Landslides are known to travel further than expected from the coefficient of friction of their material. In some cases, this is just because the ratio of the height lost to the horizontal distance travelled (H/L), which is compared to the coefficient of friction, is not computed from the centre of mass of the deposit, as it should be, but from the distal end. Simple spreading of the landslide mass can then explain the excess runout. However, spreading alone is not able to explain the spectacular runout of most landslides, for which the centre of mass does travel further than predicted for a frictionally-controlled slide. The long travel distance of the centre of mass cannot be explained by dry granular models. As it is well known that water reduces solid friction in debris flows, and that significant amounts of water are present in many landslides, it is proposed here that water is the main cause for the unexpectedly high mobility of landslides. Water in the debris also introduces a viscous dissipative stress which can account for the relatively channelled behaviour of landslides over topography. The difference between landslides and debris flows is wholly gradational and related to the water content.
展开▼
