New Orleans and Hurricane Katrina
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New Orleans, much of which sits below sea level, is surrounded by the Mississippi River to the south, Lake Pontchartrain to the north, and the Gulf of Mexico to the east. Construction of the levees between New Orleans, the River, and the Lake began in 1879. The earthen barriers were originally erected to prevent damage caused by seasonal flooding and to allow the city to expand beyond the natural levees on which it had been initially constructed.

Unfortunately, the levees interfered with the normal process of the River depositing sediment and building up the land of the delta marshlands during the periodic floods. Interrupting a process that created the land of the Mississippi Delta over the course of thousands of years caused the land to dry out. In turn, the swampy lands of Southern Louisiana shrank like a sponge, the land began to sink and entire barrier islands disappeared as the land of the vast delta slowly settled into the sea.

The subsidence of the land of southern Louisiana can be attributed to one or more of the following:

the leveeing of the Mississippi River,
the pumping of ground water from under the city,
the failure to address the environmental impact of development on the Mississippi Delta,
environmental damage caused by oil and gas production,
the failure to maintain or upgrade the levee and flood wall system despite many studies that warned of impending disaster. [1]
Interestingly, the land of New Orleans and the surrounding communities was not below sea level when the communities were built. The area began to sink precipitiously only after the current levee system was erected in the 1940s and 1950s and accelerated when the shipping canal flood walls were completed in the mid 1960s.

Shea Penland, a geologist at the University of New Orleans and contractor for the U.S. Army Corps of Engineers, which maintains the levees, attributes one third of the land subsidence to the large number of canals through the delta. Barge traffic and tides erode the earth around the edge of the canals, and salty Gulf water seeps in along them, slowly salinating the ground and killing the vegetation that helps hold the land together. [2]

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Flood walls and levees
To the surprise of some experts, the earthen levees in New Orleans city did not fail during Hurricane Katrina. Instead, the flood walls lining the shipping canals gave way and flooded the city and surrounding areas with the water from Lake Ponchartrain, which was then several feet above sea level. These flood walls are little more than two feet thick and were engineered in the 1960s to withstand only a Category 3 hurricane. Additionally, destruction of Cypress trees and other vegetation that previously thrived in the brackish waters where the mouth of the Mississippi River meets the Gulf of Mexico crippled the natural salt water filtration system. This damage escalated the process of erosion and removed the natural storm protection system that historically helped weaken storms before they struck heavily populated inland areas.

The final trigger to the catastrophe was hurricane damage to these flood walls that contained the water of the shipping canals that traverse the city. According to Col. Richard Wagenaar of the Army Corps of Engineers, three flood walls were breached in five separate places along the Industrial Canal, London Avenue Canal and the 17th Street Canal. [3]

Possible failure mechanisms being investigated by engineers include:

Overtopping by storm surge and consequential undermining of flood wall foundations, or other weakening by water of the wall foundations,
Storm surge pressures exceeding the strength of the floodwalls, and
Impact by vessels such as barges which had broken free of their moorings.
While it could be considered miraculous that the flood walls held at all during the Category 4 (or arguably Category 5) storm, they were no match for the secondary storm surge from Lake Pontchartrain. This failure of the flood walls which protected the city along the shipping canals caused a major flood that inundated the city and some of the surrounding Parishes. By August 31, the water level in New Orleans reached the level of Lake Pontchartrain, with close to 90% of New Orleans under water.

The pumping stations that were designed to remove flood waters from the city were flooded and could not be operated until dried out and supplied with power. However, even if most had continued to operate, their pumping capacity would have been insufficient to entirely protect from the flow through 5 floodwall breaches, which took only a day to flood some 80% of the city.

The levee system sustained heavy damage other than the five breaches [4]. Aerial evaluation revealed that up to 90 percent of the levee system was damaged. This forced the evacuation of New Orleans before the possible storm surge from Hurricane Rita.

Visit: NewOrleans-HurricaneKatrina.com
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