decrease in its velocity and, consequently, its ability to carry sediment. The heavier, coarser materials, such as sand and silt, are deposited immediately adjacent to the river channel where the flow loses energy quickly. This continuous accumulation of sediment over many flood cycles builds up elevated ridges along the riverbanks.
The composition of natural levees typically includes a mix of sand, silt, and mud, with coarser particles found closer to the channel and finer sediments settling further away. These natural embankments usually exhibit a gentle slope away from the river, forming elongated ridges that parallel the watercourse. While natural levees can vary in height, sometimes reaching several meters above the floodplain, their formation takes hundreds to thousands of years. Their presence can also lead to the riverbed building up, occasionally causing the river to flow at a higher elevation than the surrounding floodplain.
Artificial Levees: Construction and Role
Artificial levees are engineered structures primarily built to protect communities, agricultural lands, and infrastructure from flooding. These human-made barriers prevent water from inundating surrounding areas, safeguarding existing developments that would otherwise be at risk during high water levels.
The materials used in artificial levee construction vary, but they commonly include compacted earth, such as soil and clay, which forms a wide base that tapers to a narrower top. Some levees may also incorporate stronger materials like concrete, stone, wood, or metal, particularly in areas requiring greater resilience or where space is limited. For instance, floodwalls, a type of levee, are often made of concrete and are used in urban settings. To prevent erosion from strong currents or waves, the surfaces of artificial levees are often covered with vegetation, like Bermuda grass, or reinforced with revetments made of stones or concrete.
These constructed levees serve to confine the flow of a river, which can result in higher and faster water flow within the channel. They are strategically designed to withstand specific flood events, though exceptionally high water levels can still pose a challenge, potentially leading to overtopping or breaches. Artificial levees have been utilized for millennia, with ancient civilizations in regions like the Indus Valley, Egypt, and China building them to manage water and protect settlements. Today, these structures are found globally, playing a significant role in flood control and land management.