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Can river silt save Bangladesh?

Bangladeshis are already feeling the effects of climate change, especially those living on the low-lying river delta near the mouths of the Ganges. Frequent floods and stronger storms have been inundating the area with more water than many local rice growers can cope with, driving thousands to seek work in overcrowded cities.

But some rice and shrimp farmers are holding out hope that the very waters that have oft plagued them might hold the solution – in suspension. River silt, carried down in suspension from the ancient Himalayan mountains hundreds of miles away, might be able to build up land elevation to save the area's livelihoods and homes, The New York Times reports today.

In Bangladesh, deposited sediment has been slowly filling the marshy delta outward into the Bay of Bengal for the past few decades, but engineers are examining ways that it might be used to build the land up rather than out, according to the Times. The goal is to allow water to flood some especially low areas and let it out – through dams and channels – slowly. As the murky water slows, sediment drops out and filters to the bottom, leaving a new bed of silt after the water recedes. Residents in Beel Bhaina discovered this phenomenon after releasing stagnant floodwaters through an embankment; to the surprise of many, the Times reports, three feet (or more) of silt had been deposited, boosting land higher above sea level.

"Usually people are trying to get rid of the sediment," says John Gray, a hydrologist at the U.S. Geological Survey. "But here, people are using it as a resource." But Gray is skeptical that it's a viable answer. One major obstacle, he tells ScientificAmerican.com, is that it would require yet more flooding to achieve decent amount of sediment buildup.

"If they're really talking about building land," he says, "the experimental patches are a good start, but they're really going to have to start working on a much large scale."

Gray praises the effort, noting that it's "laudable" to try to "take something that most consider a nuisance" and turn it into an asset. But he warns that if massive quantities of water are diverted, unexpected consequences, such changes in river behavior, are likely to occur. Gray isn't even sure that it's a surefire stopgap against rising sea levels, which are predicted to top three feet by the end of this century. "The timing of accretion [deposit of the sediment] and the timing of the climate change, he says, "might not be compatible."

Centuries of engineering have gone into clearing river channels and mouths of silt to make them more navigable and easier to control. But as rivers have been carved out and dammed for easier navigation, sediment rushes out to sea, rather than getting deposited in the delta. This means many deltas worldwide, including the Mississippi River Delta in Louisiana, the Nile Delta in Egypt, the Yangtze Delta in China and others, are disappearing, leaving costal towns and cities more vulnerable to increasingly strong storm surges. If Bangladesh succeeds in bracing its floodplains this way, it might be a new safeguard for costal areas across the globe. But the proof will be in the (rice) pudding.

Image of Bangladeshi farmers harvesting rice in flooded fields courtesty of the International Rice Research Institute via Flickr

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