Breakdown: Why the Mississippi River constantly erodes its banks
MEMPHIS, Tenn. (WMC) - The Mississippi River is remarkable because, every day, almost 20 million people in 50 cities depend on it for their drinking water. The rich soils within its vast watershed produce 90% of our country’s farm exports.
However, the Mississippi River Delta and coastal Louisiana are disappearing at an astonishing rate: a football field of wetlands vanishes into open water every 100 minutes, according to Restore the Mississippi River Delta.
It carved out its present route to the Gulf about a thousand years ago and the U.S. Army Corps of Engineers (USACE) is determined that the meandering river now stay on course.
The Mississippi flows south for more than 2,300 miles to reach the Gulf of Mexico at powerful rate between 200,000 and 700,000 cu. ft. per second. With this force, the river constantly erodes its banks, particularly within the last thousand miles because, unlike upper regions, the lower Mississippi contains no locks or dams. It is uncontrolled flow to the sea.
Why is this erosion bad? The Mississippi Delta is disappearing faster than any other land on Earth, according to nextcity.org. Since the 1930s, Louisiana has lost over 2,000 square miles of land, an area roughly the size of Delaware.
To slow down further erosion, the Corps spends six to seven months each year reinforcing the banks along vulnerable stretches of the lower Mississippi. This reinforcing uses a technique uses a large piece of equipment known as a mat-sinking unit (MSU).
The MSU is used to install great sheets of concrete mattress on the riverbank to shield it from erosion and sloughing caused by channel currents and turbulent water associated with river flood stages.
While the MSU helps lay these concrete mattresses, the MSU does not manufacture them. Rather, the pavers are cast at river-side plants in relative proximity to where the boat is operating — specifically, in Memphis, Tennessee, St. Francisville, Louisiana, and Vicksburg, Mississippi.
At the plants, the concrete tiles are joined to form the 4-ft.-by-25-ft. squares. The squares then are loaded onto barges and convoyed to the floating MSU for assembly and sinking. Each supply barge holds 585 squares, which is about 950 tons of concrete. When a barge arrives, it is tied up to the MSU and the building blocks are unloaded for real-time utilization. The empty barge then is returned to the plant for another load as a full barge takes its place. Most days on the river, five barge-loads of the concrete squares are off-loaded, joined and sunk.
While some can last longer (or even shorter), the designed life of a mat is 50 years, so the MSU will eventually return to each location and overlay a new mattress on the remnants of the old ones.
Mat sinking is not a new practice. In fact, it dates back to the 19th century, when Corps crews would weave together a “mat” of willow trees, float them out into the river, then sink them with stones. The seasonal task, which usually spans the low-water months, was conceived to prevent erosion, protect key areas of the riverbank and ensure safe and reliable navigation.
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