New Rules for Catastrophic River Avulsion

Sometimes rivers suddenly jump out of their banks, abandon channels and erode floodplains to find new paths. These rare events, called avulsions, can cause devastating floods and threaten lives and livelihoods around the world.

Avulsions are difficult to predict; The factors that cause a river to change direction rapidly are not fully understood. recently to work published in Nature It describes the physical rules that show why certain rivers avulse and provides a scheme for quantifying what path an avulsed river might take.

“We are on the verge of scientifically understanding river erosion in a way that we have not been able to achieve in the last century,” the geomorphologist said. Douglas Edmonds One of the study’s authors is at Indiana University Bloomington.

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

Eyes in the Sky

Studying the topography around rivers can be difficult. Because rivers are often surrounded by thick vegetation, traditional aerial or satellite imagery cannot provide visualization of the elevations of banks or the shape of the land. But laser beams can penetrate vegetation and bounce off the Earth’s surface, creating a 3D model that can be used to measure elevation. Lidar. When NASA launched the ICESat-2 satellite equipped with advanced lidar technology to survey the polar ice sheets and the rest of the Earth’s surface in 2018, Edmonds and his colleagues realized that the agency had inadvertently created the perfect tool for studying the topography around rivers.

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

Rivers generally avulse for one of two reasons: Sediment accumulates in the riverbed and lifts the river above its banks (known as elevation), or the slope next to the river is steeper than its bed, offering a faster path downhill (known as slope). advantage).

Scientists had thought that both mechanisms could independently cause avulsions. But after analyzing 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, excessive elevation leads to avulsion, while in coastal areas avulsions are predominantly due to slope advantage.

The height and slope advantage “seemed to scale oppositely. “And the only way it works is if they’re inversely related,” he said. James GearonHe is also a doctoral candidate at Indiana University Bloomington and the study’s lead author. Where the elevation value was high, the slope advantage was low and vice versa.

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

Researchers define a new avulsion criterion as the mathematical product of two measurements.

Showing the Way

Avulsion-triggered flooding is different from other types of flooding. “It could be quite significant in terms of its volume and duration because the river has to reestablish its path,” Edmonds said. “It might take a long time.” Predicting the river’s new channel before avulsion begins is key to limiting damage and losses.

To mimic the pathfinding behavior of a river, researchers used probabilistic modeling and developed an algorithm that combines two factors: inertia and slope. Their algorithm chose the path with the greatest gradient and least change of direction.

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

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

Until recently, data on river avulsions was limited to small experiments and a handful of field observations, Ganti said. documented avulsions worldwide. “We now have a large data set that actually covers the entire world.” He added that the ICESat-2 data used by the new study “could potentially be a game changer.”

The authors hope their work will help reduce avulsion-related hazards, especially in the Global South, where avulsions are more frequent but disaster management resources are fewer. They would also like to see future flood models take avulsion into account. “Flood models are missing this important piece, but I think our understanding of ruptures is catching up,” Edmonds said.

—Sushmita Pathak (@sushmitza), Science Writer

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

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