Sea-level rise could make rivers more likely to jump course

Date:
August 13, 2020
Source:
California Institute of Technology
Summary:
River avulsions -- catastrophic floods triggered when a river
charts a new path to the sea -- could occur more frequently on
rivers as sea levels rises.



FULL STORY ========================================================================== Sea-level rise due to climate change is poised to shift the way that
rivers naturally chart their path to the shoreline. The nature of that
change will depend on both the rate of sea-level rise and the sediment
load carried by the river, according to new models by Caltech researchers
and their colleagues.


========================================================================== Periodically, a river will change its course to the sea, forming a
new path through a process called river avulsion. River avulsions are
a natural part of a river's life and are responsible for building
new land and nourishing wetland ecosystems with water, nutrients,
and sediment. However, river avulsions are also catastrophic natural
hazards. They were responsible for some of the deadliest floods in human history, including the 1887 Yellow River floods and the 1931 China floods, which together claimed an estimated six million lives.

On some rivers, like the Mississippi, engineers have built bypass channels
and levees to counter the dangers of river avulsion.

"A river avulsion is a catastrophic flood that is also crucial for
sustaining coastal land," says Austin Chadwick, lead author of a paper
about the new model that was published by Proceedings of the National
Academy of Sciences on July 13. "But the question is: how will sea-level
rise affect these catastrophic floods?" Chadwick worked on this research
as a doctoral student at Caltech with Professor of Geology Michael Lamb
and Vamsi Ganti of UC Santa Barbara.

Sea level has been rising overall since the start of the 20th century,
and at an accelerating pace. In 2014, the Intergovernmental Panel on
Climate Change (IPCC) projected that sea level could creep up as much
as three feet by the end of this century. Earth scientists have had an
ongoing debate about how this could affect river avulsions on deltas,
the fertile landscapes created where a river meets an ocean or lake --
a critical question, given that deltaic landscapes host roughly 10
percent of the human population.

"Avulsions are the earthquakes of rivers," Lamb says. "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." Lamb, Chadwick,
and Ganti combined theory, numerical modeling, and field observations
to explain how often river avulsions will occur, and how their frequency
would respond to a changing climate.

They found that the occurrence of future avulsions depends mainly on
two factors: the rate of sea-level rise, and the amount of silt and sand carried by a river.

On most deltas, including the Mississippi River delta, sea-level rise
is expected to cause more frequent catastrophic river avulsions. This is because, as sea level rises, rivers respond by depositing more of their sediment into the channel, which raises the riverbed relative to the neighboring land, making the river unstable. Eventually, a levee breach
will force the river to find a shorter and steeper path to the sea. More frequent avulsions could flood coastal communities like New Orleans,
Louisiana, that are already vulnerable to inundation by sea-level rise.

Sea-level rise could affect 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," Chadwick
says. In such a scenario, existing river-management infrastructure (dams
and levees) could be rendered obsolete, and costly avulsion-mitigation
efforts would need to shift upstream.

Chadwick, Lamb, and Ganti hope that this work could help guide river
management on densely populated deltas, offering them a framework for calculating what to expect over the coming decades.


========================================================================== Story Source: Materials provided by
California_Institute_of_Technology. Original written by Robert
Perkins. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Austin J. Chadwick, Michael P. Lamb, Vamsi Ganti. Accelerated river
avulsion frequency on lowland deltas due to sea-level
rise. Proceedings of the National Academy of Sciences, 2020; 117
(30): 17584 DOI: 10.1073/ pnas.1912351117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200813103120.htm

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