River deltas and that sinking feeling threaten environment

Kalyan Ray in New Delhi

The world faces  a heightening   risk of  floods  because major river deltas are  sinking  into the ground while  the sea level  is rising  thanks to global warming.
India is no exception. The threat of flooding is the highest in the Krishna River Delta which flows through Karnataka.

As many as 24 major river deltas including the  four Indian deltas are sinking. The scientists picked up 33 river deltas  all over the world for analysis and found that two dozen of them are in dire straits.

The problem arises as  their elevation of the deltas are  becoming lower than that of their  respective sea levels.

In India, the Krishna Delta is sinking is the most and  compares closely with other troubled deltas like the Yangtze and Yellow Rivers in China and Po in Italy.

According to a study --  published in Nature Geosciences on September 21 – delta land will be more vulnerable to serious flooding by 50 per cent if ocean levels increase as expected under moderate climate change scenarios.

Led by US researchers at the University of Colorado,  Boulder, the study shows most of the low-lying deltas are sinking owing to  human activity. This  subjects millions of peoples’ lives  to potentially hazardous conditions related   flood risks.

Without human interference, deltas naturally accumulate sediment as rivers swell and spread over vast areas of land.

But large dams constructed upstream and river diversions have held back the sediment layers that would normally increase the delta’s elevation. The rate of deltas sinking is faster than the global sea level rise which makes  it easier for  ocean water to flow back into river channels following a  storm surge.  In what may be construed as a verification of their claims, the scientists showed as much as 85 per cent of these deltas experienced severe flooding in recent years, resulting in temporary submergence of roughly 100,000 square miles of land.

Four Indian deltas – created by Brahmani, Godavari, Krishna and Mahanadi – were studied besides two promiment deltas created by Indus in Pakistan and Ganges in Bangladesh.

The Krishna Delta is the worst case and  is placed under the “greater peril” category. “The sediment trapping in upstream reservoirs on Krishna is much greater and presently little to no sediment is presently added to its delta surface,” lead author of the study James Syvitski told Deccan Herald. Krishna’s rate of ‘surface aggradation’ (accumulation of sediment) is lower compared to the rate of sea level rise around its delta.

Classified as “delta in peril”, Ganges Delta is a tad better than the Krishna.

Asked about the Gangetic Delta in West Bengal, Syvitski said that the trend is possibly even worse as very little sediment flows into that delta anymore compared to what it once experienced. Some sediment flows to the larger delta complex in Bangladesh.
The Gangetic Delta experienced flooding from ocean surges and a massive loss of life in both countries.

 Other Indian deltas do still get sediment added either to the coastal zone (Godavari) or their delta surface (Brahmani and Mahanadi). But the rates of aggradation compared to offshore relative sea level rise put them at a far lower risk.

The Colorado, Nile, Rhine, Pearl, Mekong, Mississippi, Tigris, Niger and Irrawaddy Rivers are some others with sinking deltas.

Typically rivers remain at a higher plane than the ocean because of a natural process called ‘aggradation’ through which sediment deposits over hundreds of years elevate the river. But the deltas are now  sinking because of  two man-made reasons. The upstream trapping of sediments by reservoirs and dams, man-made channels and levees that whisk sediment into the oceans beyond coastal floodplains and the accelerated compacting of floodplain sediment by the extraction of groundwater and natural gas.