More than 1,800 people died after Hurricane Katrina hit the Gulf coast on Aug. 29, 2005. Another 2.16 million people were evacuated from their homes and many have not yet returned.

The disaster hit the states of Louisiana, Mississippi, Florida, Georgia and Alabama states, affecting an area the size of Britain. Many of the same places were hit by another strong hurricane, Rita, on Sep. 24, 2005.

In addition to Katrina's fierce winds, which reached around 130 mph (210 kph), a storm surge of more than 20 feet (6 metres) swelled the coastal waters, tearing oil rigs from the sea bed, washing many of the area's famous casino boats inshore, flooding large areas of Biloxi and Gulfport, Mississippi, and causing terrible damage up and down the coast.

This surge put intolerable strain on the flood defences that surrounded New Orleans in Louisiana, and more than 75 percent percent of the historic city was under 6 to 20 feet (2 to 6 metres) of water within hours of the hurricane making landfall.

In all, some 215,000 homes were damaged in New Orleans, which had a population of about 485,000 before Katrina. In addition, many of the area's airports, roads, schools, bridges, warehouses and hospitals were damaged and temporarily closed after being hit by the hurricane.

The storm caused an estimated $75 billion of damage. Ports in the area, which once dealt with a quarter of U.S. imports and exports, were closed for a time because of damage. Oil production in the Gulf of Mexico fell by 95 percent as a result of Katrina.

U.S. President George Bush declared the states of Louisiana, Mississippi, Florida and Alabama major disaster areas, and the Department of Homeland Security began to offer federal assistance to those affected.

But all levels of government soon came under fire from survivors, analysts and the media, who questioned the speed of the response and asked why so many of those left to face the storm were mostly poor and black.

The work of the Federal Emergency Management Agency (FEMA) came under the strongest attack, and its director, Michael Brown, who had been criticised as under-qualified, resigned two weeks after the storm.

The experience of the storm, the struggle to get away from the flooding, and the battle to make a new life has inevitably been tough for many people.

Researchers on mental health found that about half of New Orleans residents - 49 percent - were suffering from anxiety and mood disorders five to seven months after Katrina devastated the city.

That's a higher rate than after most natural disasters, and significantly higher than residents of the U.S. Gulf Coast in Mississippi and Alabama. About a quarter said they were suffering the symptoms, researchers from the University of Michigan School of Public Health in Ann Arbor said, on a par with similar disasters.

The academics concluded that the slow government response to the hurricane in New Orleans created extra and avoidable stress for the people who lived through the storm there

Another research team, from the University of Mississippi, found post-traumatic stress was much lower among people with large networks of friends and family, while those who had lower incomes or were unemployed before Katrina were most susceptible to mental disorders.

New Orleans is getting back on its dancing feet, but the city has changed. There's a shortage of homes to rent, and prices have rocketed.

This has made it hard for people who want to come back and rebuild their houses, unless they perch trailers on their property.

While the tourist hotspots - the mainstay of the city's economy - look like the storm never hit, lower-income neighbourhoods are struggling to get people to return.

Evacuees were scattered in far-flung places like Houston and Dallas in Texas and Atlanta in the state of Georgia, making it difficult to come back for visits or to rebuild homes.

Many were worried there wouldn't be any services in their community - a justified concern since it took 14 months to get drinking water back to the Lower Ninth Ward, one of the worst-affected neighbourhoods. Opening hours for shops have been cut, and hospitals, child care facilities and schools not back to normal, with teachers in short supply.

School enrolment figures indicate that fewer families with children have returned.

New Orleans residents told researchers their top reason for hesitating in coming back was concern there wouldn't be enough police.

Many homeowners had problems with insurance claims, and premiums have soared, another obstacle to reconstruction.

There were heated debates in the storm's aftermath about what kind of town to rebuild, with some arguing it was an ideal time to do away with public housing developments that had become steeped in crime and others arguing against what they saw as the privatisation and gentrification of the city.

The demographics of the city changed, with the black population reduced, although white residents still make up less than half of the population.

Much of the intensive clean-up and reconstruction work was done by immigrant Latinos - over 80 percent, according to research published in American Anthropologist journal. Many of the migrant workers reported being forced into long work days with little rest for low pay, being crammed into small rooms for the duration of the job. As a final insult, some employers shopped undocumented migrants to the authorities for being illegal once they'd used their labour.

New Orleans was vulnerable to a hurricane such as Katrina because it lies below sea level, on land east of the Mississippi River and south of Lake Pontchartrain.

In addition, the relatively flat bottom of the Gulf of Mexico makes the area very vulnerable to the storm surges - water pushed towards land by high winds - that accompany hurricanes and tropical storms.

According to the U.S. National Hurricane Center, the danger posed by storm surge is related to the angle of a coastline's continental shelf. If this is very long and shallow, as with the Gulf of Mexico, a hurricane can force water far into coastal communities. Louisiana is especially vulnerable because of its lack of barrier islands or hills.

New Orleans relies on 350 miles (560 km) of levees and other flood-prevention measures, the majority of which were built by the U.S. Army Corps of Engineers.

However, under the sustained pressure of the hurricane's storm surge, the levees broke in two places, allowing water to pour into the city.

At the height of the storm, Lake Pontchartrain, which is normally just a foot above sea level, peaked at 8.6 feet above sea level.

Analysts initially believed the flooding was caused by a combination of overtopping - when flood waters go over the top of a levee - and breaches, where the fabric of the levee fails and allows water to flow though.

But a report by the Interagency Performance Evaluation Task Force, a group of 150 engineers and experts from government, academia and industry, contradicted early theories about the levee failure.

It claimed the collapse of the 17th Street Canal floodwall, which contributed to the flooding of much of central New Orleans, was caused by a weakness in its construction. The pressure of the Katrina floodwater had pushed the concrete floodwall back, causing a tear in the area where the wall met the earthen levee on which it was built. When the waters rushed into this tear, the floodwall gave way.

This finding has worried analysts, who point out that several miles of floodwalls that were damaged but not destroyed by Katrina are of similar construction.