Tuesday, March 17, 2015

Waitin' On the Levee

When Hurricane Katrina was still just a Weather Channel threat predicted to strike the historic city of New Orleans, and orders for evacuating the city were still days in the future, my thoughts turned 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 over fifty years ago (forty-plus years before Katrina) by the U.S. Army Corps of Engineers to prevent Mississippi drainage from switching to the steeper Atchafalaya channel.

Geologically, the Mississippi River has switched channels many times to build the Mississippi delta. Today, a channel change would mean stranding the port economy of New Orleans, leaving farmers and industries along the lower reaches of the Mississippi without the water they need. The expensive levee system erected along the Mississippi (extensively rebuilt following Katrina) 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, necessitating extensive relocations, and causing 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.
The Mississippi Flood of 1993 may have been a 100-year event.

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 overrunning 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. 

Breaches of the levee structure in New Orleans during Katrina in 2005 did extensive damage to property, but this very flooding may have saved the city—for the time being—from the disaster of the river switching channels.

In John McPhee's marvelous essay, "Atchafalaya," published in his book The Control of Nature, McPhee 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. —John McPhee, "Atchafalaya"

The flood of 1973 was a "40-year event," meaning that one might be expected, statistically and on average, every forty years. That the next flood came only thirty-two years later takes nothing away from this assessment: the river's overbank flow during and following Katrina had contributions from the release of impounded water from behind upstream levees, and energy from these successive releases owes much to the continued attempts by the Army Corps of Engineers to deny the river’s natural inclination to jump to the Atchafalaya drainage.

So inundation of the city from Katrina, a Category 5 storm, caused primarily by over fifty levee breaches, might have produced overbank flooding on the upper reaches of the river sufficient to allow the Atchafalaya to finally capture the Mississippi, had stress not been relieved farther down the river.

Besides, "[n]othing 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." The city of New Orleans may have survived the storm surge of Hurricane Katrina—devastating though it was—only to be hit sometime down the road with a second economic disaster. 

It could happen anytime an up-river rainstorm causes the Corps of Engineers to lose their 52-year battle for control of the Mississippi River.