Earlier this year a paper published in Nature Communications by researchers in Indiana University's Earth and Atmospheric Sciences and Geography Departments added new context to a significant yet often overlooked river event: avulsions.
Using data from more than 60 rivers from around the world, the team studied river channels' shapes, direction, and downstream changes to make predictions about avulsion behavior, a fast-moving process whereby a river abandons an existing channel and forms a new one. This work was funded by the National Science Foundation with partial support from the Environmental Resilience Institute.
As a river flows downstream, sediment carried in the water is deposited in two primary locations: in the river channel and on both embankments. If the river deposits sediment in the channel faster than it deposits sediment on the embankments, the surface of the water will slowly rise above the surrounding floodplain, a phenomenon known as "superelevation."
An avulsion event occurs when a superelevated river overflows its banks and moves to an entirely different, previously unoccupied river channel.
"It's pretty much the most dramatic event you get in natural rivers," said Jeffery Valenza, an IU PhD candidate and lead author of the paper.
Destructive, too. Water displaced during an avulsion event can migrate several kilometers before receding into a new river channel. This unpredictable movement can be disastrous for communities or ecosystems that depend on direct access to waterways—or worse, are suddenly subjected to catastrophic floods. In 2008, an avulsion event on the Kosi River displaced millions of people living near the Indo-Nepal border.
"The entire river is moving to a new position and that usually involves major amounts of flooding," said IU professor Doug Edmonds, a co-author on the paper. "All of the water in that river is now flowing over the floodplain as it tries to establish a new pathway."
Despite the dangers they pose, avulsions are an underrepresented topic of study in the earth sciences. Avulsion events are rare and difficult to observe in the field. A floodplain may only experience an avulsion event once over the course of multiple centuries. Human intervention and large-scale engineering to minimize the impacts of seasonal flooding have also reduced the frequency of river avulsions.
These obstacles made an approach using standard field research techniques impractical. Instead, Valenza and his team relied on data analysis to examine a total of sixty-three avulsion events from three separate locations over a 33-year interval.
The team then created a unique “fingerprint” classification for each avulsion based on the degree to which the event disturbed the surrounding floodplain as well as the amount of sediment that was deposited in the flooded area. Using the individual avulsion “fingerprints,” the team was able to categorize each event more precisely.
The team’s findings also allowed them to identify several trends that have significant ramifications for communities on avulsion-prone floodplains. For example, Valenza observed a common link between the location of an avulsion event and the severity of the ensuing flooding: avulsions that occur further from a mountain front will result in more extensive flooding.
“When you move further away from a mountain front, avulsions take longer, and they’re messier,” Valenza explains. “By the time the water reaches another channel it could occupy, it has flooded out over the entire floodplain.”
While the paper does not directly consider the effects of climate change on river dynamics, Valenza maintained that increasingly erratic precipitation patterns will make avulsions more common and more severe in the future. Further investigation into the relationship between global warming and river dynamics is needed to better understand their interplay.
“You need to know how a river behaves right now in order to predict how they might behave in the future,” Valenza said.
Dr. Taehee Hwang and Samapriya Roy were also involved in this research.