Tuesday, August 30, 2005
Levees And Riots In New Orleans
If the levee break is as bad as feared, there may be a chain of other events in the city. Water is heavy, and the modern infrastructure of New Orleans has never been tested under weeks of this sort of test. Buildings may shift and other levees may be undermined and fail if waterlogged for too long. A couple of gas explosions could produce frightening effects if they jiggle the soil enough.
They have to clear the city quickly. They have to. After about three days no one has any idea of what may happen. Here is an article from last year that describes some of the problems. The levee system was greatly expanded in the 1970s, and it undercut the wetlands:
And there's another reason why scientists worry more about hurricanes every single year. There's always been a huge natural buffer that helps protect New Orleans from storms. There are miles of wetlands between here and the Gulf of Mexico: they slow hurricanes down as they blow in from the sea. But that buffer is disappearing. Every year, a chunk of wetlands the size of Manhattan crumbles and turns into open water.The Barking Dingo posted an article about stupid encroachments on those wetlands. The whole story is complicated, but the major point is that when they stopped the Mississippi from flooding, they stopped the natural feeder system of the wetlands. So, to continue:
Joe Suhayda explains, "So the hurricane can move closer to the city before it starts to decrease. So in effect, the city is moving closer to the Gulf as each year goes by."
And he says, it's partly because of those levees along the Mississippi River. When they stopped the river from flooding, they also prevented the wetlands from getting the regular doses of floodwater and mud that they need to survive. Studies show that if the wetlands keep vanishing over the next few decades, then you won't need a giant storm to devastate New Orleans — a much weaker, more common kind of hurricane could destroy the city too.
New Orleans has protected itself from past floods partly with the levees, but the city also operates one of the biggest pumping systems on earth. There are giant turbines all across town, and every time there's a major rain, they suck up the water and pump it out. Combes says that system won't work after a huge hurricane.But one thing the article did not cover is that enough water sitting inside the levee system for too long will change the underlying soil characteristics. The land has been artificially dried and so it is artificially stable. Weeks of waterlogging will inevitably produce more fluid sections and pockets between more stable chunks (clay/gravel deposits) and soil substrates. You could see rapid subsidence in some areas and land rising in others. You could see some sideways slippage of clay substrates. Roads will buckle and buildings may shift. The mixture of soils that compose a delta can become quite unstable under certain conditions.
All through history mankind has been building cities on river deltas, and history resounds with stories of their sudden downfalls. Modern cities are actually lighter than most older ones with massive stone construction, but the underlying problem is the same. A sudden disruption of an artificially-created but balanced slow deterioration of the delta formation starts a chain reaction.
For more information see this Time article:
What is threatening New Orleans is a combination of two man-made problems: more levees and fewer wetlands. The levees installed along the Mississippi to protect the city from water surges have had a perverse effect: they have actually made it more vulnerable to flooding. That's because New Orleans has been kept in place by the precarious balance of two opposing forces. Because the city is constructed on 100 feet of soft silt, sand and clay, it naturally "subsides," or sinks, several feet a century. Historically, that subsidence has been counteracted by sedimentation: new silt, sand and clay that are deposited when the river floods. But since the levees went up—mostly after the great flood of 1927—the river has not been flooding, and sedimentation has stopped.
The upshot is that New Orleans has been sinking as much as 3 ft. a century.Science Daily, 2000. Also from 2000, Risk & Insurance. This explains the subsidence problem well, plus explains how the newer larger buildings are constructed with special types of pilings to stabilize them in unstable soils. Basically, drying soil makes it heavier, and buildings on top of that push down. That is why chunks of the city are sinking.
Another Radioworks article. This debate has been going on forever, but once you stopped delta formation by stopping the flooding of the Mississippi, you changed the balance. This can't be undone. The grasses and other flora that stabilize the land nearest the sea are under constant attack by high tides and storms. Without the new influx of soil and freshwater that the floods bring, they too sink and are beaten back by the sea:
Houck says before people built these walls, the giant Mississippi helped build America. Every day, the river and its tributaries washed millions of pounds of soil from all over the country down to the Gulf of Mexico.You just can't artificially replace inputs that huge. You can't. All the talk of mitigation projects avoids the underlying reality, which is that we have destabilized the land itself, so the sea must win. Among some of the ideas that have been floated (sorry, it just seemed irresistible) have been rerouting the Mississippi and building artificial islands in the Gulf to act as breakwaters. Here's one of the projects that has been attempted to restore the fresh/salt water balance in the wetlands. Very interesting.
"You can imagine what it would take in dump trucks to bring half a million tons of silt to south Louisiana," says Houck. "Well, it would take about two hundred thousand, two-and-half ton dump trucks every day, driving from Minnesota, from Rapid City, from Pittsburgh, from Denver. And in so doing, it brings down these enormous, enormous loads of earth to the mouth of the Mississippi."
Every year or so, Houck says, it would rain so much that the river would gush out of its banks, and all that mud and goo would spread out along the coast.
"And that's what built south Louisiana," Houck says. "The Mississippi built five million acres of land. A huge amount of land was wetland."
But when French settlers showed up in the 1700s, they tried to stop the Mississippi from flooding: they started building these walls. Eventually, the U.S. Army took over the job, and every time they thought they'd conquered nature, the river proved them wrong. So the army built more walls and they built them higher, they've built two thousand miles of levees as of today along the Mississippi River and its branches. And Houck says, the army has finally won the war—they've tamed the Mississippi.
"And so," describes Houck, "the project was—from an engineering point of view— brilliant, brilliant. It was hugely successful. From an environmental standpoint, it was a disaster."
There are questions as to whether all of this can work at all, but we will have to choose between abandoning New Orleans or trying to artificially create some sort of new, greater balance. However, all the plans I have ever read don't address the real problem, which is that the soil deposition is not occurring any more, and New Orleans will continue to sink. Shoring up the wetlands will help, but there are very scientists who believe it can fix the problem. We have changed the balance of the Mississippi forever.
One thing that strikes me from a historical point is that people keep doing the same thing over and over again. River mouths are great places to build a city, but not stable places. We think we are so smart, but the engineering projects of the 40s, 50s and 60s have often turned into the engineering or societal problems of the 80s, 90s, 00s. We have gotten to be good enough engineers that our own efficiency can be a danger to us.
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