A levee is an engineered or natural embankment, typically constructed from compacted earth, that runs parallel to a river or coastline. Its primary function is to contain water flow and reduce the frequency and extent of flooding in the adjacent floodplain. Levees provide flood protection, shielding populated areas, infrastructure, and agricultural land from high water events. When the pressure of floodwaters exceeds the levee’s structural capacity, a breach occurs. This failure releases a massive, sudden surge of water that can devastate the protected area.
Physical Causes of Levee Failure
Levee failure is classified into three physical mechanisms that compromise the structural integrity of the earthen embankment. The most frequent failure mode is overtopping, which occurs when the water level rises above the crest of the levee. The water flows down the landward slope, rapidly eroding the soil material from the top down, quickly leading to a breach.
Internal erosion, often termed piping or seepage, begins beneath the surface. Water pressure forces water to flow through the levee foundation, eroding soil particles and creating underground tunnels or “pipes.” This process often manifests on the landside as “sand boils,” where water carrying fine soil erupts to the surface, signaling instability that can lead to a sudden collapse.
The third mechanism is structural instability, which includes slope failure or slumping. Excessive saturation of the soil mass, particularly on the landward side, reduces the soil’s shear strength, causing a section of the levee to slide away. This instability can be exacerbated by poor foundation conditions, such as weak or liquefied subsoils, or by high uplift pressures.
Immediate Flood Wave Characteristics
Once a levee fails, the breach rapidly expands, creating a wide-open gap for the flood volume to escape. The geometry of the breach, including its final width and lateral erosion rate, depends on the levee’s height and the erodibility of its soil material. In cohesive soil structures, the lateral erosion rate can be approximately 27 meters per hour, allowing a breach to develop fully in a matter of hours.
This rapid opening creates a massive hydraulic head that pushes a high-velocity flood wave into the protected area. Flow velocity through the breach can reach speeds of approximately 1.63 meters per second (5.3 feet per second), generating hydrodynamic forces that can sweep away vehicles and demolish structures. This surge is more destructive than typical river flooding because the water rushes in quickly and reaches significant depths rapidly.
The force of the outflowing water causes extensive sediment scouring directly behind the breach location. This high-energy erosion carves out deep, crater-like depressions in the earth, sometimes called “kolk lakes.” A single levee break can displace massive volumes of soil that are then transported downstream.
This scouring process is immediately followed by the deposition of sediment as the water spreads out and slows down. The transported material is deposited as a layer of sand and mud over the floodplain, creating sand deltas that can be several meters thick. This rapid inundation and deposition fundamentally alter the landscape, carrying and redistributing a substantial portion of the river’s sediment load.
Long-Term Environmental and Health Consequences
After the initial flood surge subsides, standing floodwaters introduce long-term environmental and public health hazards. Floodwaters become contaminated with raw sewage from overwhelmed wastewater treatment systems, industrial chemicals, and hazardous materials released from flooded sites. This contamination pollutes local water sources and saturates agricultural land, introducing pathogens and toxins.
The presence of standing water for prolonged periods creates conditions for the proliferation of biological threats. This includes the growth of mold and fungi within flooded buildings, leading to long-term respiratory problems for residents. The waterlogged environment fosters increased populations of disease vectors, such as mosquitoes and rodents, which elevate the risk of transmitting vector-borne illnesses.
The risk of hospitalization for affected populations remains elevated following a major flood event. Beyond immediate injuries, chronic conditions worsen, with a higher incidence of hospital admissions for cardiovascular, respiratory, and digestive diseases. The prolonged stress and displacement also result in long-term mental health disorders within the community.
The physical damage to the land, especially agricultural areas, is persistent. Floodwater scours away nutrient-rich topsoil and deposits infertile sand and silt, leading to long-term soil productivity loss. The flood causes infrastructure paralysis, as power grids, water purification plants, and transportation networks can remain submerged and inoperable for weeks. This sustained loss of basic services impedes recovery efforts and exacerbates the humanitarian crisis.