Initiation mechanism of diluted debris flows in hilly areas: A case study of the Laomaoshan region

Dilute debris flows in hilly regions pose persistent threats to human life and property, as evidenced by catastrophic events in the Laomaoshan area of Dalian City, China, yet their underlying initiation mechanisms remain insufficiently understood. This study systematically investigates the mechanisms through an integrated approach combining physical model experiments, PFC3D numerical simulations, and theoretical mechanical analysis. Results indicate that the initiation is a progressive, gradation-controlled process driven by mesoscopic structural degradation. We propose a novel four-stage hydro-mechanical framework to elucidate this process: (1) Rainfall Infiltration, characterized by rapid matric suction dissipation and the initial weakening of strong contact force chains; (2) Initial Slip, driven by porosity-induced dilation and enhanced localized hydraulic forces at the slope toe; (3) Fissure Development, where fluid-solid velocity disparities exert substantial hydrodynamic drag on the soil skeleton, inducing retrogressive sliding; (4) Overall Mobilization, culminating in the complete collapse of the internal force network and rapid channelized fluidization. Furthermore, mesostructured soil dictates vulnerability, with discontinuously graded soils exhibiting higher susceptibility to sudden, liquefaction-like failure upon saturation. By linking mesoscopic internal yielding with macroscopic hydrodynamic entrainment, this study establishes a physically grounded framework. This coupled weakening–erosion mechanism distinguishes dilute debris flows from traditional viscous debris flows, offering a robust theoretical basis for improved prediction and early warning in analogous geomorphic environments.