They failed for a combination of reasons. Some were overwhelmed by the enormous storm surge caused by Hurricane Katrina, while others failed because of weaknesses in their design and the soft soil supporting them. The flood defenses had also developed over many years rather than being designed as one integrated system.
A levee is basically a compacted pile of earth that is built along a waterway to prevent flooding. It has to be sturdy enough to hold up against the water and high enough for the water not to overflow it. The word comes from the French lever, to raise, because the ground is being raised up to keep the water away. Levees have been made for thousands of years because they are a good way of protecting land that would otherwise be vulnerable to flooding.
New Orleans was founded in 1718 by a Frenchman called Jean-Baptiste Le Moyne de Bienville. He was working for the French Mississippi Company and founded the city near the mouth of the Mississippi River. He named it after Philippe II, Duke of Orleans. Whoever controlled this area could control access between the Mississippi River system and the Gulf of Mexico. New Orleans became the capital of the French territory of Louisiana and started to grow. Huge numbers of enslaved Africans were brought to the area to work on plantations. The United States took control of New Orleans when it bought the Louisiana Territory in 1803, and the city continued to grow. It became a huge center of trade and slavery. The mixture of different peoples and cultures made New Orleans one of the most distinctive cities in America. It became the birthplace of jazz, and its Mardi Gras is world famous.
As New Orleans grew, people wanted to build on the marshy, low-lying land surrounding the original city. To do this, they needed pumps to pull the water out. Unfortunately, draining the land caused some of the soft soil underneath to compact and sink. Over time, large parts of New Orleans ended up below sea level. The city is also surrounded by water, including the Mississippi River and Lake Pontchartrain. This meant that New Orleans became increasingly dependent on its flood defenses. Although people usually refer to them as “the levees,” the city was actually protected by a complicated system of earth levees, concrete floodwalls, canals and pumps.
In 2005, Hurricane Katrina approached New Orleans. It had reached Category 5 while it was over the Gulf of Mexico, although it had weakened to Category 3 by the time it made landfall in Louisiana. However, it was an enormous storm and it pushed a huge storm surge towards the coast. The flood protection system failed in many places and roughly 80% of New Orleans was flooded. More than a thousand people died in Louisiana, with drowning being one of the major causes of death. Because large parts of the city were below sea level, the water couldn’t simply drain away again and had to be pumped out. So, why didn’t the flood defenses do their job?
The first problem was that they had not been designed as one complete system. The levees and floodwalls had been built in different sections, at different times, using different designs. Different organizations were also responsible for different parts of the system. Some sections were higher than others and some were stronger than others. A flood protection system is only as strong as its weakest points because water finding a way through one section can flood the area behind it.
The second problem was the design of some of the floodwalls. Some canals were lined with concrete walls supported by long steel sheet piles driven into the ground. When the water level rose, it pushed sideways against these walls. Engineers had underestimated how the soft soil underneath them would behave under this pressure. At some locations, a gap opened between the floodwall and the soil on the water side. Water entered the gap and increased the pressure against the wall. Eventually, the soil underneath could no longer resist the force and sections of the floodwall and levee moved sideways and collapsed. Shockingly, some floodwalls failed even though the water had not reached their tops.
Other parts of the flood protection system failed in a different way. The storm surge was so high that water flowed over the tops of some levees and floodwalls. The water then rushed down the other side and began to wash away the earth supporting them. Once enough soil had been eroded, sections of the flood defenses collapsed and enormous amounts of water could pour through the resulting gaps.
This meant there wasn’t one single reason why the levees failed. In some places, Katrina simply overwhelmed the defenses. In others, the defenses failed at water levels they should have been able to withstand because of weaknesses in their design and the soil supporting them. The fact that the flood defenses had developed piecemeal over many years made the whole system more vulnerable.
After Katrina, engineers completely reconsidered the way New Orleans was protected. Billions of dollars were spent building a much more integrated system of stronger levees and floodwalls, enormous pumps, barriers and gates that can be closed when a storm approaches. Katrina was an enormous natural disaster, but the flooding of New Orleans was not caused by the hurricane alone. It was also the result of engineering and organizational failures, and the lessons learned from those failures have changed the way flood defenses are designed. And this is what I learned today.
Sources
https://www.science.org/content/article/how-new-orleans-levees-buckled#
https://ready.nola.gov/hazard-mitigation/hazards/infrastructure-failure-levee-failure
https://en.wikipedia.org/wiki/Investigations_of_levee_failures_during_Hurricane_Katrina
https://en.wikipedia.org/wiki/Levee
https://en.wikipedia.org/wiki/New_Orleans
https://en.wikipedia.org/wiki/Hurricane_Katrina
Photo by John Hill: https://www.pexels.com/photo/aerial-view-of-city-buildings-and-river-13976391/