Controls on Shallow Landslide Area, Depth, and Shape

Friday, 19 December 2014
David Milledge, University of Durham, Durham, DH1, United Kingdom, Dino G. Bellugi, Massachusetts Institute of Technology, Cambridge, MA, United States, Jim A McKean, USDA Forest Service, Whitefish, MT, United States, Alexander Logan Densmore, University of Durham, Durham, United Kingdom and William E Dietrich, University of California Berkeley, Berkeley, CA, United States
The area and volume of a shallow landslide are fundamental controls on both its hazard and its geomorphic importance. Observed shallow landslides from a range of environments are similar in size and geometry. They generally: 1) have depths of 0.1-3 m and areas of 101-104 m2; 2) are longer than they are wide, and wider than they are deep; and 3) have depths that scale as approximately the square root of area. Here we derive a simple model that includes the forces acting on each margin of a potential landslide to explain each of these observed landslide characteristics. The model extends existing methods based on earth pressure theory to make them more appropriate for natural slopes, and represents root reinforcement as an exponential function of soil depth. We find that the model predicts a critical depth in both cohesive and cohesionless soils, resulting in a minimum size for failure which is consistent with observed area-frequency distributions. Furthermore, the differential resistance on the boundaries of a potential landslide is responsible for a critical landslide shape which is longer than it is wide. Finally, our results show that minimum area increases as approximately the square of failure surface depth, consistent with observed landslide depth-area data.