Twenty-four of the thirty-three deltas in the world are sinking, and many of these are in India.

This alarming finding comes from a study of the world’s deltas using satellite data. The results were published online in the journal Nature Geoscience.

The study found that most of the deltas are “sinking at rates many times faster than global sea level rise.”

Deltas that are sinking relative to the sea level are prone to flooding. As per projections made by the Intergovernmental Panel on Climate Change (IPCC), sea level is expected to rise by another 21 to 71 cm by 2070 due to global warming and this will put millions of people living along the deltas at great risk.

According to James P.M. Syvitski from the University of Colorado, the first author of the study, the deltaic area at risk of flooding would increase by 50 per cent over the twenty-first century if the global sea level continues to rise rapidly.

Low sediment inflow

The study found that reduction in sediment inflow into the deltas played a major role in making the deltas vulnerable to flooding. The vulnerability arises as the deltas which lack sediment supply are unable to stay well above the sea level.

Delivery of vital sediments has either been reduced or totally eliminated in most of the deltas by construction of dams and compaction of delta sediments through water and oil extraction.

These activities, which have been going on for the past 50 years, tend to reduce the height of deltas with respect to the sea level.

The fate of deltas

The study found that three of the Indian deltas — Brahmani in Orissa, and Godavari and Mahanadi in Andhra Pradesh — are at a greater risk as the rate of sediment aggradation (raising the level of the delta through sediment deposition) no longer exceeds relative sea-level rise.

While there are no Indian deltas in the peril category (where the aggradation and compaction of the delta sediments overwhelms the rate of sea-level rise), the Krishna delta in Andhra Pradesh falls in the last category of greater peril.

In the case of the “greater peril” category, the deltas have virtually no sediment deposition (aggradation) and/or have very high accelerated compaction of sediments.

Here the relative sea-level rise is about 3 mm per year. The maximum relative sea-level rise of 13-150 mm per year is seen in the Chao Phraya delta in Thailand.

Though the relative sea-level rise is around 3 mm per year even in the case of the Mahanadi delta, there has been some amount of deposition of sediments leading to a small increase in the delta level surface relative to the sea level. The relative sea-level rise has been about 1.3 mm per year in the case of the Brahmani and Godavari deltas.

Reduction in sediment inflow into the deltas is mainly due to human activity. Dams constructed across many major rivers tend to reduce the silt from flowing.

Another major reason is the reduction of flow in the active distributary channels that supply the river with water and sediments. Accelerated compacting of floodplain sediment caused by the extraction of groundwater and natural gas is yet another cause.

The repercussions

Reduced sediment inflow into deltas could have severe repercussions Nearly 85 per cent of the world’s major deltas have experienced flooding during the last decade.

The flooding could either be from coastal inundation from surges, flooding from rivers overflowing the embankment, or flooding from intense rainfall within the delta.

In 2007-08, many deltas in the world, including Brahmani, Mahanadi, Krishna, and Godavari experienced flooding killing more than 100,000 lives and displacing millions of habitants.

The paper shows that more than 100,000 square km of deltas in the world are vulnerable to flooding from storms, surges or river floods as they have elevations of less than 2 metres above sea level.