THE destruction of New Orleans by Hurricane Katrina in 2005 showed the importance of keeping levees—the artificial banks that contain the flow of partly canalised rivers—in tip-top condition. In practice, though, that is hard. Levees fail for many reasons, not all of them associated with violent storms, and there are so many of them (100,000 miles-worth in America alone) that keeping an eye on all of them is an almost impossible task. It is good, therefore, to have a backup plan to block up unexpected holes before they can cause too much damage.

The traditional approach is to throw bags filled with sand or rocks into a breach. Such bags, though, are heavy and unwieldy—particularly if they have to be filled far from the breach and then carried there. William Laska of the Science and Technology Directorate at America's Department of Homeland Security has therefore sought out alternatives. He has found several technologies that have a common theme: they all use water itself to help stem the flood.

The largest of the new devices is designed to block deep breaches. The Portable Lightweight Ubiquitous Gasket (PLUG) is a sausage-shaped balloon made of polyester and PVC, and fitted with motorised pumps. When dropped into a river (usually by helicopter), PLUG's pumps switch on automatically and begin forcing water into the balloon through a valve. The air thus displaced is expelled through a second valve until the device is 80% full, at which point the pumps shut themselves down.

Filled thus far, PLUG is still buoyant and will float wherever the current carries it. If it has been dropped in the right place, that will be towards the breach it is designed to fill. And there, if all goes well, it will stick—blocking the hole in a manner suitable to its rather contrived acronym.

Prevention, however, is always better than cure, and the second of Mr Laska's devices is designed to stop levees being breached in the first place. In this case the acronym of choice is REPEL (Rapidly Emplaced Protection for Earthen Levees). REPEL is made of the same material as PLUG, but instead of being thrown into the water and carried to its destination by the current, it is laid out flat on a levee that is in danger of being overtopped and thus eroded by the river it is supposed to contain.

In normal circumstances merely placing a layer of protective material on top of a levee in this way would not do much good. The force of the water would quickly wash it away. However, REPEL has a series of tubes that sit on top of it and can be pumped full of water in a manner similar to that employed by PLUG. The weight of these tubes holds the protective layer in place, while the gaps between the tubes permit the overspill to escape. Some flooding from that overspill results—but not as much as if the spill were allowed to erode and destroy the levee itself.

Yet simply halting floodwaters and preventing short-term damage is not enough. Neither PLUG nor REPEL can be used permanently. It is the third of Mr Laska's acronyms, REHAB, that allows engineers to make permanent repairs.

The Rapidly Emplaced Hydraulic Arch Barrier, made of the same material as the other two devices, can be put in place around a plugged breach to keep it sealed and dry once the PLUG has been removed. First, the arch is filled with air and floated to the desired location. Then, once it is in place, it is partly flooded and allowed to sink to the riverbed around the breach, making a tight seal. That done, a second set of pumps evacuates the gap between the arch and the PLUG, allowing workers access to the site.

Not only is installing REHAB easier and faster than building a conventional temporary dam, it is also less wasteful. A conventional temporary structure is demolished after use. REHAB can simply be floated away and reused—it will have rehabilitated itself, in other words.