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Landslides: Slope stability, triggers, failure dynamics, and morphology - Failure

            The physics of landslides during failure depend on the type of landslide. Coherent blocks behave differently from disrupted, incoherent slides, and saturated slides behave differently from dry slides.  This section briefly overviews the effect of water on landslide dynamics, focusing on debris flows, and the failure process of rock avalanches.

Effects of water on landslide dynamics

            Landslides that have high water contents behave as Non-Newtonian fluids and have little to no shear strength (this includes mudflows). Behavior depends on the amount of water present, as water decreases the shear strength. Materials with low water contents will behave in a brittle fashion; increasing the water content progressively results in ductile, then plastic, then fluid like behavior. Because of the effects of water on debris flows, the mechanics of the flow are in between that of fluid dynamics and that of soil dynamics.

            Modeling the dynamics of debris flows is difficult due to the interaction between particles in water and the interaction of the debris flow with its channel. Simple experiments on the effect of shear rate on the shear strength of water-clay mixtures show shear thickening behavior. Unlike water, which shows a linear relationship between shear stress and strain rate, and does not show an increase in shear strength with increasing strain rates, the shear strength of clay-water mixtures increases non-linearly with strain rate. Increasing clay content also increases the shear strength of the mixture. 

Fig 3
Figure 4: The dependence of shear strength on shear rate for clay-water mixtures. Each line is labeled with the percent clay. Figure is from De Blasio (2011).  

           The flow behavior of a mudflow is modeled as a Bingham fluid. Bingham fluid flow consists of a basal shearing layer with velocity increasing until the boundary with an upper plug layer is reached. The upper plug layer moves at the same velocity as the top of the shearing layer. Figure 5 shows Bingham fluid flow on a slope. Channel morphology also controls the flow behavior, with decreasing channel volume increasing velocity, and increasing channel volume decreasing velocity.

Fig 4

Figure 5: Bingham fluid flow on a slope. Figure is from De Blasio (2011). 

 Rock Avalanches
            Rock avalanches, also known as sturtzstroms, are the largest failure events on the planet.  They originate as a gravitational failure of a plane or wedge (in many cases likely triggered by an earthquake) that disintegrates into a fragmented mass. Possible mechanisms of disintegration include impact with the ground surface, particle impacts, and bending of the slab beyond the tensile strength of the rock. The disintegrated mass then travels rapidly across long distances, with run-outs that are much greater than any other types of failure. The mechanism for transport across long distances is debated. Some have argued that a layer of compressed air supports the slide and provides a lubricated layer for sliding (Shreve, 1959) while others argue that acoustic fluidization may sustain fluid-like flow for long distance travel (Collins and Melosh, 2003).

Figure 6: Satellite image of the Saidmarreh landslide in Iran. This sturtzstrom occurred about 10,000 years ago, and was likely triggered by an earthquake. About 20 cubic kilometers of material were transported up to 14 km from the source. Image from

Morphology and mapping
            Failures that involve the transport of material, such as slides and flows, have three distinct parts, the source region, the run out region, and the deposition area. The source region is characterized by erosion, the run out region can be both erosional and depositional, and the deposition area is characterized by the deposition of the rock or sediment. The morphology of the depositional area is dependent on the type of failure. Disrupted failures can produce inverse grading, and compression ridges may appear when velocity in the center of the slide was greater than that on the sides of the slide. In cases where the slide moved as a coherent block, the block will be found in the depositional area with some internal deformation.

            While distinguishing between the distinct failure parts in the field may be relatively simple, doing so for large scale mapping using aerial photography is difficult. Many landslide inventories have problems with landslide amalgamation, and do not differentiate between source areas and deposition. Accurate and complete landslide inventories are needed to train hazard models (See Coseismic landslide hazard mapping methodologies-Von Voigtlander). Incorporating the physics of landslide failure and transport will improve mapping of source areas and improve predictions of where landslides will occur and what they will impact.


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