exacerbate the problem. Here's an explanation from Time...
>snip 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. That's bad news for a city that is already an average of 8 ft. below sea level. Making things worse: sea levels worldwide are rising as much as 3 ft. a century on account of global warming. The lower New Orleans plunges, the worse it will be when the big one hits.
New Orleans' other major man-made problem is that its wetlands and its low-lying barrier islands are disappearing. The Louisiana coast is losing 16,000 acres of wetland each year, mostly as a result of population expansion into once pristine areas, destructive oil and gas drilling, pollution and land loss through lack of sedimentation. As it turns out, wetlands and barrier islands aren't just nice to look at; they are also a key natural barrier to hurricanes. (Every 2.7 miles of wetland absorbs a foot of storm surge.) As the wetlands go, the chance of a hurricane blowing the city away grows.
So environmentalists and engineers are frantically coming up with plans to save New Orleans. One idea is to raise levee walls to increase their effectiveness against storm surges. Another is to create large-scale diversions that would allow the Mississippi to flood in a controlled manner—and through sedimentation add thousands of acres a year of new land. Yet another would be to take immediate steps to reverse the loss of sensitive wetlands. Adding land through sedimentation is one of the best ways of restoring wetlands. Among other possible schemes: cutting back on shipping routes that harm marshes, installing wave absorbers to reduce wetland erosion and rebuilding damaged barrier islands. >more
http://www.time.com/time/reports/mississippi/orleans.htmlWetlands are so important! Besides controlling floods, they help to clean water and provide a habitat for a large variety of plant and animal species.
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