Modeling how wet and dry granular layers interact during erosion

Consider making a sandcastle: Adding a tiny bit of water allows the sand to adhere to itself, allowing you to mold it into structures.

Similarly, during erosion, small amounts of liquid affect the cohesive forces in granular materials. The distinct layers of material have contrasting cohesion, but cohesive and non-cohesive granular materials are often studied separately; in real-life erosion, they interact. Braysh et al. simulated these interactions.

“Gaining a general physical picture of these interactions is crucial because they govern phenomena across wildly different scales, from large-scale natural hazards like soil erosion and landslides to fine-tuned industrial powder processing,” said author Serge Mora.

The researchers modeled how the erosion process changes as the dry, noncohesive layer’s flows affect its underlying cohesive layer, taking parameters like surface tension and friction into account. Despite the complexity of the interaction, they uncovered a universal scaling law that describes how the erosion rate evolves, significantly simplifying the underlying physics to inform hazard assessments in the geosciences.

The authors plan to expand their study to include various particle sizes and shapes and to compare their simulations to experimental data.

“As climate change destabilizes slopes and mountain environments, catastrophic events — like the massive debris flows and landslides recently seen in Nepal — remind us how crucial it is to model how a moving flow entrains the soil beneath it,” Mora said. “Our work helps build the fundamental physical basis to better predict these erosion dynamics.”

Source: “Wet granular bed eroded by a dry granular flow,” by Lama Braysh, Patrick Mutabaruka, Franck Radjai, and Serge Mora, Journal of Rheology (2026). The article can be accessed at https://doi.org/10.1122/8.0001212 .