Floods pose a huge threat to the large populations of West Bengal, India and the state of Bangladesh, particularly in the highly fertile fluvio-deltaic plains of the Ganges and Brahmaputra. The two river systems drain 2 million km2 of the Eastern Himalaya of annual monsoon rains and snow melt, the first flowing west to east and the latter from east to west at the apex of the low-lying Bengal Basin. The 400 million people subsisting in the 105 thousand km2 onshore basin make it the world’s most populous delta plain with one of the highest population densities, averaging 1,100 per square kilometre in 2019. The risk of catastrophic flooding is generally ascribed to unusually high monsoonal precipitation and snow melt, combined with storm surges from the Bay of Bengal that funnels tropical cyclones. But either can bring inundation. Another factor has recently been proposed as an addition to flood hazard: earthquakes near the basin (Chamberlain, E.L and 12 others 2024. Cascading hazards of a major Bengal basin earthquake and abrupt avulsion of the Ganges River. Nature Communications, v. 15, online article 4975; DOI: 10.1038/s41467-024-47786-4). It seems they can completely and suddenly change the flow networks in such a complex system of major channels.
Using remotely sensed data Elizabeth Chamberlain, currently at Wageningen University in the Netherlands, and colleagues from Bangladesh, the US, Germany and Austria have detected an immense abandoned channel in the Ganges River. They reckon that it resulted from a sudden change in the river’s course. Such avulsions in the sluggish lower parts of a river system are generally caused by the flow becoming elevated above the flood plain by levees. When they burst free the channel may be abandoned. This one is 1.0 to 1.7 km wide and may have been the main Ganges channel at the time of avulsion. The main channel now flows about 45 km north of the abandoned relic. The event must have been sudden and irreversible as the relic channel contains a much thinner layer of fine mud deposited by stagnant water than in other abandoned channels that became ox-bow lakes. That implies rapid uplift and complete drainage from the channel. Throughout the Bengal Basin the immense high-water discharge and heavy sediment load seems generally to have infilled most abandoned channels, so this one is an anomaly.
Fieldwork near the old channel reveals fracturing of earlier riverbed sediments some of which are filled by intrusions of sand in the form of dykes up to 40 cm wide. Sand dykes are produced by liquefaction of sandy alluvium by seismic waves to slurry that can be injected into fractures pulled apart by seismic movements. The channel is now about 3 m below the level of the floodplain, suggesting subsidence since the avulsion event. Optically stimulated luminescence dating of sediment grains from the uppermost channel sands yielded ages averaging around 2.5 ka, marking the time when the sudden event took place. The authors consider that it marked a major reorganisation of the Ganges River system, involving catastrophic flooding. The nearest seismically active area is about 180 to 300 km to the east and northeast. Seismic modelling suggests that for liquefaction and fracturing to have affected the area of the abandoned channel the earthquake must have been of magnitude 7.5–8.0, possibly in the subduction zone that roughly follows the Bangladesh-Myanmar border. It may have had similar, yet to be demonstrated, effects throughout the eastern Bengal Basin.
There are no historic records of more recent massive earthquake-induced flooding of the Bengal Basin. However, global warming and growing human intervention in the Ganges-Brahmaputra river systems, such as large-scale dredging and industrialisation could make such events more likely. Other basins close to seismically active fault systems, such as the Yangtze and Yellow River basins of China, also face such risks.
Many thanks to Piso Mojado for giving me the tip about this paper
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