Monday, January 16, 2012
Simulated effect of soil depth and bedrock topography on near-surface hydrologic response and shallow landslide triggering by Lanni, McDonnell, Hopp and Rigon
We have just submitted a paper that, looked from a certain perspective, can be thought on the evolution of soil moisture content in presence of variable soil depth. In fact, variable soil depth, jointly with the fact that increases in hydraulic conductivity follows the increase in water pressure, and that hydraulic conductivity itself can often be considered negligible when the soil is unsaturated, delays the formation of a widespread water table in a hillslope. Therefore the effective contributing area above a point of a catchment is usually not the total upstream area but just a part of it. This obviously has consequences on the propagation of instabilities along a slope.
The Abstract of the paper:
This paper explores the effect of hillslope hydrological behavior on slope stability in the context of transient subsurface saturation development and landslide triggering. We perform a series of virtual experiments to address how subsurface topography affects the location and spatial pattern of slip surface development and pore pressure dynamics. We use a 3D Darcy-Richards equation solver (Hydrus 3-D) combined with a cellular automata slope stability model to simulate the spatial propagation of the destabilized area. Our results showed that the soil-bedrock interface and in particular, bedrock depressions, played a key role on pore pressure dynamics, acting as an impedance for the downslope drainage of perched water. Filling and spilling of depressions in the bedrock surface microtopography induced localized zones of increased pressure head such that the development of pore-pressure fields—not predictable by surface topography—lead to rapid landslide propagation. Our work suggests that landslide models should consider the subsurface topography in order to include a connectivity component in the mathematical description of hydrological processes operating at the hillslope scale. Quantitative soil- landscape methods combined with physically-based landslide models may improve our ability to predict shallow landslide potential.
Among the original stuff presented in this paper, there is a tentativ to move away from the concept of instable points to the one of instable regions. The traditional (simplified) approach based on the infinite slope stability (Ning Lu is discussing it here) is in fact used in modern GIS based program like SHALSTAB (here on ARCGIS or here on opensource GIS) or SINMAP to determine the instabilities of single points, which we try here to generalize a little.
The paper draft is available here, if you are interested in. A related discussion can also be found in the previous papers by Lanni et Al., 2011 and in the draft by Cordano and Rigon, 2012.