Sometimes rivers suddenly jump their banks, abandon channels and wash over floodplains in search of new routes. These rare events, called avulsion, can cause catastrophic flooding and threaten lives and livelihoods worldwide.

Avulsions are difficult to predict – the exact factors that cause a river to rapidly change course are not well understood. A recent one study published in Nature identifies physical rules that indicate why certain rivers open their mouths and offers a blueprint for measuring the path an avulsed river might take.

“Scientifically, we are about to understand fluvial avulsion in a way that we haven’t been able to for the last century,” says the geomorphologist Douglas Edmonds at Indiana University Bloomington, one of the authors of the study.

Using satellite images, the researchers mapped avulsions around the world and confirmed an intuition long held by the scientific community: Avulsions tend to occur in places with large topographic changes such as mountain fronts or near coastlines. Of 174 mapped river avulsions since 1984, 74% occurred in these areas.

Eyes in the Skies

Studying the topography around rivers can be tricky. Rivers are often surrounded by thick vegetation, so traditional aerial or satellite imagery cannot provide visualizations of the heights of banks or the shape of terrain. But laser beams can penetrate vegetation and bounce off the Earth’s surface, creating a 3D model that can be used to measure height using a technique called suffering. So when NASA launched the ICESat-2 satellite in 2018, equipped with advanced lidar technology to map polar ice caps and the rest of Earth’s surface, Edmonds and his colleagues realized the agency had inadvertently created the perfect tool for studying the topography around rivers.

Using ICESat-2 and digital elevation models, the researchers were able to obtain clear topographic data, including the elevation and slope of channels and banks, for 58 avulsion sites.

Rivers typically swell for one of two reasons: Sediment builds up on the river bed and raises the river above its banks (known as superelevation), or the slope next to the river is steeper than the bed, providing a faster path downhill (known as slope). advantage).

Researchers had believed that both mechanisms could independently cause avulsion. However, after analyzing the ICESat-2 data, the authors concluded that these two factors actually work together but play different roles depending on the location of the river. Near mountains, elevation drives avulsion, while avulsion in coastal areas is primarily caused by slope advantages.

Superelevation and slope benefits “seemed to scale oppositely. And the only way that works is if they’re inversely related,” said James GearonPhD student also at Indiana University Bloomington and lead author of the study. Where the value of superelevation was high, the slope advantage was low and vice versa.

“It’s not something that was expected,” the geomorphologist said Vamsi Ganti from the University of California, Santa Barbara, who was not part of the study. “But the authors make a really good case for it, and it’s very data-driven,” he added.

The researchers define a new avulsion criterion as the mathematical product of the two measures.

Shows the way

Floods triggered by avulsion are different from other types of floods. “It could be quite significant in terms of its volume and its duration because the river has to reestablish its path,” Edmonds said. “It could take a long time.” Predicting the river’s new channel before the desilting begins is key to limiting damage and loss of life.

To replicate the path behavior of a river, the researchers used probabilistic modeling and developed an algorithm that incorporated two factors: inertia and slope. Their algorithm chose a path that favored the largest gradient and the smallest change in direction.

“What we showed is that just based on these two simple rules, inertia and slope dependence, you can completely recreate the direction the river is going,” Gearon said. When tested on 10 real avulsions, the paths predicted by the algorithm overlapped almost completely with the observed paths.

The model could be used to predict future avulsion paths, but “the incredibly important piece of information that we can’t yet predict is, along a given river, where will the next avulsion be?” Edmonds said.

Until recently, data on river avulsion were limited to small experiments and a handful of field observations, says Ganti, who has documented avulsions around the world. “Now we actually have a large data set that spans the entire world.” The ICESat-2 data the new study uses could “potentially be a game changer,” he added.

The authors hope their work will help mitigate avulsion-related hazards, particularly in the global south, where avulsions are more common but disaster management resources are fewer. They would also like to see future flood models account for avulsion. “Flood models are missing this important piece, but I think our understanding of avulsions is catching up,” Edmonds said.

—Sushmita Pathak (@sushmitza), Science writer

Quote: Pathak, S. (2024), New rules for catastrophic river erosion, Eos, 105, https://doi.org/10.1029/2024EO240523. Published on November 21, 2024.

Text © 2024. The authors. CC BY-NC-ND 3.0
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