Bengaluru: The Ganga-Brahmaputra-Meghna (GBM) delta could not only be impacted by rising sea levels but also the Farakka barrage and the central government’s river-linking plans.
The GBM delta occupies an area of about 150,000 square kilometres and covers parts of Bangladesh and eastern India. With over 1,000 people per square kilometre, it is one of the most populous deltas in the world.
The delta is also one of the most climate-vulnerable regions, given that it routinely experiences flooding and heavy rainfall. Between 1968 and 2012, water level in the region had increased by 3 mm on average annually against the global average of 2 mm each year during the same period.
A recent study titled ‘Water level changes, subsidence, and sea level rise in the Ganges–Brahmaputra–Meghna delta’ has shown that sea-level rise around the GBM delta could reach 85-140 cm by the end of this century.
This is double the rise previously projected under a greenhouse gas emission mitigation scenario in the Fifth Assessment Report of the Inter-governmental Panel on Climate Change in 2013.
Mélanie Becker, one of the authors of the paper, told ThePrint that while estimates of water level changes had been drawn in the past, this particular study is unique because it takes into account various local processes like impact of water pumping and sediment compaction — the process by which sediment volume is reduced — which can provide a better understanding of regional trends in water level changes.
It should be noted that the IPCC report did not take into account land subsidence — a phenomenon wherein there is a sudden sinking or gradual downward settling of the Earth’s surface.
While this process is natural and occurs due to glacial or sedimentary adjustments, tectonics and sediment compaction, Becker explained that it could also be “considerably amplified at the sites of high local subsidence due to groundwater pumping and/or other anthropogenic factors”.
Himanshu Thakkar, coordinator, South Asia Network on Dams, Rivers and People (SANDRP), told ThePrint, “In delta regions, there is always a sort of fight that goes on between the river and the sea. In this fight, the river uses two weapons — water and silt.”
Thakkar noted that deltas receive a lot of silt from rivers and this deposition combats processes like land subsidence.
“Now, what is happening with the Ganga is that large quantities of the silt gets trapped upstream of the Farakka barrage. So, Farakka is a bottleneck and downstream areas don’t receive a lot of the silt,” he explained, adding it is one of the major reasons for land subsidence in the delta.
“EIAs [Environmental Impact Assessments] for dam-building activities don’t consider impacts like sinking and shrinking of deltas,” he said.
The Farakka barrage also causes other problems like flooding in Bihar. “On request of Bihar Chief Minister Nitish Kumar, the Union Water Resources Ministry had set up a committee to study the impact of Farakka barrage on the floods in Bihar. But the CWC (Central Water Commission) could not provide data which the committee required to study this impact,” he said.
Thakkar was also a member of the expert committee.
How river linking can impact the delta
There is also reason for further alarm given that plans for linking the Ganga and the Brahmaputra are in place as part of the National River Linking Project (NRLP).
NRLP aims to connect 44 rivers via 9,600 kilometre of canals.
According to a paper titled ‘River linking in India: Downstream impacts on water discharge and suspended sediment transport to deltas’, NRLP will result in a 39-75 per cent reduction in sediment flow in the Ganga and a 9–25 per cent reduction in the Brahmaputra.
Overall, sediment deposition in the delta is also set to decrease from 3.6 to 2.5 mm annually, which is large enough to further accelerate sea-level rise in the delta.
“Given the Farakka barrage already traps a lot of sediments, the plan for river linking will mean very high levels of degradation of the delta,” Jagdish Krishnaswamy, a senior fellow at the Suri Sehgal Centre for Biodiversity and Conservation, Ashoka Trust for Research in Ecology and the Environment, told ThePrint.
Krishnaswamy is also a coordinating lead author of the special IPCC report on climate change, desertification, degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems.
“There is a non-linear relationship between the amount of water flowing in the rivers and the amount of sediment the rivers carry,” Krishnaswamy said.
What this means is that even if the flow of water is reduced by small to moderate quantities, the amount of sediment carried is decreased by a large amount.
For example, if water flow is reduced by 10 per cent in the Ganga and the Brahmaputra because of the river linking project, the sediment flow into the delta could potentially reduce by about 50 per cent, he explained.
Groundwater extraction and land subsidence
Not only river linking, groundwater extraction can also affect the delta.
Thakkar explained the impact of groundwater extraction on land subsidence. When the amount of water withdrawn from the ground exceeds the rate of groundwater recharge, it can lead to sinking of the ground and an increased flow of saline water into the delta that can affect the remaining groundwater.
“Since the new study [Water level changes, subsidence, and sea level rise in the Ganges–Brahmaputra–Meghna delta] shows that the GBM delta is sinking at a much higher rate than what we had previously thought, we need to keep sediment flowing into the delta to prevent its submergence and consequent impact on people in terms of displacement,” Krishnaswamy said.
He added there were mitigation measures like reduction of groundwater pumping that can be adopted within the delta region.
ThePrint reached the Hydrology Directorate of the CWC and the Ministry of Environment, Forest and Climate Change for more details. This report will be updated when responses are received.