Long read, but the most comprehensive, well-written article that I have yet seen on what the USACE has to deal with in controlling the flow of these two rivers.

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quote:
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. 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. By the route of the Atchafalaya, the distance across the delta plain was a hundred and forty-five miles—well under half the length of the route of the master stream.
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 equalled 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.

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Posted by beachdude

Member since Nov 2008

5640 posts

Posted on 5/11/11 at 8:32 pm to LegalTiger

Very good essay. I read it in book form when the volume The Control of Nature was published. If you've never read anything by John McPhee, this one is a great place to start.

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Posted by LegalTiger

Member since Nov 2006

1098 posts

Posted on 5/11/11 at 9:30 pm to beachdude

quote:
Calculating maximum storms, frequency of storms, maximum snowmelts, sustained saturation of the upper valley, coincident storms in scattered parts of the watershed, the Corps reached for the figure that would float Noah. The round number was three million—that is, three million cubic feet per second coming past Old River. This was twenty-five per cent above the 1927 high. The expanded control system, with its variety of devices, would have to be designed to process that. Various names were given to this blue-moon superflow, this concatenation of recorded moments written in the future unknown. It was called the Design Flood. Alternatively, it was called the Project Flood.
Bonnet Carre was the first spillway—completed in 1931, roughly thirty miles upriver from New Orleans. The water was meant to Spill into Lake Pontchartrain and go on into the Gulf, dispersing eight and a half per cent of the Project Flood. Bonnet Carre (locally pronounced “Bonny Carey”) would replace dynamite in the defense of New Orleans. When the great crest of 1937 came down the river—setting an all-time record at Natchez—enough of the new improvements were in place to see it through in relative safety, with the final and supreme test presented at Bonnet Carre, where the gates were opened for the first time. At the high point, more than two hundred thousand feet per second were diverted into Lake Pontchartrain, and the flow that went on by New Orleans left the city low and dry.

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The Atchafalaya, continuing to grow, had become, by volume of discharge, the second-largest river in the United States. Compared with the Mississippi, it had a three-to-one advantage in slope. Around 1950, geologists predicted that by 1975 the shift would be unstoppable. The Mississippi River and Tributaries Project would be in large part invalidated, the entire levee system of southern Louisiana would have to be rebuilt, communities like Morgan City in the Atchafalaya Basin would be a good deal less preserved than Pompeii, and the new mouth of the Mississippi would be a hundred and twenty miles from the old. Old River Control was authorized in 1954.
The levees were raised again. What had been adequate in 1937 was problematical in the nineteen-fifties. New grades were set. New dollars were spent to meet the grades. So often compared with the Great Wall of China, the levees had more in common with the Maginot Line. Taken together, they were a retroactive redoubt, more than adequate to wage a bygone war but below the requirements of the war to come. The levee grades of the nineteen-fifties would prove inadequate in the nineteen-seventies. Every shopping center, every drainage improvement, every square foot of new pavement in nearly half the United States was accelerating runoff toward Louisiana. Streams were being channelized to drain swamps. Meanders were cut off to speed up flow. The valley’s natural storage capacities were everywhere reduced. As contributing factors grew, the river delivered more flood for less rain. The precipitation that produced the great flood of 1973 was only about twenty per cent above normal. Yet the crest at St. Louis was the highest ever recorded there. The flood proved that control of the Mississippi was as much a hope for the future as control of the Mississippi had ever been. The 1973 high water did not come close to being a Project Flood. It merely came close to wiping out the project.