River Kosi became very unstable after embankments were made on either side of it
Rivers lining tropical and desert regions are more likely to change directions, according to a new study. These rivers expose the surrounding areas to floods as they abandon their usual route for a new one, it added.
In India, a large flood forced the Kosi River to abandon its established channel for an older one in 2008, displacing 3 million people and claiming more than 250 human lives.
The findings can help predict where such events – called avulsions – are likely to occur and how climate change may alter the typical locations, the researchers said in the report published in the journal Science.
Avulsions are rare, occurring only once a decade or century, or even less, the report said.
The infrequent nature of avulsions, compared to more frequent extreme weather events and the continuous effect of sea-level rise, makes them less discussed despite their catastrophic effects, researchers Paola Passalacqua and Andrew J Moodie from the University of Texas at Austin wrote in a related perspective in Science. They were not involved in the study.
Researchers from the United States combed through satellite imagery from 1973-2020 and historical maps to document 113 avulsions worldwide.
Satellite data helped the team pinpoint where rivers changed their courses. In 33 instances, rivers changed routes in the bases of mountains while descending onto unconfined valleys or open oceans, the study pointed out. Kosi river belongs to this category.
In the other two categories, the change occurs in the delta regions. One is along backwater zones, part of the river that flows differently because of the effects of the downstream sea.
They documented 50 such instances occurring on low sloping deltas along some of the world’s largest waterways, like the Orinoco, Yellow, Nile and Mississippi Rivers.
The last category, which makes up the remaining 30 events, occurs in rivers with extreme sediment load, the study stated.
The common thread tying all three categories is sediments, Rajiv Sinha, Professor, Department of Earth Sciences at the Indian Institute of Technology Kanpur, told Down To Earth (DTE). He was not involved in this study.
Sediments are known to fill up river beds, forcing rivers to seek new channels during floods, according to experts.
Climate change can alter where rivers jump course in delta regions, the study predicted. For example, rising sea levels can push avulsions farther inland in the backwater zone.
This needs to be confirmed by running a model that simulates different climate change scenarios, Sinha pointed out.
“Around 330 million people live on river deltas worldwide, and many more live along river corridors,” Vamsi Ganti, University of California Santa Barbara and the study’s co-author, said in a statement.
Therefore, he added, it is essential to understand how river mobility will change in response to climate change and anthropogenic interference.
Kosi River case study
The study has compiled data from several different locations worldwide, Sinha said. But the downside is that it has not captured the complete picture, he added.
For example, the study has not looked at the role of embankments — barriers created along rivers to protect against floods — in triggering avulsions, he said.
Kosi-like systems bring a lot of sediments from the Himalayas. After embankments were made on either side of the river in the 1950s, it became much more unstable, Sinha highlighted.
The avulsion in the Kosi River, according to him, is not happening naturally. “Before the embankment, the river could distribute sediments along the 200-kilometre stretch. Now that has been reduced to 10 km,” he said.
The flow of sediments has not changed, but the area available for its movement has gone down, he explained.
Various engineering interventions to protect communities and their livelihoods have existed for centuries, Passalacqua and Moodie said.
Temporary solutions like embankments, for example, exist. Such temporary solutions contribute to a false sense of protection and even amplify degradation at the system scale by limiting natural sediment dispersal, they added.
It helps to create additional channels for the river and divert a part of the flow, Sinha noted. This distributes the flow of the water and sediments across channels, dissipating floods and avulsion, he added.
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