A combined conceptual model for the effects of fissure-induced infiltration on slope stability

Abstract

In humid and subhumid Mediterranean environments the disruption of the vegetation cover by in particular shallow landslides limits the area in which erodible material is exposed to overland flow. On short temporal scales the hydrological system that allows for the generation of critical pore pressures or soil moisture conditions on the potential shear surface determines the occurrence of these landslides. Hence the relative contribution of landsliding to land degradation processes can be quantified in terms of its magnitude and frequency through a semiphysical hillslope model that links the relevant hydrological processes to a stability analysis. For the development of such a combined model for hydrology and slope stability a conceptualization of the process system is needed. Through the implementation of the resulting model in a GIS environment the effect of topography, to which the occurrence of landslides is intrinsically linked, can be incorporated. In addition, the spatial variation of hydrological and geomechanical parameters can be incorporated in the model. This is important since the occurrence and extent of shallow landslides is directly dependent on the net rainfall input in the hydrological system, as defined by the land cover of the area.

In this paper a conceptual model is presented that combines a description of the hydrology with an assessment of the slope stability. The conceptual framework of this model is based on field observations in the Alcoy area (SE Spain). In this area shallow landslides occur on steep, unsaturated slopes in marly deposits of Miocene age, at the boundary between regolith and bedrock (1–2 m depth). Given the low matric permeability of the marl it has been assumed that preferential flow along distinct sets of fissures by-passing slower matric percolation might account for the observed response time of landslides to rainfall events. The fissures in the regolith are either relict primary bedrock structures (discontinuities) or are formed by weathering, creep and shear; they are supplied with water by subsurface flow through the more permeable rootzone. With the combined slope stability model, which is programmed in the meta-language embedded in the PCRaster GIS package, a sensitivity analysis has been performed to assess the impact of fissure flow on the occurrence of landslides in a small catchment of 1.2 km² near Alcoy.