Mediterranean Storms (Proceedings of the 4 th EGS Plinius Conference held at Mallorca, Spain, October 2002)  2002 by Editrice LANDSLIDE RAINFALL TRIGGERS IN THE SPANISH EASTERN PYRENEES J. Corominas, J. Moya, M. Hürlimann Department of Geotechnical Engineering and Geosciences. UPC. Jordi Girona 1-3, D-2 building. 08034 Barcelona. Spain e-mail: Jordi.Corominas@upc.es ABSTRACT Three different rainfall patterns for triggering landslides have been observed in the Spanish Eastern Pyrenees: (a) high intensity and short-lasting rains, trigger shallow landslides, debris flows and some rock falls; (b) moderate intensity long lasting rainy episodes reactivate mudslides and both mid-size rotational and translational slides; and (c) small rainfall episodes produce surges in continuously moving landslides. The geological and hydrological features for such a behaviour are briefly discussed. 1. INTRODUCTION Rainfall is the most frequent landslide-triggering factor in many regions in the world. Many attempts have been made to predict landslide occurrence through rainfall records. However, there is no agreement on how rainfall thresholds should be defined and on the role of the antecedent rainfall. In one of the first comprehensive reviews, Caine (1980), proposed the following intensity-duration rainfall threshold, for both shallow landslides and debris flows: I = 14.82 D -0.39 (1) Where I, is the rainfall intensity in millimetres per hour and D is storm duration in hours. Equation (1) can be used for rainy periods lasting between 10 minutes and 10 days. The concept implicit in this equation is that the average rainfall intensity needed to trigger debris flows is much higher for short lasting rains than for rainstorms extending several days. These results have been confirmed by other researchers (Wolle & Hachich, 1989; Kim et al. 1991). However, the applicability of such a threshold in pervious soils has been questioned (Brand 1995; Au 1998). For other climatic regions, different rainfall thresholds have been proposed (i.e. Wieczorek, 1987; Larsen & Simon, 1993) which differ significantly from that proposed by Caine. 2. RAINFALL TRIGGERS OF PYRENEAN LANDSLIDES Our experience studying recent landslides in the Eastern Pyrenees has shown the relationship between rainfall and occurrence of landslides is complex. We have reconstructed the temporal activity of landslides based on technical reports, field reconnaissance and dendrogeomorphological analysis. According to the nature and permeability of slope materials, we have observed several patterns of landslide-triggering rainfall events,: 2.1 Shallow landslides In slopes covered with pervious colluvium and weathered bedrock formations, high-intensity short-lasting rainfall is able to trigger debris slides, debris flows and rock falls. The analysis of the isohyets and of the areal extent of the shallow slope failures in three recent events (November 6-7, 1982; September 8, 1992 and December 17-18, 1997) has allowed the establishment of a rain threshold of 190 mm in 24-36 h for the Eastern Pyrenees (figure 1). In all of the cases, no significant rainfall was recorded during the weeks previous to the occurrence of the failures. Instead, long lasting rain episodes of low to moderate intensity do not trigger shallow landslides. This is due to the presence of large interparticle voids in colluvium and macropores (animal burrows, decayed root channels, soil pipes) in weathered argillaceous soils that drain water infiltrated from moderate-intensity rains very rapidly. Only high high-intensity rains allow the concentration and the build up of pore water pressures that lead to the slope failure.