New Orleans is built on a former swamp which has been drained over time; it is sandwiched between the Mississippi river, Lake Pontchartrain and the Gulf of Mexico. The weight of buildings has slowly compressed the soils upon which they are built, lowering the height of the land in this already vulnerable location. This means that many parts of New Orleans are below sea level. Indeed, historically, the Mississippi river used to flood the area where New Orleans now stands, mankind’s intervention has stopped this. The Mississippi river is held back from flooding by a series of levees, large mounds of earth or fences of concrete that raise the level of the river. These levees protecting New Orleans were only designed to offer protection in category 3 storms.
Hurricane Katrina was a category 5 storm that started over the Bahamas as a category 1 storm on August the 23rd 2005, which gained strength as it crossed the Gulf of Mexico and reached New Orleans on August the 28th. It formed in the Gulf of Mexico, where ocean waters reach temperatures above 27°C and to sufficient depth for hurricane formation. Warm ocean waters are evaporated, rise and cool to give the clouds which form a hurricane.
Indeed, ocean waters near to New Orleans always top 27°C in late summer.
Katrina was tracked across the Gulf of Mexico by the United States National Hurricane Centre, the Mayor of New Orleans, Ray Nagin, listened to the advice of the National Hurricane centre and ordered the mandatory evacuation of the city on the 27th of August. Despite this the response of FEMA (the Federal Emergency and Management Agency) was criticised for being slow and not everyone heeded the evacuation notices and over 100,000 people remained in the city. Over seventy countries pledged monetary donations or other assistance after the storm. Kuwait made the largest donation of $500 million.
Find out more
The BBC's animated guide to Hurricane Katrina
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