Levee
The word levee first appeared in New Orleans a few years after the city's founding in 1718. It comes from the French verb lever, meaning to raise. Early documents from 1770 describe the town as secured by a raised bank called the Levée. The Latin root levare also appears in records dating back to 954. That document mentions a millpond near Bagnolo di Po created by a levee with a road running over it. Another ancient usage dates to 778 when Duke Hildeprand of Spoleto granted a canal for a mill construction. The modern English term dike likely derives from the Dutch word dijken. Construction of dikes was well attested as early as the 11th century. The Westfriese Omringdijk completed by 1250 connected existing older dikes into one continuous barrier. Roman chronicler Tacitus recorded that the rebellious Batavi pierced dikes to flood their land and protect their retreat in 70 CE. In Anglo-Saxon England the word existed as dick or ditch depending on regional pronunciation. Offa's Dyke remains a combined structure while Car Dyke functions primarily as a trench today. Some regions like Lincolnshire still use terms such as Rippingale Running Dike to describe drainage channels.
The Indus Valley civilization built levees across Pakistan and North India to support agrarian life for the Harappan peoples. Ancient Egypt constructed systems along the left bank of the River Nile more than 3,000 years ago. These structures stretched from modern Aswan to the Nile Delta on Mediterranean shores covering distances exceeding hundreds of kilometers. Mesopotamian civilizations and ancient China also developed large-scale levee networks during their respective eras. A historical example protected Mēxihco-Tenōchtitlan and Tlatelōlco during the early 1400s under Nezahualcoyotl. This tlahtoani supervised construction separating brackish Lake Texcoco waters from fresh potable supplies. After European destruction of Tenochtitlan flooding became major problems leading to most lake draining by the 17th century. Levees usually form by piling earth on cleared level surfaces with broad bases tapering to narrow tops. Temporary embankments or sandbags often sit atop these permanent earthworks during emergencies. Sections sometimes set back from rivers create wider channels while flood valley basins divide into multiple zones. A spetchel describes a specific type made from stones laid horizontally with thin turf beds between each layer.
The Mississippi levee system represents one of the largest such systems found anywhere in the world today. It comprises over thousands of miles extending some thousands along the river itself. Construction began by French settlers in Louisiana during the 18th century to protect New Orleans. First Louisiana levees stood about three feet high covering distances around fifty miles along riversides. The U.S. Army Corps of Engineers extended the system starting in 1882 from Cairo Illinois to the delta mouth. By mid-1980s they reached present extents averaging six feet in height though some reach heights up to thirty feet. One continuous levee extends southwards from Pine Bluff Arkansas for distances reaching several hundred miles. These structures have often been compared to the Great Wall of China due to their sheer scale. Cellular confinement technology using geocells serves as best management practice recommended by engineers. Particular attention focuses on surface erosion prevention and protection of downstream slopes. Reinforcement provides tensile force to soil better resisting instability during extreme water events. Artificial levees can lead to elevation of natural riverbeds over time depending on sediment accumulation rates. Alluvial rivers with intense bed load accumulations tend toward this behavior notably seen in Yellow River and Mississippi examples.
Levees are very common on marshlands bordering Bay of Fundy in New Brunswick and Nova Scotia Canada. Acadians who settled the area created original construction of many dykes for farming fertile tidal marshlands. These structures feature hinged sluice gates called aboiteaux opening on falling tides to drain freshwater. They close on rising tides preventing seawater entry behind protective barriers. In Lower Mainland around Vancouver British Columbia dikes known locally as sea walls protect Richmond city on Lulu Island. Additional dikes safeguard Pitt Polder land adjacent to Pitt River and other tributary systems. Coastal flood prevention levees also line inland coastline behind Wadden Sea devastated by historic floods. Peoples and governments erected increasingly large complex systems stopping even storm floods. Biggest examples exist in Netherlands where huge levees aggressively take back land below mean sea level. Spur dykes or groynes typically man-made hydraulic structures sit perpendicular to banks protecting against erosion. Two common types include permeable and impermeable variations depending on materials used for construction. These structures widely applied along coastlines prevent direct wave impact from damaging shorelines directly.
Both natural and man-made levees can fail through overtopping erosion structural failures or saturation conditions. Most frequent dangerous outcome involves a breach leaving large openings allowing water to flood protected lands. A breach may occur suddenly or gradually caused either surface erosion or subsurface weakness within structure itself. Fan-shaped sediment deposits radiating away from breach point described technically as crevasse splay. Natural levee gaps remain until filled again by subsequent building processes increasing future breach likelihood. Sometimes levees said to fail when water overtops crest causing flooding without damaging structure itself. Soil erosion found among most important factors causing breaches according to numerous studies conducted globally. Briaud et al tested erodibility of soils using Erosion Function Apparatus tests during Katrina Hurricane events. Hughes and Nadal studied combined effects of wave overtopping and storm surge overflow on erosion generation. Electrical resistivity tomography detects critical saturation areas in embankments acting as early warning systems. Negative impacts arise where large scale structures modify natural processes inevitably creating drawbacks or consequences. Levees interrupt floodplain ecosystems developed under seasonal flooding conditions affecting biodiversity significantly. Increased height requirements emerge as more upstream construction increases recurrence intervals for high-water events requiring taller barriers.
Levees interrupt floodplain ecosystems that developed under conditions of seasonal flooding throughout history. Reduced flooding recurrence facilitates land-use change from forested floodplains to agricultural farms across regions. When levees keep floodwaters inside narrow channels delivery occurs downstream over shorter time periods raising river stages higher. Same volume delivered over shorter interval means increased pressure on downstream communities and infrastructure. Breaches produce high-energy flooding similar to dam breaks carving deep holes channels into nearby landscapes. Impacted areas far from breach may experience flooding resembling natural events while damage near breach remains catastrophic. Prolonged flooding follows after failure since water pours out blocking return to river channel entirely. Flooding persists waiting for slow infiltration evaporation processes to clear affected zones completely. Natural flooding adds sediment layers making crust sink deeper into mantle like floating wood pushed down further. Momentum continues downward movement even when new sediment layers stop being added resulting in subsidence. Coastal areas see land dipping below sea level ocean migrating inland saltwater intruding freshwater aquifers. Where large rivers spill into oceans velocity slows reducing ability transport sand silt effectively. Sediments settle forming deltas extending coastline seaward naturally but levees cut off this process entirely. Elevated river velocity delivers sediment directly to deep water where wave action cannot redistribute it properly.
Common questions
When did the word levee first appear in New Orleans?
The word levee first appeared in New Orleans a few years after the city's founding in 1718. Early documents from 1770 describe the town as secured by a raised bank called the Levée.
Who built the Indus Valley civilization levees across Pakistan and North India?
The Harappan peoples of the Indus Valley civilization built levees across Pakistan and North India to support agrarian life. These structures were constructed more than 3,000 years ago along the left bank of the River Nile in ancient Egypt.
How high are the Mississippi levees today compared to early construction heights?
By the mid-1980s the Mississippi levee system reached present extents averaging six feet in height though some reach heights up to thirty feet. First Louisiana levees stood about three feet high covering distances around fifty miles along riversides during the 18th century.
What is an aboiteaux used for in Acadian dykes on Bay of Fundy marshlands?
Acadians created hinged sluice gates called aboiteaux opening on falling tides to drain freshwater from farming fertile tidal marshlands. They close on rising tides preventing seawater entry behind protective barriers in New Brunswick and Nova Scotia Canada.
Why do levees cause land subsidence where large rivers spill into oceans?
Levees cut off natural sediment deposition processes causing elevated river velocity to deliver sediment directly to deep water where wave action cannot redistribute it properly. This results in coastal areas seeing land dip below sea level as ocean migrates inland and saltwater intrudes freshwater aquifers.