Deciphering Controls of Pore‐Pressure Evolution on Sediment Bed Erosion by Debris Flows.

Pore‐fluid pressure (PP) plays an important role in bed erosion, but the mechanisms that control PP evolution and the resulting feedbacks on flow dynamics are unclear. Here, we develop a general formulation, allowing quantification of the propensity for PP evolution of saturated and unsaturated bed sediments. We conduct erosion experiments by systematically varying grain composition and water content of beds, for investigating effects of PP evolution on flow erosion. With increasing water content, PP shows a slight rise in deforming beds with drained behavior but significant larger rise in undrained beds. Regardless of bed composition, the erosion rate of beds presents a synchronous change tendency with PP evolution due to the loss in basal friction. PP instigates positive feedback that induces a remarkable gain of flow velocity and momentum on wet beds with undrained behavior. Our results help explain observations of volume growth and long run out of debris flows. Plain Language Summary: Debris flows are common geophysical flows consisting of debris grains and muddy water. Debris flows can grow significantly in volume and mobility as they pick up loose sediment from gully bed and banks. The destructive potential of debris flows increases with increasing flow volume and run out. This brings about great challenges for effective early warning of debris flows, design of prevention measures and mapping of hazard zones related to human settlements. It is commonly believed that flow momentum is consumed by carrying static bed sediments. However, flows can gain momentum by overriding wet bed sediments. This can be explained by pore‐pressure generation as debris flows move across wet beds. The increase of measured pore‐fluid pressure is limited for beds with a low water content, but substantial for beds with a higher water content, which strongly affects the erosion rates of bed sediments. Flow velocity and momentum on wet beds are observed to increase significantly but slightly for dryer beds as a result of the pore‐pressure feedback. These findings indicate that the debris composition of the catchment, the water content of bed sediment and the pore‐pressure development should be evaluated when making predictions on debris‐flow hazard. Key Points: Propensity for pore pressure evolution of bed sediments during debris‐flow erosion is evaluated by a Deborah numberSignificant pore pressure and accompanying intense erosion occur for wet bed sediments with undrained behaviorEnhanced pore pressure of wet bed sediments reduces flow basal friction, increasing flow mobility and runout [ABSTRACT FROM AUTHOR]