A3 Landslides and Gravitational Flows

Topic

A rock slide or gravity-driven flow occurs when the shear stress exerted by gravity on a slope exceeds the slope's shear strength; this gravitational shear stress is amplified by destabilizing factors such as high pore-water pressure within the slope, the loss of cohesion caused by weathering over time, or a sudden surcharge (added load) on the slope. A well-established mechanical failure criterion defines the precise threshold at which a slope fails, and the slope's Factor of Safety measures the ratio between its available strength and the stress it is actually bearing.

Once slope movement begins, the material's rheology determines the flow style: a rigid rock slide moves according to rigid-body kinematics, displacing as a coherent block, whereas a lahar or debris flow behaves like a viscoplastic material—remaining solid until the applied stress exceeds a threshold, at which point it begins to flow like a liquid. Large rock avalanches also exhibit anomalously high mobility, traveling a horizontal distance far greater than simple friction between the rock and the ground would suggest based on the fall height; this is likely because interstitial pressures within the avalanche itself, or dynamic liquefaction of the material, reduce effective friction while the avalanche is in motion.

Submarine landslides are particularly dangerous because they can generate tsunamis by abruptly displacing a large mass of water on the seafloor. Destabilization of sediments accumulating on a submarine slope can be triggered either by the dissociation of gas hydrates trapped within the sediments or by the additional load imposed on the slope by an earthquake. The runout distance of a debris flow scales with the square root of the total volume of material mobilized by the flow—a well-established empirical relationship that allows for estimating the hazard posed by a future debris flow based on an inventory of deposits left by previous flows in the same region, thereby closing the link between the gravitational shear stress with which this description began and the final runout of each landslide or gravity-driven flow.

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