For thousands of years, the Mississippi River
meandered through its valley unhindered, flooding lowlands and creating
oxbow lakes when it changed course. But as men began to settle and
develop the valley, they tried to place restrictions on the whims
of the river and make it conform to their needs. Sometimes the river
would cooperate, but many times it would not. Caving banks would
claim buildings or valuable farmlands, the shifting channel would
leave prosperous ports high and dry, and numerous floods devastated
crops and economies.
More than a century ago, the Vicksburg District
began working with the Mississippi River and its tributaries seeking
their cooperation, but the present comprehensive program began after
the disastrous flood of 1927. The work of the past half century
has included cutoffs, floodways, reservoirs, and levees. Much of
the ongoing work on the river-bank stabilization, dikes, and dredging-is
performed to keep the river in the desired channel for navigation
and flood control.
One of the largest of the Vicksburg District's
operations is the annual revetment program. Using articulated concrete
mattresses cast at fields in Greenville, Mississippi, and Delta
Point, Louisiana, the district repairs and fortifies banks against
the destructive, gnawing current of the river. For the revetments
to be effective, they need at least two qualities- economy and strength.
The present use of articulated concrete mattresses, is the culmination
of 100 years of engineering evolution.
One of the earliest forms of revetment in the
Lower Mississippi Valley was the willow mattress, woven by hand
on site and weighted into place with stone. The willow mats were
successful for several reasons. The main ingredient, willows, was
abundant in most places along the river, and young willows could
be woven into a mat that was flexible enough to conform to the irregularities
of the bank. But the willows had their drawbacks-the tediousness
of weaving the mats, eventual deterioration of the material, and,
by 1910, the scarcity of willows.
Experiments with reinforced concrete as a revetment
material began in Vicksburg in 1914, and the first successful reinforced
articulated- concrete mattress was developed in 1917 and patented
by D. H. Shearer. The 3-inch-thick concrete slabs were reinforced
with a wire mesh extending on all sides. The 16-block mattresses
are now cast in units of 13 uniform slabs and barged to the worksite
as needed.
Prior to the placement of the mat, the bank of
the river is graded to a stable slope using bulldozers and massive
floating draglines. The units of reinforced concrete slabs are then
assembled on the sloping deck of the sinking unit into mattresses
156 feet wide. The sinking unit is then moved out from the bank,
allowing the completed sections to slide from the deck onto the
bank. When mattress placement is complete, the graded area above
the revetment is protected with a covering of stone riprap. This
entire process costs about the same per square foot as laying good
carpet in a home.
The articulated concrete mattress has attracted
the interest of engineers from around the world. Each year, numerous
foreign visitors come to the Vicksburg District during the work
season to observe the process from beginning to end-from the casting
of the mats through their assembly and placement on the banks of
the mighty river. However, despite international interest, there
is only one existing mat sinking unit, unique to the Mississippi
River and operated by the Corps' Vicksburg District.
Though the river may seem tame to some, it still
has a will of its own, fighting to go where it pleases. However,
as long as there is a need, the Vicksburg District will continue
to work with the Mississippi, developing the required technology
to meet the needs of new challenges. |