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A floodplain is a wide, flat area of land either side of a river in its lower course. The floodplain is formed by both the processes of erosion and deposition.

Lateral erosion is caused by meanders and their associated river cliffs and the slow migration of meanders downstream.

Deposition occurs on the slip-off slope (the sediment deposited on the inside of a meander) and when the river floods and the increase in friction causes the flood waters to deposit the sediments they are carrying.

Levées are natural embankments of sediment along the banks of a river. They are formed along rivers that carry are a large load and periodically flood.

Sometimes natural levées are heightened and reinforced by humans to protect settlements, industry and infrastructure on the floodplain from floods.

Dams formed of metal tripods and self-filling water bladders could be air-dropped to rapidly repair levees breached by storm damage, say US government researchers.

When Hurricane Katrina struck New Orleans in August 2005, the levees created to protect the city from flooding were breached and the resulting deluge left 80% of the city underwater.

Should a similar disaster strike the US again, the researchers hope that make-shift dams, dropped from the air, could stem prevent the resulting flood.

Solid structures, resembling tripods, would be dropped into the gap left by a levee breach. After the legs are secured to the riverbed, empty water bladders would be dropped in the river just in front of the structure, slightly upstream. These tube-shaped bags would then fill themselves with water using internal pumps and fit into the gaps in the structure.

Mary Ellen Hynes, director of research for critical infrastructure at the US Department of Homeland Security, who is leading the project, says levee repair strategies are a priority because there are about 160,000 kilometres (100,000 miles) of levees across the US. The density of the US coastal population is also predicted to increase by 11% between 2003 and 2015.

The self-filling bladders, which were originally developed by the US Department of Defence as temporary jetties, could also be used to protect cities from storm surges. The tubes could be dropped into estuaries downstream of a city, just before a storm hits, to prevent water from rushing up the river and causing flooding.

Robert Bea of University of California at Berkeley says the idea could prove very useful, assuming the bladders can be kept securely in place.

He says the water bladders could have two important advantages over the type of sandbag air-dropped in to fix the levees in New Orleans in 2005. Firstly, they should be able to plug a breach more quickly. Secondly, they should be fairly easy to remove. "Man, did we encounter problems with those sand bags," he told New Scientist. "We had to go in and lift out everything piece by piece, shovel-full by shovel-full."

However, Bea also advocates rebuilding natural defences, such as beaches and wetlands, for long-term protection. He says engineered solutions should only be seen as a short-term fix.

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