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Hurricane Katrina

When Hurricane Katrina reached Category 5 in 2005, some experts predicted that the levee system might fail completely if the storm passed close to the city. Although Katrina weakened to a Category 4 before making landfall on August 29 (with only Category 3 strength in New Orleans on the weaker side of the eye of the hurricane), the levees designed to withstand Category 3 storms suffered multiple breaks the following day (see Effect of Hurricane Katrina on New Orleans) flooding 80% of the city.

The walls of the Industrial Canal were breached by storm surge via the Mississippi River Gulf Outlet, while the 17th Street Canal and London Avenue Canal experienced catastrophic breaches, even though water levels never topped their flood walls. Louisiana State University experts presented evidence that some of these structures might have had design flaws or faulty construction.

There are indications that the soft earth and peat underlying canal walls may have given way. In the weeks before Katrina, tests of salinity in seepage pools near canals showed them to be lake water, not fresh water from broken mains. The 5.5 mile (9 km) long I-10 Twin Span bridge heading northeast between New Orleans and Slidell was destroyed. Apparently, a bit farther east, the shorter Fort Pike Bridge crossing the outlet to Lake Borgne remained intact. By mid-October, one side of the Twin Span had been repaired and was ready to reopen to two-way traffic. But there are doubts whether the levees can even be restored to pre-Katrina condition by next summer (2006).

On September 5, 2005, the Army Corps of Engineers started to fix levee breaches by dropping huge sandbags from Chinook helicopters. The London Avenue Canal and Industrial Canal were blocked at the lake as permanent repairs started. On September 6, the Corps began pumping flood water back into the lake after seven days in the streets of New Orleans, even though it was fouled with human and animal corpses, sewage, heavy metals, petrochemicals, and other dangerous substances. Filters on the pumps kept out large debris.

Aerial photography suggests that 25 billion gallons (95 bn liters) of water covered New Orleans as of September 2, which equals about 2% of Lake Pontchartrain's volume. Due to a lack of electricity, the city was unable to treat the water before pumping it into the lake. It is unclear how long the pollution will persist and what its environmental damage to the lake will be, or the hazards from the mold and contaminated mud remaining in the city.

On September 24, 2005, Hurricane Rita again overtopped levees#redirect and breached walls, allowing water back into the nearly-dry city. The Corps of Engineers had most of it pumped out by the second week in October.

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