Sunday, August 28, 2005

Katrina could alter Louisiana geography, Mississippi river flow


As Hurricane Katrina stalks the historic city of New Orleans, and massive evacuations are ordered, my thoughts turn to an already-stressed structure located several hundred miles upstream of New Orleans at the distributary channel of the Atchafalaya with the Mississippi River. The Old River Control structure was built by the Army Corps of Engineers to prevent the Mississippi drainage from switching to the steeper Atchafalaya channel.

Geologically, the Mississippi River has switched channels many times to build the Mississippi delta. Today, that change would mean stranding the port economy of New Orleans, with farmers and industries along the lower reaches of the Mississippi without the water they need. The expensive levee system erected along the Mississippi would no longer be needed, while a new levee system would have to be built on the Atchafalaya.

In addition, the Atchafalaya River could not accept the Mississippi flow without massive flooding of the basin's bayous, extensive relocations, and the upheaval of the social and economic patterns of the area. Since the completion of Old River Control in 1963, therefore, the Corps of Engineers has striven to prevent the river from jumping channels.

But the water will not be denied forever.

Since 1963, the coastal salt marshes, an important buffer for New Orleans against Gulf hurricanes, have diminished as the basin subsided. The Mississippi River continues to raise its bed in a natural process of stream-bed deposition, even as the surrounding ground sinks lower. The result is a city not only mostly below sea level, but also well below river level. Only the levees (whose bases have also been sinking) prevent the Mississippi from over-running its banks and flooding the streets, even in the driest season.

Also since 1963, the Mississippi has experienced several devastating floods. During the high waters of the Flood of 1973, water undercut the Old River Control structure and nearly swept away an entire sidewall. Rather than lose the control structure, the Corps let the water run through into the Atchafalaya basin, restoring the 70% flow to the lower Mississippi River only after the flood waters subsided. The record-breaking flood of 1993, even though its effects were mostly felt along the upper reaches of the river, also required the control to be let run, which further undercut the structure.

In John McPhee's marvelous essay, "Atchafalaya," published in his book The Control of Nature, he relates the first-hand account of the damage taken by the control structure in 1973.
When Dugie himself went to look at the guide wall, he looked at it for the last time. "It was dipping into the river, into the inflow channel." Slowly it dipped, sank, broke. The foundations were gone. There was nothing below it but water. Professor Kazmann likes to say that this was when the Corps became "scared green." Whatever the engineers may have felt, as soon as the water began to recede, they set about learning the extent of the damage. The structure was obviously undermined, but how much so, and where? What was solid, what was not? What was directly below the gates and the roadway? With a diamond drill, in a central position, they bored the first of many holes in the structure. When they had penetrated to basal levels, they lowered a television camera into the hole. They saw fish.

The flood of 1973 was a "40-year event," meaning that one might be expected, on average, every forty years. The inundation heading toward Louisiana on the fringes of Katrina, a Category 5 storm, could produce local flooding on the lower reaches of the river sufficient to allow the Atchafalya to finally capture the Mississippi. "Nothing says capture has to happen at Old River," McPhee was told by a local. "It could happen anywhere the two channels are close enough. It probably will someday."

New Orleans could survive the storm surge of Hurricane Katrina—however devasting—only to be hit with a second economic disaster, if the up-river rain causes the Corps of Engineers to lose their 42-year battle with the river at Old River Control.

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Blogger samraat said...

4/03/2010 10:57 PM  

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