"Communities around the country continue to be at risk," engineering professor Gerald Galloway of the University of Maryland told a news conference.

"Major urban centers like Sacramento, like Los Angeles, remain protected only at the margin. Engineers and scientists have got to be fully involved in this decision process, this political process," added Galloway, a retired brigadier general in the US Army Corps of Engineers.

No government in recent years has had the courage to make the changes needed, Galloway and other experts told a meeting in St Louis of the American Association for the Advancement of Science.

And they said rules on declaring floodplains, insuring them and notifying potential homebuyers often did not ensure that people understand which areas are at risk.

Past efforts to prevent flooding, such as building levees, have made the risks greater, said Nicholas Pinter of Southern Illinois University.

"It encouraged development in areas that were previously cornfield," Pinter said. "But it also physically makes the flooding process worse."

He said studies had shown Mississippi River levees had driven potential flood levels 10 feet to 12 feet (up to 4 metres) higher than what they were before.

GREAT MIDWEST FLOOD

A perfect example of what can go wrong was the 1993 Mississippi Flood, considered the most devastating single flood in US history.

Heavy rainfall overwhelmed the Mississippi and its tributaries, flooding 17,000 square miles (44,000 sq km) of land in nine states, killing 50 people and causing $15 billion in damage, according to Norbert Schwartz of the Federal Emergency Management Agency's mitigation program.

The St Louis area was badly hit but Adolphus Busch, chairman of the Great Rivers Habitat Alliance, said that more than 14,000 acres (5,700 hectares) of floodplain land in the region has been developed since then.

"By 1997-1998 we started to see huge, wholesale developments again going on out there," Busch said.

But perhaps a worse risk exists in California's Sacramento-San Joaquin Delta, said Jeffrey Mount of the University of California, Davis.

He predicts a 2-in-3 probability of a catastrophic levee failure over the next 50 years in the 700,000-acre (280,000-hectare) estuary that makes up the delta.

"In California we know we have two kinds of levees - those that have failed and those that will fail," Mount said.

He said the ground was subsiding and sea levels rising, compounded by "the wild card in California, seismicity, which can undo this system in a heartbeat."

"So you have a policy vacuum, and what rushes into a policy vacuum in California because of land prices? Urbanisation. We are reinventing Katrina all over again."

And the United States can probably expect a lot more rain, coming in heavy storms, said Anthony Arquez of the National Oceanic and Atmospheric Administration.