Dealing with the titanic force of America’s largest river is one of the perpetual responsibilities of the US Army Corps of Engineers. Beginning in 1866, Congress authorized the Corps to improve its navigability by dredging, removing snags, and removing trees from the river’s banks. “The river flows over sand and clay, not bedrock, and it’s always cutting a new channel,” said Col. Brian D. Sawser, commander of the corps’ Memphis district. “Since 1921 for reasons of navigation and commerce we have been trying to keep the canal in place.” The Corps is mandated to maintain the channel at 9 feet deep and 300 feet wide.
One of the long-standing items in the body’s toolkit is the wing jacks, which you say typically extend hundreds of feet into the river, perpendicular to the shoreline. Most often built of piles and stone, they force water into the center of the river, speeding up its flow, causing it to wash away sediment and remove sandbars. “The approximately 1,000 levees built by the Corps over decades have greatly reduced the amount of dredging required,” Sawser says. Early ENR articles describe them as “contraction works”.
The most imposing control structures are the system of levees that span the lower Mississippi for 1,000 miles from Cairo, Illinois to the Gulf of Mexico, as well as its tributaries and waterways. They constitute the largest artificial structure on earth. Throughout the 19th century they were built under the jurisdiction of local levee boards, and their conditions varied. The levee system has reduced the floodplain of the river’s alluvial valley by 90% from its original size of 37,500 square miles.
The Corps was given partial oversight of the levees in 1879, with the formation of the Mississippi River Commission, consisting of three members of the Corps, one representative of the National Oceanic and Atmospheric Administration, and three civilians, two of whom must be engineers civilians The Commission provides policy advice, engineering direction, and oversees the planning and implementation of improvements affecting navigation and flood control.
The transformative event in the river’s history was the flood of 1927. Months of unusually heavy rains in the Midwest and South filled it to its banks, eventually causing levees to break or overflow in 17 places, forcing 630,000 people to leave their homes, flooding 25,000 square meters. kilometers and left approximately 500 people dead. To prevent future catastrophes, the Corps proposed and Congress passed a comprehensive flood control program, giving the federal government full responsibility for the Mississippi River for the first time.
The Flood Control Act of 1928 had four elements: dikes; waterways to divert excess flows; channel stabilization; and improvements to the tributary basin. The first water channel to be built was the Bird Point-New Madrid Floodway in southeastern Missouri. Completed in 1932, its levee contains embedded pipes that can be filled with explosives and detonated to breach the levee and divert water into the 205-square-mile waterway. It has been activated twice, in 1937 and in 2011.
The largest flood control complex on the river is the Old River Control Structure, a collection of four structures in central Louisiana. A sluice system that regulates the flow of water from the Mississippi to the Atchafalaya River, prevents the Mississippi from changing course. The first three components were completed in 1963 and the fourth added in 1986 after a 1973 flood heavily damaged much of the original structure. The Bonnet Carre Spillway, a 1.5-mile-long, mechanically operated reservoir located 12 miles west of New Orleans, allows the river’s overflow volume to flow into Lake Pontchartrain. The landfill has been opened 15 times since 1937 and has been left open for between 13 days and 75 days.
Another river control tactic, subject to endless debate, is man-made cutting channels through meanders. Proponents said they improved navigation channels and reduced flood heights. Detractors argued that they caused excessive bank failures and upset the balance of a river. The Commission banned them in 1884. But a pro-cut faction within the Corps, led by General Harley B. Ferguson, gained momentum in the 1920s. Ferguson was appointed chairman of the Commission in 1932, and over the next decade the Corps dug 14 man-made cuts, shortening the river by 132 miles. Although they initially provided navigational improvements, they caused the river to respond over the following decades in ways that were ultimately considered negative because of how sediment was moved. River morphology was still an evolving science at the time.
Perfection of the coating process
The body’s biggest ongoing task is to maintain the integrity of the dikes. From 1881, the preferred approach was the manufacture and installation of huge wooden mattresses. Boatloads of willow boughs were tied into bunches (fascines) with wire and assembled into mattresses 150 feet to 300 feet wide and 1,000 feet to 1,200 feet long. The mattresses were manufactured in the water by teams of 100 workers at the site where they were being placed. Once assembled, they were ballasted with stone and sunk, extending from the waterline down the seawall slope and 50 feet along the flat river bed. The operation involved four barges for workers’ housing, numerous barges for supplies of willow branches and stone, a barge-mounted crane, a pump barge for hydraulic hose maintenance and a concrete mixing plant and paving for the casting of a concrete slab that extends along the dike. slope from the waterline.
Willow supplies dwindled as the land along the river was deforested. The rich valley bottoms were cleared for agriculture, and the logging fueled fleets of wood-burning steamboats.
At the beginning of the 20th century, a new type of coating was developed in Japan. Articles describing a flexible concrete mattress pad appeared in Engineering News in 1912 and 1913. A 1918 article in ENR by Japanese engineer B. Okazaki promoted them as more economical since they resisted rubbing, which over time it damaged and destroyed the wooden mattresses.
The body began to engage in jointed concrete carpet linings in the 1920s, but wooden carpets also remained in use. By 1930, the corps was manufacturing concrete mats in two sizes. Some mattresses featured 1ft x 4ft rectangular slabs strung with cables, while others had 5ft x 7ft blocks. By 1928, concrete mats represented an important part of the paving work. An ENR article from that year stated: “The floating mixing plant travels up and down the line of barges, filling successive mats. … Two ten-hour shifts were being worked … the time 1.5 minutes for mixing and 15 seconds each for unloading and filling.” After curing for six days, the 140-foot by 250-foot mats were transferred to a steel mat ship fitted with rollers and gradually lowered into place in the river. At that time, the sinking plant could place an average of 70,000 square feet of mat per day. They remain the standard form of lining, with 400 corps members laying an average of 18 million square feet of mat per year.
Seepage is a threat to the integrity of levees. Two tools to combat it are terrestrial berms and relief wells. Earth berms are auxiliary embankments, 25 m or 50 m wide, at the foot of a dike. Relief wells, located on the landward side of levees, send water to ditches or small channels that run further inland.
Wood siding and concrete mats can only be installed at certain times of the year. “The lining season is late summer to late fall, when the river is low enough,” Sawser explains. In 1934, the Corps experimented with a new form of lining “which can be used in an emergency to combat an unexpected attack on the bank during high river stages.” A blanket of gravel is placed on the bank, followed by a layer of tetrahedron-shaped concrete blocks, which act as reinforcement to prevent the gravel from being washed away.
A promising new practice is floodplain reconnection. One of the largest so far involved a failed 16,000-acre soybean farm near Monroe, La., surrounded by a levee. The Nature Conservancy and the US Fish and Wildlife Service planned to breach the levee and let the Ouachita River, a tributary of the Mississippi, reconnect the property. Shortly before carrying it out in 2009, nature did it for them. The site, now part of the Upper Ouachita National Wildlife Refuge, can draw 300,000 acre-ft of water during flood periods. “The Conservancy is now working on several other opportunities for floodplain reconnection projects where we already have a conservation footprint or wetland easement,” says Shelly Morris, the Conservancy’s director of freshwater conservation for Kentucky and Tennessee .
This variety of tools and tactics has reduced the risk of the river wreaking havoc during periods of high water. “The Mississippi moved more water downstream in 2011 than it did in 1927, without a levee breach,” Sawser says.
