The Mississippi River is the longest river in the world and encompasses the third-largest watershed, draining 41 percent of the continental United States including an area of 1,245,000 square miles (3,224,550 km2). The river transports 230 million tons of sediment, including the sixth-largest silt load in the world. Before the Europeans came and began altering the river, this silt used to cover the flood-plains with this fertile material during the semiannual floods and carry more downriver to be deposited on the Mississippi River delta. Levee construction along the lower Mississippi River system began with the first settlers who came to the region, and has continued until the present-day levee system, the main parts of which include 2,203 miles (3,580 km) of levees, flood walls, and other control structures. Of this, 1,607 miles (2,586 km) of levees lie along the Mississippi River, and another 596 miles (959 km) are along the banks of the Arkansas and Red Rivers in the Atchafalaya basin. Additional levees are built along the Missouri River.
The first levee along the Mississippi River was built around the first iteration of New Orleans between 1718 and 1727, and consisted of a slightly more than mile-long (5,400 feet; 1,646 m), 4-foot-high earthen mound that was 18 feet (5.5 m) wide at the top, with road along the crown. This levee was meant to protect the residents of the newly founded city from annual floods and pestilence that would last from March until June of each year. New Orleans had only recently been inhabited—Louis XIV of France had commissioned the explorer Pierre Le Moyne, Sieur d'Iberville, to establish a colony near the mouth of the Mississippi River to control the Mississippi valley and the lumber and fur trade moving down the river. D'Iberville's younger brother, Jean Baptiste Le Moyne, Sieur de Bienville, established New Orleans in 1718 in a bend of the river to control the portage between the river and Lake Pontchartrain. The site of New Orleans was surrounded by water on all sides. Lakes Ponchartrain, Maurepas, and Bayou Manchac and the Amite River divide it from higher land on the north, and the Mississippi River wraps around its other sides. The site of New Orleans on the natural levee of the Mississippi on the Isle of Orleans has always been precarious, and the city has been inundated by floods from the river on three sides, and by storm surges from hurricanes on the other side about every 30 years since its founding. The first levee built in 1718-27 did not stop the floods, and the city was destroyed by a hurricane in 1722. On September 2324 a hurricane almost completely destroyed the newly founded capital city. The storm had 100-mile-per-hour (161 kph) sustained winds and a storm surge of 7-8 feet (2-2.4 m) that overtopped the four-foot (1.2-m) high levee. Almost every building in the city was destroyed or severely damaged. If city planners has taken this warning when the city consisted only of several dozens of buildings, much future damage could have been avoided. Instead, more and higher levees were built, with successive floods by storms destroying or severely damaging the city in 1812, 1819, 1837, 1856, 1893, 1909, 1915, 1947, 1956, 1965, 1969, and 2004. The old levees did not hold in 1722, the new levees did not hold in 2004 during Hurricanes Katrina and Rita, and the levees broke repeatedly during the high-water events in between.
The early river levees along the Mississippi consisted of earthen mounds, generally with a slope of 1:2. The local and state governments made it a policy that local farmers had to build their own levees on the property they owned along the Mississippi. Haul methods for bringing the dirt to make the levees were primitive, typically with horse and carriage, yielding only 10-12 cubic yards (7.5-9 m3) per day. The federal government became involved in 1820 with legislation that focused mostly on navigation along the river and did not consider flood control. As the levees were built at breakneck pace, the river became constricted, causing the bed of the river to raise itself continuously in a process called aggradation. This happens because if the river is not allowed to migrate laterally, it cannot move out of the way of the sediment it is carrying and depositing, and cannot widen the channel, so therefore it raises the bed as this sediment is deposited. Disastrous floods along the lower Mississippi in 1844, 1849, and 1850 resulted in passage of the Swamp Acts of 1849 and 1850. These acts gave Louisiana, Mississippi, Arkansas, Missouri, and Illinois swamp and overflow lands within their boundaries that were unfit for cultivation. These lands were sold, and the revenues generated were used to construct levees and complete drainage reclamation of the purchased lands. Between 1850 and 1927 the levees along the lower Mississippi had to be continuously heightened because of this river avulsion caused by the construction of the levees.
In 1850 Congress appropriated $50,000 to complete two topographic and hydrographic surveys to promote flood protection along the Mississippi River. One survey was completed by a civilian engineer, Charles Ellet Jr., and the other by army engineers A. A. Humphreys and Henry Abbot. The Humphreys-Abbot report recommended three possible methods for flood control including cutting off the bends in the river, diversion of tributaries creating artificial reservoirs and outlets, and confining the river to its channel using levees. Since the first two options were considered too expensive, the third was enacted, with long-lasting consequences. Their levee design called for freeboards at 311 feet (1-3.4 m) above the level of the 1858 flood.
The Civil War (1861-65) saw the levees disregarded and they fell into a state of disrepair, made worse by the large floods of 1862, 1865, and 1867. New floods in 1874 prompted the creation of a Levee Commission to complete a new survey of the state of the levees and recommend how to repair the system and reclaim the floodplain. The Levee Commission made a stark assessment, citing major defects in the system and huge costs to repair and improve it. They documented that previous levees were built in faulty locations, with poor organization, insufficient height, poor construction, and inadequate inspection and guarding. They estimated that it would cost $3.5 million to repair the existing system and $46 million
Map of the lower Mississippi River from the mouth of the delta to southern Missouri showing the thousands of miles of levees constructed along the river in the past century to build a new, complete levee system to reclaim the floodplain from the river.
In 1879 Congress created the Mississippi River Commission (MRC), as organized by James B. Eads.
The commission consisted of three officers from the U.S. Army Corps of Engineers, three civilians, and one officer from the U.S. Coast and Geodetic Survey. The MRC conducted surveys and suggested many modifications and new additions to the flood-control and navigation projects along the river. They made a policy in 1882 to close the breaks along the levee and to construct a line of levees with sufficient height and grade supposedly to contain the frequent floods along the river. They did not have long to wait to see the faults in their model.
The flood of 1890 destroyed 56 miles (90 km) of levees, and the MRC began to raise the levees from 38 to 46 feet (11.5 m to 14 m). During this phase of massive reconstruction the federal government and private citizens added more than 125 million cubic yards of soil to the levees (96 million m3), but much of this was lost to the river by mass wasting processes including slumping and bank caving. Efforts were made to reinforce the banks with various revetments, but then the flood of 1912 destroyed much of the levee system that was meant to protect the adjacent floodplain. The response of the commission was to raise the levees again, to three feet above the 1912 flood line. The lesson was not yet learned that raising the levee and constricting the river causes the bed to aggrade and rise as well.
The first federal flood control act was passed in 1917, authorizing for the first time levees to be built for flood control, along both the Mississippi and its tributaries. The federal government would pay two-thirds of the costs of levees if the local interests would pay the balance. During the 1920s levee construction was stepped up to a higher pace with the mechanization of earth-moving technology, with introduction of large cranes, moving tower machines, and cable-way draglines that could move dirt orders of magnitude faster than the traditional horse and cart.
The year 1927 came, and with it, the greatest flood in recorded history along the lower Mississippi River valley. Many of the levees built to the MRC standards failed up and down the river, with enormous consequences in terms of loss of life, displaced people, and loss of property that was supposed to be protected by the levees. The government responded with the 1928 Flood Control Act, passing legislation to improve the grade of the levees and make models of different flood scenarios, including the creation of several large floodways that could be opened to let water out of the river in high flow times. Some of these floodways were quite large, such as the Birds Point-New Madrid floodway, which is about 35 miles (56 km) long, 3-10 miles (5-16 km) wide, and designed to divert 550,000 cubic feet (15,576 m3) per second of flow from the Mississippi during floods. Further downriver the West Atchafalaya floodway was designed to carry half of the modeled projected flood of 1,500,000 cubic feet (139,400 m3) per second. The Bonne Carre floodway was built upriver from New Orleans, designed to restrict the flow to downstream by diverting the water and protecting New Orleans. Levees were redesigned, moved to locations where their projected life span was from 20 to 30 years, and thought to be stronger. As construction on the new levee and floodway system continued, new floods, such as the 1929 flood, disrupted operations, but the construction methods continued to improve, and the levees were built, forming much of the present-day levee system.
In 1937 a large flood emanated from the Ohio River watershed, raising the waters to levels such that the Birds Point-New Madrid floodway was used, opening the floodway by dynamiting the Fuse Plus levee. This released huge volumes of water and eased the flood downstream. One of the lessons from the 1937 flood was that roads should be added to the levees to aid in moving material from place to place during floods. In 1947 the MRC began redesigning levees to be stronger to avoid failure, recognizing the importance of compaction for reducing the chances of levee failure.
Levees fail by three main modes: underseepage of water beneath the levee, where the pressure from the high water opens a channel causing catastrophic failure; hydraulic piping, in which the water finds a weak passage through the levee; and overtopping when the water flows over the top of the levee and erodes the sides. Levees can also fail when the river current scours the base of the levee during high-flow conditions, as happened in many of the Mississippi River floods, and this causes slumping and massive collapse of the levee. Mass wasting is also promoted by long-term floods in which the water gradually saturates the pores of the levee, weakening it, causing massive liquefaction and catastrophic failure, leading large sections of the levee to collapse at the same time. Most levee failures happen during times when the flow has been high for long periods, since this increases the pore pressure, scouring, and liquefaction potential of the levee.
By 1956 the MRC was modeling floods with twice the previous discharge, examining the ability of the river and levee system to handle a discharge of 3,000,000 cubic feet (2,300,000 m3) per second. Then the flood of 1973 hit the Mississippi River basin with one of the highest floods recorded in 200 years. The flood set a record for the number of days the river was out of bank, causing more than $183,756,000 in damages. In terms of flood management, the flood of 1973 brought the realization that building levees, wing dikes, and other navigational and so-called flood-control measures had actually decreased the carrying capacity of the river. This meant that for any given amount of water, the flood levels (called stages) would be higher than before the levees were built.
The catastrophic floods of 1993 provided another test of the levees, and the new system failed massively. The constriction of the river caused by the levees led to numerous cases of levee failure, overtopping, crevasse splays, collapse, and massive amounts of damage as had never been seen along the river. Approximately two-thirds of all the levees in the upper Mississippi River basin collapsed, were breached, or were otherwise damaged by the floods of 1993. Dozens of people died and 50,000 homes were damaged or destroyed, with total damage estimated at most than $15 billion.
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