Much of New Orleans lies below sea level. The city has a system of canals and levees topped with concrete floodwalls to keep water out. These are designed to withstand a category three hurricane, but when Katrina - a category four storm - hit, they were quickly overwhelmed. Within 24 hours, 80% of the city was flooded.

In pictures

The operation to drain the city began nearly a week later.

Repairing the breach: Helicopters dropped huge sandbags into the breach in the 17th street canal. Earthmovers built a causeway allowing trucks to bring in stones to strengthen the repairs.

Steel dam: A temporary steel barrier has been built at the mouth of the canal, sealing it off from Lake Pontchartrain.

Pumping out: Once all breaches are sealed, pumps will start to drain floodwater out into the lake, a process which could take 80 days.