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Disappearing deltas could spell disaster
20 February 2006
From New Scientist Print Edition.
Jeff Hecht

Hurricane Katrina, which ravaged New Orleans and left more than half a million people displaced, showed just how dangerous living in a flood-prone delta can be. For some researchers this came as no surprise. They think that by 2050, millions more living in low-lying river deltas will be equally vulnerable to rising sea levels, sinking land and storms.

Jason Ericson of the University of New Hampshire in Durham, now at the Virginia Department of Conservation and Recreation in Richmond, and his colleagues identified five other deltas that could face similar, if not more devastating, disasters: the Bengal delta in Bangladesh, the Yangtze delta in China, the Mekong delta in Vietnam, the Nile delta in Egypt and the Godavari delta in India.

At a delta, "the shoreline is a balance between sea level and sedimentation", says Daniel Stanley, an expert on the marine geology of deltas at the Smithsonian Institution in Washington DC. Any reduction in the amount of sediment reaching the deltas can alter this balance, causing deltas to subside.

While deltas are naturally vulnerable because they form in low-lying coastal regions, human activity is adding to the risk. "The subsidence issue is related to population," says Robert Nicholls of the Tyndall Centre at the University of Southampton, UK. It matters because the people live there, but their very presence adds to the risk.

Most deltas are subsiding because dams and canals upstream are preventing sediment from reaching the deltas. To make matters worse, people are also pumping out water and petroleum from beneath the deltas, causing the sediment-starved land to settle further. This is causing the waterline in most deltas to rise faster than the average rise in global sea level of about 1.5-2 millimetres per year.

Ericson and his colleagues found that all 40 of the major deltas they studied were at risk: 68 per cent of them were sinking because they were being starved of sediment, 20 per cent were subsiding, mainly because of the pumping of water and oil, and in 12 per cent of the deltas, rising sea level was the biggest problem.

Five of the six deltas most threatened by rising sea level are also at risk from storms, the only exception being the Nile delta. Storm surges can overwhelm an already low-lying region. In the Bengal delta, for instance, storm surges can go right over the hurricane shelters, says Greg O'Hare of the University of Derby, UK.

The deltas in greatest danger lie in the arc from China to India, where populations are high, and powerful typhoons and tropical cyclones are common. Geologically active mountains and heavy rains make south-east Asia the source of half the world's sediment supply, making large deltas commonplace in the region, says Janok Bhattacharya of the University of Texas in Dallas. Intense development has robbed some deltas of water as well as sediment. For instance, with some 50,000 upstream dams catching water, China's Yellow River did not flow at its mouth on 230 days of the year in 1998, compared with just 20 days in 1992, he says.

Ericson's study, which will appear in the journal Global and Planetary Change, calls for an increased surveillance of the river deltas using technology such as remote sensing to prevent unprecedented displacement of people. "Greater awareness of potential threats to such systems is a precursor to the design of responses that maximise the protection of life, infrastructure and economic development," the researchers say.

From issue 2539 of New Scientist magazine, 20 February 2006, page 8

Bengal delta, Bangladesh: 3,430,000 at risk Residents of this densely populated delta are pumping out so much groundwater that in some areas the land is subsiding by almost 25 millimetres a year. Submarine geology amplifies storm surges at the northern end of the Bay of Bengal, some of which have exceeded 10 metres. Such problems and a lack of warning made the Bengal delta the site of the two deadliest storms of the last century: a storm killed about 300,000 people in 1970 and 138,000 were killed in 1991. Mekong delta, Vietnam: 1,910,000 at risk Also densely populated, this sediment-starved delta is on the southern fringe of the Asian typhoon zone and on average is hit once a decade by a typhoon. These storms can be devastating. Typhoon Linda, which developed rapidly over the South China Sea in 1997, was blamed for more than 3000 deaths as it passed along the southern edge of the Mekong delta. The storm flattened villages, wiped out crops, and left tens of thousands of people homeless. Nile delta, Egypt: 1,300,000 at risk Though it is not at risk from storms, the Nile delta suffers heavily from human activity. The completion of the Aswan High dam in 1964 has had even more devastating consequences. The dam stopped the delivery of silt carried by the river's seasonal floods and the silt now lies trapped in the dam's reservoir. And beyond the dam a dense network of drainage and irrigation canals is preventing much of the remaining sediment from reaching the coast. Yangtze delta, China: 484,000 at risk Shanghai, a coastal city on the delta, has records of typhoons dating back to the 7th century, though their frequency increased rapidly over the 20th century. "Shanghai's defences are as good as London's - one of the best in the world," says Robert Nicholls of the University of Southampton, UK. They are also better maintained than the defences that failed in New Orleans. This extensive protection does not extend into rural parts of the deltas, however. Mississippi delta, USA: 480,000 at risk Hurricane Katrina showed just how vulnerable this low-lying region is. The delta also scores high on another measure of risk: 20 per cent of the delta could be lost to subsidence and rising sea levels by 2050. Tens of square kilometres of wetlands to the south of New Orleans were turning to open water each year even before Katrina hit, as petroleum pumping combined with sediment starvation was causing the land to sink and subsidence within New Orleans has left large areas of the city below sea level. Godavari delta, India: 453,000 at risk This delta on the east coast of India could lose an even larger fraction of land than the Mississippi delta - about 23 per cent by 2050. Groundwater pumping is a major cause of subsidence, and storms are a constant threat to a region that has few coastal defences. More than a thousand people died when a tropical cyclone hit the delta in November 1996. In the Mahanadi delta, further north along the coast, some 10,000 people died in 1999 when hit by a "super cyclone". Note: populations at risk by 2050

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