For as much as we rely on the Mississippi River for trade, transportation and agricultural irrigation, the world's third-largest tributary system has only recently been tamed. One multiple occasions, the mighty Mississippi has overflown its banks, flooding into the surrounding valley, destroying property and lives.
After an especially damaging series of floods in 1912, 1913 and 1927, US Congress pushed through the Flood Control Act of 1928. This legislation empowered the Army Corps of Engineers to reign in the river's might through the strategic use of levees, floodgates, targeted dredging, and most importantly, bank revetments. The Mississippi, like the Nile and Amazon, does not follow a set route to the sea. Its banks continually erode and reform as its waters shift sediment downstream. Revetments prevent this natural occurrence and keep the river on-course by covering soft soil embankments with articulated mats of reinforced concrete.
Each individual unit of the revetment mat is constructed from nearly a cubic yard of concrete measuring 1.2m wide, 7.6m long and 7.6cm thick. These blocks are then strung together with steel wire tendons to form a 43m wide mat which is loaded onto a "laying" barge — part of the USACE's unique "Mat Sinking Unit" — and towed out to its designated resting place.
The Army Corps of Engineers website explains:
The articulated concrete mattress (mat) arrives on location by barge from one of the mat-casting fields along the river in Tennessee, Arkansas, Mississippi and Louisiana. A fleet of 50 mat supply barges, some loaded and on location and some empty and awaiting loading by the mat-loading crew at the casting field, are towed up and down the river by Corps or contract boats.
On location, the mooring barge and spar barge are perpendicular to the shore and the work barge (mat boat) is parallel to shore and tied off to the mooring barge. The work boat positions a supply barge to be tied off to the back of the mat boat and the mat-laying operation is ready to begin.
The four overhead cranes move the 16-block sections of mat from the supply barge across to the mat boat where workers, using a pneumatic "mat-tying" tools, wire the sections together and connect to 3/8-inch launching cables running from the mat boat to the bank. The 4- by 25-foot sections (squares) are tied together with 35 other squares to form one launch. A typical blanket of mat will consist of from 12 to 24 launches. Each supply barge holds 585 squares of mat, consisting of 950 tons of concrete.
In order to get the mat anchored firmly on the bank, anchors are driven in the ground. The crew will hook the mat cables to dozers (tractors) waiting on shore that serve at temporary anchors. The mat boat will then move away from the bank launching the concrete mattress in the process. The mat boat can move riverward [along the] mooring barge and then spar barges are utilized to allow the mat boat to continue out for the remainder of the channel mat length. The entire plant moves upstream… and begins the first launch of a new channel mat.
To date, some 580km of shoreline in the New Orleans are alone have been matted. And while this has certainly kept the river from meandering, it has also effectively turned the mouth of the Mississippi into a gigantic concrete sluice. How this will affect the long term health of both the river and the larger Gulf region remains to be seen. [US Army Corps of Engineers - Mammoth - The Atlantic]