Levees broke in New Orleans following Hurricane Katrina because the storm surge was too massive, and the levees were poorly designed, according to USA Today. The U.S. Army Corps of Engineers issued a 6,000-page report identifying poor soil conditions that undercut levees protecting New Orleans. Some structures were overtopped by water, but three major levees collapsed due to soil failure, according to the U.S. Geological Survey.
Most of the levees failed when water rose too high and then rose on the dry side of the levee. Water then penetrated the levee from the opposite side of the storm surge and flooded the city. Levee breaches destroyed 169 miles of the 350-mile system near the city.
Undercut levees with faulty soil were breached at weak transition points between levees of different heights. Many of the bad transition points had weaker soil at lower depths than higher levees next to them. Flooding inundated nearly 80 percent of New Orleans following Hurricane Katrina's storm surge.
Science teams used sophisticated laser mapping and GPS systems to determine how the levees failed. The full investigation team scoured the city from Sept. 28 to Oct. 15, 2005. The final report was released in June 2006.
The report cost $19.7 million, and the Army Corps of Engineers spent billions of dollars to repair and upgrade levees following the storm. More than 1,500 people lost their lives in New Orleans because of Hurricane Katrina.