Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 5:20 PM GMT on May 13, 2011
America has an Achilles' heel. It lies on a quiet, unpopulated stretch of the Mississippi River in Louisiana, a few miles east of the tiny town of Simmesport. Rising up from the flat, wooded west flood plain of the Mississippi River tower four massive concrete and steel structures that would make a Pharaoh envious--the Army Corps' of Engineers greatest work, the billion-dollar Old River Control Structure. This marvel of modern civil engineering has, for fifty years, done what many thought impossible--impose man's will on the Mississippi River. Mark Twain, who captained a Mississippi river boat for many years, wrote in his book Life on the Mississippi, "ten thousand river commissions, with the mines of the world at their back, cannot tame that lawless stream, cannot curb it or define it, cannot say to it "Go here," or Go there, and make it obey; cannot save a shore which it has sentenced; cannot bar its path with an obstruction which it will not tear down, dance over, and laugh at." The great river wants to carve a new path to the Gulf of Mexico; only the Old River Control Structure keeps it at bay. Failure of the Old River Control Structure would be a severe blow to America's economy, interrupting a huge portion of our imports and exports that ship along the Mississippi River. Closure of the Mississippi to shipping would cost $295 million per day, said Gary LaGrange, executive director of the Port of New Orleans, during a news conference Thursday. The structure will receive its most severe test in its history in the coming two weeks, as the Mississippi River's greatest flood on record crests at a level never before seen.
Figure 1. Two views of the Mississippi River. Left: the meander paths of the Mississippi over time, as published in "Geological Investigation of the Alluvial Valley of the Lower Mississippi River" (Fisk, 1944). Right: The Army Corps of Engineers' view of Mississippi River peak flow rates during a maximum 1-in-500 year "Project Flood" (U.S. Army Corp of Engineers, 1958.) The places outlined in red are where the Corps has built flood control structures capable of diverting a portion of the Mississippi's flow.
A better path to the Gulf
The mighty Mississippi River keeps on rollin' along its final 300 miles to the Gulf of Mexico south of New Orleans--but unwillingly. There is a better way to the Gulf--150 miles shorter, and more than twice as steep. This path lies down the Atchafalaya River, which connects to the Mississippi at a point 45 miles north-northwest of Baton Rouge, 300 river miles from the Gulf of Mexico Delta. Each year, the path down the Atchafalaya grows more inviting. As the massive amounts of sediments the Mississippi carries--scoured from fully 41% of the U.S. land area--reach the Gulf of Mexico, the river's path grows longer. This forces it to dump large amounts of sediment hundreds of miles upstream, in order to build its bed higher and maintain the flow rates needed to flush such huge amounts of sediment to the sea. Thus the difference in elevation between the bed of the Mississippi and the Atchafalaya--currently 17 - 19 feet at typical flow rates of the rivers--grows ever steeper, and the path to the Gulf down the Atchafalaya more inviting. Floods like this year's great flood further increase the slope, as flood waters scour out the bed of the Atchafalaya. Without the Old River Control Structure, the Mississippi River would have carved a new path to the Gulf in the 1970s, leaving Baton Rouge and New Orleans stranded on a salt water estuary, with no fresh water to supply their people and industry.
History of the Old River Control Structure
The Mississippi River has been carving a path to the ocean since the time of the dinosaurs, always seeking the shortest and steepest route possible. Approximately once every 1000 years, the river jumps out of its banks and carves a new path. In John McPhee's fantastic essay, The Control of Nature, we learn:
The Mississippi's main channel of three thousand years ago is now the quiet water of Bayou Teche, which mimics the shape of the Mississippi. Along Bayou Teche, on the high ground of ancient natural levees, are Jeanerette, Breaux Bridge, Broussard, Olivier--arcuate strings of Cajun towns. Eight hundred years before the birth of Christ, the channel was captured from the east. It shifted abruptly and flowed in that direction for about a thousand years. In the second century a.d., it was captured again, and taken south, by the now unprepossessing Bayou Lafourche, which, by the year 1000, was losing its hegemony to the river's present course, through the region that would be known as Plaquemines. By the nineteen-fifties, the Mississippi River had advanced so far past New Orleans and out into the Gulf that it was about to shift again, and its offspring Atchafalaya was ready to receive it.
For the Mississippi to make such a change was completely natural, but in the interval since the last shift Europeans had settled beside the river, a nation had developed, and the nation could not afford nature. The consequences of the Atchafalaya's conquest of the Mississippi would include but not be limited to the demise of Baton Rouge and the virtual destruction of New Orleans. With its fresh water gone, its harbor a silt bar, its economy disconnected from inland commerce, New Orleans would turn into New Gomorrah. Moreover, there were so many big industries between the two cities that at night they made the river glow like a worm. As a result of settlement patterns, this reach of the Mississippi had long been known as "the German coast," and now, with B. F. Goodrich, E. I. du Pont, Union Carbide, Reynolds Metals, Shell, Mobil, Texaco, Exxon, Monsanto, Uniroyal, Georgia-Pacific, Hydrocarbon Industries, Vulcan Materials, Nalco Chemical, Freeport Chemical, Dow Chemical, Allied Chemical, Stauffer Chemical, Hooker Chemicals, Rubicon Chemicals, American Petrofina--with an infrastructural concentration equaled in few other places--it was often called "the American Ruhr." The industries were there because of the river. They had come for its navigational convenience and its fresh water. They would not, and could not, linger beside a tidal creek. For nature to take its course was simply unthinkable. The Sixth World War would do less damage to southern Louisiana. Nature, in this place, had become an enemy of the state.
The Atchafalaya steadily took more and more of the Mississippi's water to the Gulf of Mexico during the 20th Century, until by 1950, it had captured 30% of the great river's flow, becoming the 4th largest river in the U.S. by volume discharge. The Army Corps of Engineers stepped in, and in the late 1950s began construction of a massive structure that resembled a dam with gates to control the amount of water escaping from the Mississippi to the Atchafalaya. This "Low Sill Structure", completed in 1963, consisted of a dam with 11 gates, each 44 feet wide, that could be raised or lowered. The entire structure was 566 feet long. A companion "Overbank Structure" was built on dry land next to the Low Sill Structure, in order to control extreme water flows during major floods. The Overbank Structure had 73 bays, each 44 feet wide, and was 3,356 feet long. The total cost of the two structures: about $300 million.
Figure 2. Aerial view of the Mississippi River's Old River Control Structure, looking downstream (south.) Image credit: U.S. Army Corp of Engineers.
The flood of 1973: Old River Control Structure almost fails
For the first ten years after completion of the Old River Control Structure, no major floods tested it, leading the Army Corps to declare, "We harnessed it, straightened it, regularized it, shackled it." 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.
The flood of 1973 permanently damaged the Low Sill Structure, forcing the Corps to build additional structures to control future great floods. The first of these structures was the Auxilliary Control Structure. This 442-foot long structure, completed in 1986, consisted of six gates, each 62 feet wide, and cost $206 million to build. Joining the mix in the late 1980s was a 192-megawatt hydroelectric power plant, build at a cost of $520 million.
Figure 3. The flow of water in the Mississippi River as of Friday, May 13 (red line) has exceeded 2 million cubic feet per second, and was approaching the all-time record (dashed blue line.) Image credit: USACE.
The Old River Control Structure's greatest test: the flood of 2011
Flow rates of the Mississippi at the latitude of the Old River Control Structure are expected to exceed the all-time record on Saturday, giving the Old River Control Structure its greatest test since the flood of 1973. Since there are now four structures to control the flooding instead of just the two that existed in 1973, the Old River Control Structure should be able to handle a much greater flow of water. The flood of 2011 is not as large as the maximum 1-in-500 year "Project Flood" that the Old River Control Structure was designed to handle, and the Army Corps of Engineers has expressed confidence that the structure can handle the current flood. However, the system has never been tested in these conditions before. This is a dangerous flood, and very high water levels are expected for many weeks. Unexpected flaws in the design of the Old River Control Structure may give it a few percent chance of failure under these sorts of unprecedented conditions. 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. I'll say more about the potential costs of such an event in a future post.
According to the latest information from the Army Corps the Old River Control Structure is currently passing 624,000 cubic feet per second of water, which is 1% beyond what is intended in a maximum "Project Flood." The flow rate of the Mississippi at New Orleans is at 100% of the maximum Project Flood. These are dangerous flow rates, and makes it likely that the Army Corps will open the Morganza Spillway in the next few days to take pressure off of the Old River Control Structure and New Orleans levees. Neither can be allowed to fail. In theory, the Old River Control Structure can be operated at 140% of a Project Flood, since there are now four control structures instead of just the two that existed in 1973 (flows rates of 300,000 cfs, 350,000 cfs, 320,000 cfs, and 170,000 cfs can go through the Low Sill, Auxiliary, Overbank, and Hydroelectric structures, respectively.) Apparently, the Corps is considering this, as evidenced by their Scenario #3 images they posted yesterday. This is a risky proposition, as the Old River Control Structure would be pushed to its absolute limit in this scenario. It would seem a lower risk proposition to open the Morganza spillway to divert up to 600,000 cfs, unless there are concerns the Corps has they aren't telling us about.
Figure 4. Kayaking, anyone? The stilling basin downstream of the Low Sill Structure of the Old River Control Structure, as seen during major flood stage of the Mississippi River on May 10, 2011. The flow rate is 2 - 3 times that of Niagara Falls here. Video by Lee Alessi.
John McPhee's fantastic essay, The Control of Nature
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