Bridging a 60-Year Gap: CEE at Illinois-led study documents first in-field evidence of underwater sediment transport mechanisms

When sediment builds up underwater, the increased density can cause a sort of avalanche. The movement of this cloudy, dense water carries sediment through and between bodies of water, sometimes bringing with it changes to underwater topography, water depth and aquatic ecosystems.

“This research was driven by the desire to bridge a decades-long gap between mathematical theory and real-world observation. The breakthrough was the result of an arduous eight-year journey and multiple intense field campaigns.”

Hongbo Ma, Assistant Professor, Department of Civil and Environmental Engineering

While scientists have known for 60 years that these sediment-laden underwater flows, called turbidity currents, could theoretically “ignite” on their own – picking up speed and sediment in a self-reinforcing loop as they travel – this phenomenon has never been captured outside of a laboratory setting. 

A new study led by Civil and Environmental Engineering Assistant Professor Hongbo Ma documents the first-ever field-scale evidence of self-accelerating turbidity currents (SATCs). Published in PNAS, these findings provide new credence to the decades-old theory of SATCs and create a jumping-off point for further research into their application for sustainable reservoir management and prediction of hazardous underwater events.

 

An Eight-Year Quest in a "Natural Lab"

“This research was driven by the desire to bridge a decades-long gap between mathematical theory and real-world observation,” Ma said. “The breakthrough was the result of an arduous eight-year journey (2018–2025) and multiple intense field campaigns.”

The team turned to the Xiaolangdi Reservoir along China’s Yellow River, a "natural lab" characterized by rapid sediment evolution. Operating in extreme conditions with temperatures often exceeding 40°C, Ma and his team used a fleet of research vessels to capture bed bathymetry and water and sediment samples from the reservoir. By synchronizing high-resolution topographic mapping with multi-layered water sampling, the team achieved daily-scale monitoring of the reservoir floor—capturing the "ignition" of SATCs in a way nearly impossible to record in the deep ocean.

Challenging Traditional Wisdom

Through the in-field observations, they were able to document the whole structure of the SATCs, and additionally shed light on certain misconceptions that have taken shape over the years. Most notably, they found that the currents do not require steep inclines to start accelerating but occur even on the relatively flat floors of reservoirs and submarine canyons.

“SATCs offer a highly efficient solution to the global crisis of reservoir sedimentation. By triggering these self-accelerating flows, engineers can flush sediment out of dams more effectively.”

Hongbo Ma, Assistant Professor, Department of Civil and Environmental Engineering

By integrating the in-field data with advanced theory, Ma also identified a universal threshold value that determines when a current will begin to self-accelerate. This “point-of-no-return” value provides a new framework for predicting SATC occurrence and can even inform novel engineering strategies for sustainable reservoir and dam management.

 

Photo Credit: Baohua Dong

 

Global Applications: From Dams to Deep-Sea Cables

“SATCs offer a highly efficient solution to the global crisis of reservoir sedimentation,” Ma said. “By triggering these self-accelerating flows, engineers can flush sediment out of dams more effectively.”

Understanding of the threshold number is also critical to protecting seafloor telecommunications cables, which can become damaged or even severed under the force of SATCs.

Looking forward, Ma will begin applying the new findings, including the universal threshold value, in different environments, advocating for its application in dam, reservoir, and ecosystem management.

The paper, “In situ evidence of self-accelerating turbidity currents”, can be read here. 

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Hongbo Ma is an assistant professor in the Department of Civil and Environmental Engineering at The Grainger College of Engineering, University of Illinois Urbana-Champaign. Dr. Ma's research focuses on environmental fluid mechanics, sediment-laden flows and their geomorphic expressions across diverse environments, including both mountain streams and fine-grained depositional systems. His recent study revealing how Hack's Law can help to simplify the design and construction of structures used to manage water flow and reduce flood risks along river deltas was published in the journal Science.