The correct answer is - flooding.
The Hurricane Katrina was a devastating one. It had very strong winds that were very destructive, and the rainfall was also very intense, but it was not what made the biggest damage in New Orleans, instead it was the flooding of the city.
New Orleans is a city that is on a very small elevation, close to sea level, so it is very vulnerable to the sea level rising, which unfortunately did happened with the Hurricane Katrina. Because of the strong winds, and the heavy rainfall, the sea started to flood the city and destroyed big portion of it. Lots of people lost their lives, and there was also a huge material damage.