Society and the Environment: Hurricane Katrina and New Orleans

New Orleans

Over 80 percent of New Orleans was submerged by floodwater when Hurricane Katrina struck in August, 2005.

The city of New Orleans was built on the Mississippi River Delta, about 160 kilometers upriver from the Gulf of Mexico. This city is vulnerable to flooding from the Mississippi River, Lake Pontchartrain to the north, and heavy rainfall from tropical storms. In addition, hurricanes that pass over the coast can create storm surges—waves up to 9 meters.

To help protect New Orleans from flooding, engineers and city planners built levees and flood walls along the river banks and lakeshore. They constructed pumps to move floodwater from lower-lying areas through canals into Lake Pontchartrain. Despite these measures, Hurricane Katrina overwhelmed the city in late August, 2005. Why was the damage so severe?

Analyses of the catastrophe concluded that human-made changes in the natural environment were partly responsible for the damage.

Protection from the Sea
The Mississippi River Delta formed from sediment carried down the river over several thousand years. It consists of saltwater and freshwater marshes, mud flats, and creeks, collectively known as coastal wetlands. Winds and currents move loose sediment to build up barrier islands. These islands shelter the coastal wetlands and mainland from the ocean. Plants growing in the wetlands trap sediment and help to stabilize the land. The Louisiana coast has about 40 percent of all the coastal marshes in the continental United States. These coastal wetlands are an important habitat for crustaceans, mollusks, fish, and birds. As well, they filter out pollutants from the river, absorb floodwater, and help to supply fresh water to aquifers.

Eroding the Barrier Islands
As levees were constructed to confine the flow of the Mississippi River, sediment carried by the river was flushed farther out into the Gulf of Mexico. The sediment was no longer deposited to build up more land.

Canals that were built through the barrier islands to handle river traffic increased the erosion of the coastal wetlands.Soil dug from the canals was piled on the banks, smothering vegetation that had helped to hold the banks in place. The increased speed and volume of the water in the canals washed away more soil from the barrier islands. As the barrier islands eroded, the marshes and land behind them were left exposed and were washed away.

The Impact of Katrina
Most of the damage from Hurricane Katrina was caused by rising water that overflowed or broke through the levees. A wide shipping canal funneled a 4.6 meters storm surge from the ocean into the city. The storm surge broke through the banks of the canal. Other canals, built to drain water into Lake Pontchartrain, had their flow reversed as the water level in the lake rose. Eventually, the city’s drainage system failed when most of the pumping stations were submerged.

As a result of these factors, an estimated 300,000 homes were destroyed or damaged beyond repair. More than 1300 people, 70 percent of whom were elderly, died.

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