Friday :: May 13, 2011

Controlling Nature on the Mississippi

by Mary

One of the stories John McPhee told in his classic book, The Control of Nature, was about how the Army Corps of Engineers had taken on the challenge to tame the Mississippi River first to keep it from causing devastating floods and to make sure it kept the current route to the Gulf of Mexico that passes by Baton Rouge and New Orleans. This essay has now been put online by the New Yorker.

The Mississippi River, with its sand and silt, has created most of Louisiana, and it could not have done so by remaining in one channel. If it had, southern Louisiana would be a long narrow peninsula reaching into the Gulf of Mexico. Southern Louisiana exists in its present form because the Mississippi River has jumped here and there within an arc about two hundred miles wide, like a pianist playing with one hand—frequently and radically changing course, surging over the left or the right bank to go off in utterly new directions. Always it is the river’s purpose to get to the Gulf by the shortest and steepest gradient. As the mouth advances southward and the river lengthens, the gradient declines, the current slows, and sediment builds up the bed. Eventually, it builds up so much that the river spills to one side. Major shifts of that nature have tended to occur roughly once a millennium.

As McPhee writes, since just after the Louisiana Purchase, it was noted that the Mississippi River would likely switch to a new path down the Atchafalaya River and through the next century and a half, that likelihood increased. Then man intervened and build a massive series of structures to hold the river.

The Corps was not intending to accommodate nature. Its engineers were intending to control it in space and arrest it in time. In 1950, shortly before the project began, the Atchafalaya was taking thirty per cent of the water that came down from the north to Old River. This water was known as the latitude flow, and it consisted of a little in the Red, a lot in the Mississippi. The United States Congress, in its deliberations, decided that “the distribution of flow and sediment in the Mississippi and Atchafalaya Rivers is now in desirable proportions and should be so maintained.” The Corps was thereby ordered to preserve 1950. In perpetuity, at Old River, thirty per cent of the latitude flow was to pass to the Atchafalaya.

Today, Jeff Masters has updated McPhee's story in light of this year's flood noting that the Old River Control Structure is the USA Achilles heel. This is the first significant test of this structure since the massive flood of 1973 when there was a major but not catastrophic failure of a part of the structure.

But in 1973, a series of heavy snowstorms in the Upper Midwest was followed by exceptionally heavy spring rains in the South. The Mighty Mississippi rose inexorably until the flow rate at the Old River Control Structure reached 2 million cubic feet per second--twenty times the flow of Niagara Falls--and stayed there for more almost three months. Turbulence from the unprecedented flows through the Low Sill Structure scoured the foundation and destroyed a 67-foot-high wing wall that guided water into the structure. Scour holes as big as a football field developed upstream, downstream, and underneath the structure, exposing 50 feet of the 90-foot long steel pilings supporting the structure. The structure began vibrating dangerously, so much so that it would slam open car doors of vehicles parking on top of Highway 15 that crosses over the top. Emergency repairs saved the structure, but it came every close to complete failure.

Since then the structure has been expanded and strengthened, nevertheless, this flood will truly stress the system.

While I expect that the Old River Control Structure will indeed hold back the great flood of 2011, we also need to be concerned about the levees on either side of the structure. The levees near Old River Control Structure range from 71 - 74 feet high, and the flood is expected to crest at 65.5 feet on May 22. This is, in theory, plenty of levee to handle such a flood, but levees subjected to long periods of pressure can and do fail sometimes, and the Corps has to be super-careful to keep all the levees under constant surveillance and quickly move to repair sand boils or piping problems that might develop. Any failure of a levee on the west bank of the Mississippi could allow the river to jump its banks permanently and carve a new path to the Gulf of Mexico.

To help ease the stress, the commander of the Morganza spillway has requested to open the spillway within the next 24 hours. This will be only the second time the Morganza spillway has been opened since its construction. The first time it was used was in 1973. The Army Corps of Engineers are planning to only partially open it so they can minimize the flooding downstream, while still lowering the flood peak in New Orleans.

Truly the Corp of Army Engineers will have their hands very full for the next few weeks.

(Here's a fascinating map illustrating how the Mississippi and Atchafalaya interact and the various control systems put in place to manage the river. h/t)

Mary :: 3:47 PM :: Comments (0) :: Digg It!