Sea-level rise will cause catastrophic river avulsions

River avulsions, which occur when a river jumps its course and forms a new channel, will become much more frequent as a result of sea-level rise. Researchers at Caltech found that the intensity of these hazards will depend on the rate of sea-level rise and the sediment load carried by the river.

Even though avulsions are a natural phenomenon, they can turn into catastrophic and deadly flooding disasters. For example, river avulsions caused the 1887 Yellow River floods and the 1931 China floods which collectively killed about six million of people.

“A river avulsion is a catastrophic flood that is also crucial for sustaining coastal land,” said study lead author Austin Chadwick. “But the question is: how will sea-level rise affect these catastrophic floods?”

A 2014 report from the Intergovernmental Panel on Climate Change (IPCC) projected that the global average sea level could rise by as much as three feet by the end of this century. Scientists have been investigating how this would affect avulsions on deltas where a river meets an ocean or lake.

“Avulsions are the earthquakes of rivers,” said study co-author Professor Michael Lamb. “They are sudden and sometimes catastrophic natural events that occur with statistical regularity, shifting the direction of major rivers. We are trying to understand where and when the next avulsions will occur.” 

The researchers combined theory, numerical modeling, and field observations to estimate how often river avulsions will occur with future climate change. 

The analysis showed that the occurrence of future avulsions primarily depends on two main factors: the amount of silt and sand carried by a river and the rate of sea-level rise.

On most deltas, sea-level rise will cause more frequent catastrophic river avulsions. The rivers will respond to the rising ocean by depositing more of their sediment into the channel. This raises the riverbed relative to its surrounding landscape and leaves the river unstable. Eventually, the river will find a shorter and steeper path to the ocean. 

More frequent avulsions are particularly threatening to coastal communities like New Orleans, which is already extremely susceptible to flooding.

Sea-level rise will impact not only when, but also where, future river avulsions occur. 

“If sea level rises faster than rivers can deposit sediment, then the zone of deposition and avulsion will shift upstream, introducing new avulsion hazards to upstream communities,” said Chadwick. In this case, dams and levees that exist today would become obsolete and expensive mitigation efforts will be needed.

The Caltech team hopes that this research will help guide river management on densely populated deltas, highlighting what can be expected in the coming decades.

The study is published in the journal Proceedings of the National Academy of Sciences.

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By Chrissy Sexton, Earth.com Staff Writer