Martial law has been declared in New Orleans as the city, swamped by floodwater after hurricane Katrina, faces a rising tide of crime. The US army is sending in 30,000 National Guard troops over the coming days. They will be spread across the disaster zone from southern Louisiana into Mississippi.Their first objective is to crack down on looters who have taken advantage of the chaos to break into stores and homes. The atmosphere of lawlessness has hampered efforts to help those stranded in the city. Helicopters working to evacuate the Superdome stadium had to pull back briefly after they came under fire from below. More than 20,000 people are now being slowly bussed and flown out of the refuge. They have complained of increasingly squalid conditions inside. Most will be taken to the Houston Astrodome, where the Red Cross is planning to look after them, while others need hospital care. The future looks bleak for the survivors of the storm, who have been told it could be three or four months before they can return to their home city. The American government has declared a public health emergency in the Gulf Coast area amid fears outbreaks of disease could take hold. Officials worry bodies in the water could contribute to the health risk. One of the Louisiana Senators says the death toll may climb into the thousands. Four days after the 225 km/h winds and nine metre storm surge hit the sheer scale of this disaster is still to be fully understood.