What happened that made Hurricane Katrina a huge disaster?

Early in the morning on August 29, 2005, Hurricane Katrina struck the Gulf Coast of the United States. When the storm made landfall, it had a Category 3 rating on the Saffir-Simpson Hurricane Scale-it brought sustained winds of 100-140 miles per hour-and stretched some 400 miles across

Hurricane Katrina was the eleventh storm and fifth hurricane on the year 2005 and originated over the Bahamas from the interaction between a tropical wave and the remnants of the tropical depression ten. Morning of the next day, the new depression intensified into tropical storm Katrina and generally headed westward to Florida. But it only strengthened into a hurricane in two hours before making a landfall at Hallandale Beach and Aventura. It then deepen after emerging from the Gulf of Mexico. Total property damage was 108 billion US dollars.