Dams responsible for South Asia’s sinking deltas
The Third Pole, Sundarbans
Seven of South Asia’s river deltas, including the Ganga-Brahmaputra, the Krishna and the Indus, are sinking faster than sea-level rise because of dam construction upstream
One or two people leave their homes in the Sundarbans forests of the Ganga-Brahmaputra delta each day, perhaps never to return. It’s but a small vignette of a larger tragedy being played out across South Asia’s delta regions where land is fast sinking as the sea waters rise, leaving millions of people vulnerable to disasters like cyclones and floods.
The mouth of the Ganga-Brahmaputra mega-delta in Bangladesh, the largest in the world, is dotted with 139 small islands called polders. An embankment protects each polder from daily tides. Every year, as embankments are breached and repaired several times over, the embankments rise higher and higher.
Local communities know they have little hope of seeing their families thrive in this land, said Anurag Danda, head of the climate change and Sundarbans landscape programmes of WWF-India. Every family in the Sundarbans, for instance, that can afford it sends its able-bodied members away. “They understand that they can’t be here for all time to come… Migration on a daily basis is already happening.”
The heartbreak and hopelessness finds echo across large swathes of South Asia. According to studies, the deltas of the Ganga-Brahmaputra in Bangladesh, the Indus river in Pakistan, the Krishna, the Godavari, Brahmani and Mahanadi in south India and the Irrawaddy in Myanmar are sinking faster than the rate of sea-level rise. The main reason for the deltas disappearing is the presence of hundreds of dams along the lengths of these rivers.
Of the seven, the Krishna delta is sinking the fastest and the Ganga and Indus have the largest affected areas.
Communities living in these South Asian deltas are already feeling the effects of the sinking land. As saline ocean waters creep further, inland farmlands are being damaged and ground water contaminated. A sinking delta is also more vulnerable to extreme climate events like Cyclone Aila that tore through the low-lying Ganga-Brahmaputra delta in 2009 leaving hundreds dead and many thousands homeless.
The second installment of the fifth assessment report of the Intergovernmental Panel on Climate Change notes, “Most large deltas in Asia are sinking (as a result of groundwater withdrawal, floodplain engineering, and trapping of sediments by dams) much faster than global sea-level is rising.”
Humans are sinking deltas four times faster than sea levels are rising, according toJames Syvitski, geologist at the University of Colorado, Boulder, and chairman of theInternational Geosphere Biosphere Programme (IGBP). Water, oil and gas extraction and fish farms are common causes for delta subsidence around the world but in India, Pakistan and Bangladesh dams are predominantly responsible, Syvitski toldthethirdpole.net.
The building blocks of a delta are the inland sediments that get swept down by a river and deposited around the mouth of a river every year over centuries. This process is necessary to maintain the delta, which faces strong eroding forces from the ocean. “The problem with this engineering stuff is that you move water away from the coastal zone, you move sediment away from the coastal zone through these barrages. And because you’re not getting water, you’re not getting sediment and you’re getting literally the drowning of the delta,” Syvitski said while addressing a symposium in Bangalore.
In a 2009 Nature Geoscience paper, Syvitski and his colleagues assessed 33 deltas around the world and ranked them according to risk based on the rate of sediment deposited, and the rate at which the height of the delta was falling (also known as compaction). Compaction occurs naturally when sediment brought in from rivers laid out on deltas loses its air and water content. Anthropogenic compaction occurs when water, oil and gas are extracted from beneath deltas.
Syvitski tagged deltas as being at “great risk” or “in peril” where the rate of sediment build-up is less than the rate of sea level rise and compaction is accelerated. “The land is often sinking faster than sea level is rising, and the thing is sea level is going to be rising faster than it used to,” Syvitski said.
He estimates relative sea level rise at the Ganga-Brahmaputra delta to be between 8 and 18 millimeters every year.
In Bangladesh, the Brahmaputra still continues to bring a lot of sediment into its delta but the western part fed by the Ganga seems to be moribund, added WWF-India’s Danda. “The silt that the Indian part of the delta receives is a push back from the sea than from the head waters,” he said.
The Indus delta in Pakistan is on Syvitski’s list of “deltas at greater risk”. It has seen an 80% reduction in sediment since the early twentieth century and the relative local sea level rise is now greater than one centimetre per year. About 20 dams and twice the number of canals divert the river’s waters en route to the Arabian Sea, leaving the delta parched. Only the floods of the summer of 2010 that wrought havoc in the upper Indus basin brought some relief to the delta. A 2012 study from Sargodha Universitycompared satellite images and ground data from the delta before and after the 2010 floods. It found a marked increase in the amount of vegetation, number of water bodies and area of wet soil in the otherwise withered region.
The delta of the river Krishna in southern India is in greatest danger. According to Syvitski’s study, the rate of sediment deposition on the Krishna delta has fallen from 7 millimetres per year in the early twentieth century to just 0.4 millimetres per year in the twenty first century, leaving an area of 250 square kilometres at less than two metres above sea level.
K. Nageswara Rao, professor emeritus at the department of geo-engineering atAndhra University, who first started studying the Krishna-Godavari basin in the 1970s, has seen the delta lose its mud flats to fish farms and its mangroves to seawater. In a study published in 2010, Rao and his team correlated maps of the Krishna delta since the 1930s with aerial photographs and satellite imagery. They noticed that a healthy amount of sediment was being moved onto the delta up to the 1960s, adding close to 50 square kilometres to the shore in 30 years. But the trend then reversed. With more erosion than deposition taking place after the 1960s, the shoreline receded and almost 800 square kilometres was lost to the sea.
Rao has no doubt that this is because of the numerous dams along the 1,300 kilometre river. India’s Central Water Commission conducted a survey in 2012 where it found that such large amounts of sediment had accumulated in the Nagarjunasagar and Srisailam reservoirs that they had lost about 20% of their capacity.
“From Google images of the Krishna in summertime you can observe that out of the four mouths of the Krishna, the main one, is getting closed,” Rao said.
Rao and his colleagues estimate that if sea levels rise one metre by 2100, 400 square kilometres of the Krishna-Godavari delta will be submerged. Almost 1,000 square kilometres of the Andhra coast will disappear affecting 1.7 million people.
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