Modelling mass failure in a Mediterranean mountain environment: climatic, geological, topographical and erosional controls

Extensive mass failures occur in the Alpujarras on the south flank of the Sierra Nevada of southeast Spain, especially in the Guadalfeo valley, even though rainfall is low, about 500 mm/year. There are four major controls on the occurrence of these landslides: structure, climate, topography and erosion. The tectonic control is through the structure and lithology of phyllites and mica-schists with strong lineations providing preferred seepage and failure planes. Climate controls the long-term, seasonal and short-term water-pressure heads. Topography controls the localised head-pressure gradients as a result of seepage and erosion controls the incidence of slope undercutting by fluvial action. The effects of these controls are tested using a hydrological budget model for percolation to ground water, a finite-element model for ground water flows and stability analysis for modelling the actual failures. The results suggest that in this mountainous terrain while climate may be very important for shallow failures, the erosional history and the consequent morphology are in general much more important except for the trimming induced by occasional very large run-off events.