Answer:
Reason 1: The failure of the levees allowed water to rush into the city. Engineers originally designed the levees to withstand a direct hit from a level three hurricane, but Katrina was a level four hurricane.
Reason 2: New Orleans is a city located below sea level. As a result, any water that made it through the damaged levee inundated the city and was held in place by the remaining levee walls.
Reason 3: New Orleans sits on the Gulf of Mexico coast and is directly between Lake Pontchartrain and the Mississippi River. So, once the levees failed, water rushed into the city, and within 24 hours of Katrina making landfall, 80 percent of New Orleans became flooded.