River sediment may counter Bangladesh sea level rise

Sediment flow from the Himalayas is building up land but will it compensate for sea level rise? Copyright: Flickr/amirjina

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[DHAKA] Bangladesh may avoid losing a predicted fifth of its land to rising sea levels because of accumulated sediment being washed down by Himalayan rivers, according to scientists.

Researchers at the Center for Environmental and Geographic Information Services (CEGIS), in capital city Dhaka, said that the Himalayas — relatively young mountains with a loose structure — have been losing more than a billion tonnes of sediment every year, some of which has been building up the land in the river estuaries below.

The team presented its work at a seminar on the impact of climate change on rivers organised by BRAC, Bangladesh’s largest non-governmental organisation, in Dhaka last month (22 April).

The scientists say the results challenge predictions by the Intergovernmental Panel on Climate Change (IPCC) that a one-metre sea level rise could submerge a fifth of the country.

Funded by the Asian Development Bank, the CEGIS study focussed on a 15,000-square-kilometre area where two major Himalayan rivers, the Brahmaputra and the Ganges, meet before draining into the Bay of Bengal.   

The scientists analysed how the two rivers and the land around them changedin response to the changing climate from 1943 to 2008.  

They found that the rate of sediment addition was more than the rate of soil erosion during this period — the country gained nearly 1,800 square kilometres of new land, almost five times the area of Dhaka.

The IPCC’s 2007 report estimated that a one-metre rise in sea level could sink nearly one fifth of Bangladesh’s land mass and displace 20 million people.

But IPCC scientists did not consider the sediment factor, Maminul Haque Sarker, deputy executive director at CEGIS, told SciDev.Net.

"If we can use these sediments in a planned way we can tackle 60 centimetres to one metre sea level rise over next 100 years," he said.

But IPCC chairman Rajendra Pachauri warned against "jumping to conclusions" on the basis of "one study alone".

"The IPCC looks at a range of publications before we take a balanced view on what’s likely to happen," he told the news agency AFP.

Saleemul Huq, director of the climate change cell at the UK-based International Institute of Environment and Development (IIED), said that improved understanding of sediment deposition and erosion in river deltas will help develop better adaptation strategies.  

However, the CEGIS team cautioned that future sediment deposits would not match the land additions of the last 65 years. The sediments from that period resulted from a massive earthquake measuring 8.6 on the Richter scale in Assam in northeast India in 1950, which dislodged about 45 billion tonnes of sediments.