Tuesday, November 15, 2005

New Orleans Levees and Floodwalls

by Dianne Kim (Class of '05)

The havok and destruction wreaked by hurricane Katrina brought into question the effectiveness of the levees put in place to combat such natural disasters. The levees were designed to block storm surges of category 3 storms and katrina entered New Orleans as a category 4 storm. Before redesigning a levee, engineers must consider this: Is the Mississippi Delta still capable of serving as a buffer that can absorb surges and rising ocean levels or has it been washed away beyond repair, necessitating a 300-mile wall to hold back the Gulf?

The hurricane also has a significant impact on nature and wildlife. Environmentalists had filed a suit at the U.S. District Court in New Orleans, claiming that bottomland hardwood wetlands must be spared if the Lousiana black bear is to survive and that the lands serve as breeding grounds for many species of birds in lower Mississippi. For years, environmentalists have condemned levees as artificial barriers to nature, “robbing the river of its ability to sustain itself by disrupting the natural flow and deposit of the Mississippi River’s sediments. The levees installed at the mouth of the MS river spared New Orleans but starved the wetlands of the sediment, nutrients and freshwater they need to survive.

Could the destruction of Katrina been prevented? Researchers say yes. Upon arriving at the scene one month after the hurricane left, they found more breaches throughtout the levee system than they had anticipated. The storm surge eroded sols from the base of the landward side of some levee section. If this wasn’t the scenario, water percolated under the sheet plings through layers of peat, sand and clay, and bubbled up on the other side. These levees were driven only 10 feet into the ground, whereas levees driven 25 feet into the ground kept the water at bay.

In 1998, scientists and engineers proposed a $14 billion dollar plan called Coast 2050, which outlined strategies to revive the delta and control flooding. It was rejected by Congress but considering the rate of wetland loss, land subsidence, seal-level rise and increasing frequency and severity of storms, the proposed should be on the table.

Coast 2050 proposed several different strategies: one such plan proposes connecting the barrier islands and outer marshes with levees, dams and floodgates. This would essentially create a continuous rim, or circle of safety, if you will, around the delta.

The Dutch goverment used this network but to prevent the loss of marshland, they erected an extensive series of sluices whose doors remain open yearround and close when storms approach.

Venice, the sinking city, will rely on mobile floodgates as well, but there’s a spin to it. The mobile floodgates will lie flat on the seabed under normal conditions and rise only during extremely high tides. All these efforts are put in place to appease environmentalists.

Engineers need a new vision to revive LA. The region produces one-third of the country’s seafood and wintering for 70% of the nation’s migratory waterfowl. And still, at least 25 square miles are receding every year. Katrina didn’t help either. The excess salt probably “baked” the soil, and is expected to kill off marsh grass.

So what are the lessons we need to draw from Katrina? Global-warming models show that sea level will climb 1-3 feet this century and increase the frequency and intensity of storms. Some may argue that we are currently going through a cycle but cycles run for 25-30 years and we are only 8 years into the current one. The Coastal Vulnerability Index, a fun-tuned formula made by the USGS, predicts how at risk an open coast-line is to high seas.

Though politicians are trying to save face and are in a scramble to act fast, a long-term plan is needed. Instead of working against nature, we need to work with it.

  • Scientific American, November 2005 "Protecting Against the Next Katrina."
  • www. csmonitor.com, "Greens vs. Levees"
  • www.fas.org CRS Report for Congress, "Hurricane Damage Protection"

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