A natural levee is a raised bank of sediment that runs parallel to the main channel of a river. This elongated ridge is built up over many years, creating a topographical feature bordering the watercourse. Understanding how this feature interacts with floodwaters is central to comprehending river dynamics and associated risks.
The Mechanics of Natural Levee Formation
Natural levees are constructed by the river itself when water rises and overtops its banks. As floodwater leaves the channel and spreads across the floodplain, its velocity decreases instantaneously. This reduces the water’s capacity to carry its sediment load.
The heaviest and coarsest materials, such as sand and silt, drop out of suspension first, accumulating right at the channel’s edge. Finer sediments are carried further across the floodplain before settling. Repeated flood events deposit new layers of this coarse material, gradually building up the bank’s height. This results in a gentle slope away from the river, with the thickest deposits closest to the channel.
Primary Impact: River Channel Containment
The most observable effect of a natural levee is that it elevates the river’s effective bank height, increasing the amount of water the channel can hold. This barrier confines the flow, protecting the broad lowlands of the floodplain. By raising the spillover point, the levee significantly reduces the frequency of small-to-moderate flood events.
This containment allows adjacent land to remain dry during typical seasonal rises, preserving areas for human development and agriculture. The levee increases the river’s capacity to convey water without causing widespread inundation. It also increases the speed of the water within the channel because the flood flow is restricted to a narrower path, allowing it to move quickly downstream.
Secondary Impact: Floodplain Drainage Obstruction
While the levee contains river water, it also acts as a barrier to water flow originating from the floodplain side. Water accumulating behind the levee—from heavy rainfall, tributaries, or groundwater seepage—is trapped because the levee stands higher than the surrounding land. This creates a drainage issue known as the backwater effect.
The trapped water pools in low-lying areas, or swales, located immediately behind the levee ridge. Since the levee prevents this pooled water from draining back into the lower river channel, these areas can remain saturated or flooded for extended periods. This prolonged inundation leads to localized flooding and high water tables, causing agricultural losses and infrastructure problems.
Catastrophic Failure and Breach Flooding
The containment beneficial during minor floods creates a hazard during extreme events. By confining large volumes of water, the levee forces the river to reach a much higher stage, increasing pressure on the bank structure. When the flood stage exceeds the levee height, water overtopping the crest can rapidly erode the structure from the landward side.
A sudden risk occurs when a section of the levee is structurally compromised and breaches, often due to erosion or saturation. This failure releases a massive volume of water onto the floodplain with speed and force. The resulting flood is often more devastating than a typical overbank flood because the water is released rapidly, generating a powerful flood wave. Evidence of these breaches is visible as large, fan-shaped deposits of coarse sediment, known as crevasse splay deposits.