Visco‐Collisional Scaling Law of Flow Resistance and Its Application in Debris‐Flow Mobility

Abstract Flow resistance against gravity‐driven forces is a key factor controlling debris‐flow mobility, which is an important parameter for hazard risk assessment. In practice, Manning's formula is widely used for debris‐flow mobility analysis. However, this formula depends on the boundary conditions (channel roughness) and neglects the physical mechanisms of debris flow. Based on the systematic analysis of the field observation data of 93 debris‐flow events at Jiangjia Ravine (Yunnan Province, China), this study investigates the dynamic mechanisms and sources of flow resistance of debris flow. As the flows tend to be liquefied, the fluid viscous effect and particle collisions are the main sources of flow resistance. Flow resistance can be described by a visco‐collisional scaling law. Under fixed channel boundary conditions, this law is further incorporated into Manning's formula, bridging the gap between the resistance model based on physical mechanisms and the empirical formula of flow resistance. The modified Manning's coefficient is closely related to flow regimes, giving Manning's coefficient an explicit physical meaning. The modified Manning's coefficient provides a more reasonable basis for the mobility analysis and risk assessment of debris flow.