Levee

• 1. Introduction

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  A levee is a raised ridge of earth that stands between a river and everything people have built beside it. In 1718, when New Orleans was still a young city, its first levees stood about 3 feet high and ran for roughly 50 miles along the riverbank. Today, those original French settler constructions have grown into a system covering over 3,500 miles. What began as a modest earthen berm has become one of the largest engineering works of its kind on the planet. How did something so simple become so vast? And what happens when it fails?

• 2. A Word With Ancient Roots

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  The word levee comes from the French levee, derived from the past participle of lever, meaning to raise. It entered documented use in New Orleans shortly after the city's founding in 1718 and was later taken up by English speakers. But the Latin root reaches even further back. The word levata appears in a document dated 954, referring to a millpond near Bagnolo di Po in northern Italy, created by a levee with a road running over its top. The Latin verb levare was already in use for diverting water for mills as early as 778, when Duke Hildeprand of Spoleto granted a canal to Bishop Guicpert of Rieti for that purpose.

  The word dike traces a different path. It most likely comes from the Dutch dijk, and dike construction is well documented as early as the 11th century. The 126-kilometer Westfriese Omringdijk, completed by 1250, was built by linking together older, pre-existing dikes. The Roman writer Tacitus recorded that the rebellious Batavi pierced their own dikes around 70 CE to flood the land and shield their retreat. The original Dutch word dijk indicated both the trench dug and the bank thrown up beside it, which explains why the same English term can refer to a ditch, a wall, or a drainage channel, depending on the county in which you are standing.

• 3. Rivers That Build Their Own Banks

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  Natural levees form without any human hand guiding them. When a river floods and spills across its floodplain, the fast-moving water suddenly loses speed as it spreads out. Coarser sediments drop first, landing closest to the channel's edge. Finer particles travel further before settling, which gives a natural levee its characteristic gentle slope on either side. Over many flood cycles, these deposits accumulate into a ridge that actually raises the carrying capacity of the river channel.

  In some rivers, this accumulation goes far enough that the riverbed ends up sitting higher than the land on either side of it. The Yellow River in China provides the most striking example: near the sea, the river runs so high within its levee walls that oceangoing ships appear to sail above the surrounding plain. Alluvial rivers carrying heavy loads of suspended sediment are the most likely to behave this way, as are tidal creeks where high spring tides bring coastal silts and muds surging inland. Each flood lays down another thin layer, and the levee slowly rises.

• 4. Four Thousand Years of Construction

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  People have been building artificial levees since at least 2600 BCE. The Indus Valley civilization in what is now Pakistan and northern India constructed levees on which the farming life of the Harappan peoples depended. Ancient Egypt built a system of levees stretching more than 600 miles along the left bank of the Nile, running from the area of modern Aswan all the way to the Nile Delta and the shores of the Mediterranean.

  In the early 1400s, the ruler of the city-state of Texcoco, Nezahualcoyotl, supervised the construction of a levee to protect Mēxihco-Tenōchtitlan and the neighboring city of Tlatelōlco. Its job was to keep the brackish waters of Lake Texcoco separate from the fresh water supplying the settlements. After the Europeans destroyed Tenochtitlan, the levee was destroyed as well, and flooding became a persistent problem. The result was that the majority of the lake was drained in the 17th century.

  During the Chinese Warring States period, the Qin built the Dujiangyan irrigation system on the Min River, the longest tributary of the Yangtze, in Sichuan. Some historians have argued that the enormous organizational demands of building and maintaining levee systems may have been a catalyst for the development of centralized governance in early civilizations, though others point to evidence of large-scale water-control earthworks predating centralized rule in Predynastic Egypt.

• 5. The Mississippi System

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  French settlers in Louisiana began building levees to protect New Orleans in the 18th century with structures about 3 feet tall covering around 50 miles of riverside. The U.S. Army Corps of Engineers, working alongside the Mississippi River Commission, took over and extended the system from 1882 onward, pushing coverage from Cairo, Illinois all the way down to the mouth of the Mississippi delta. By the mid-1980s, the levees had reached their present extent, averaging 24 feet in height, with some individual levees reaching 50 feet.

  One levee runs south from Pine Bluff, Arkansas, without interruption for approximately 380 miles. The whole system spans more than 3,500 miles and stretches roughly 1,000 kilometers along the river from Cape Girardeau, Missouri, down to the delta. Its scale is sometimes compared to the Great Wall of China. On the river's surface, this engineering has had a physical consequence: the artificial confinement of the Mississippi has elevated its natural riverbed over time, just as has occurred along the Yellow River.

• 6. When Levees Break

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  A levee can fail in several ways: overtopping, erosion, structural collapse, or saturation of the earthen body itself. The most dangerous failure is a breach, where a section of the levee actually breaks open and leaves a gap through which floodwater pours. A breach may happen suddenly or develop gradually through either surface erosion or weakness beneath the surface. Once a breach forms in a natural levee, the gap persists until the river's own flood-deposition processes fill it again, leaving that spot more vulnerable to future failures.

  A breached levee can leave behind a fan-shaped deposit of sediment called a crevasse splay, radiating outward from the opening. Researchers including Briaud and colleagues in 2008 and Hughes and Nadal in 2009 studied the forces of overtopping water on levee surfaces, using laboratory tests to develop correlations between overtopping discharge and a levee's resistance to scour. Another detection method, electrical resistivity tomography, works by identifying critically saturated zones inside an embankment before a failure occurs, allowing it to serve as an early warning system. In the United States, levee tolerances are often expressed as an annual exceedance probability: a standard of less than 1 percent annual chance of flooding means the structure is designed so fewer than one flood in a hundred years would be expected to breach it. The catastrophic levee failures in Greater New Orleans during Hurricane Katrina in 2005 showed how much turns on that engineering margin.

• 7. Ecological Costs

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  Levees do more than hold back water. They also intercept the sediment, nutrients, and organic material that recurring floods would otherwise spread across surrounding wetlands and floodplains. Studies of land where levees have been removed recorded increases in riparian habitats and biodiversity after floodwaters were allowed to return.

  By concentrating river flow rather than letting it spread, levees speed up and intensify flood pulses. Artificial dams and reservoirs can capture some of that energy, but they introduce their own consequences for river navigation and aquatic ecosystems. The narrowing of the effective floodplain increases the severity of extreme flood events, which in turn demands more engineering to manage, creating a cycle that has no easy exit. Along the Acadian shores of the Bay of Fundy in New Brunswick and Nova Scotia, coastal levees called dykes were built with hinged sluice gates known as aboiteaux, designed to drain freshwater on the falling tide and seal against seawater on the rising tide; that careful two-way design reflects a more measured integration of hydrology and land use than simple containment alone.