The factor that contributed the most to the flooding in New Orleans during Hurricane Katrina was the failure of the levee system. The levees were designed to protect the city from storm surge and flooding, but they were not strong enough to withstand the intensity of Hurricane Katrina. As a result, many of the levees collapsed or were breached by the storm, leading to severe flooding in the city. Other factors, such as the low-lying nature of the city and poor drainage systems, also contributed to the flooding, but the failure of the levee system was the primary cause.
