Landslide susceptibility in the Rio Aguas catchment, SE Spain

The definitions of hazard and risk in natural hazard studies are well established in the scientific literature. However, many examples of 'landslide hazard assessment' only identify the susceptibility of slopes to failure and make no statement on the frequency of occurrence that would be necessary for a complete hazard evaluation. In a research programme undertaken in SE Spain the issue of landslide susceptibility in a semi-arid, neotectonic environment was examined, with some attempt to evaluate the hazard. This work involved establishing the occurrence of landsliding within the 550 km(2) Rio Aguas catchment through remote sensing interpretation and field mapping. These data were compiled in an inventory containing the records of nearly 250 landslides that was analysed to establish the nature and extent of landslide susceptible situations. Within the catchment anticipated combinations of geological materials proved to be susceptible to failure, and relationships between landslide volume and travel angle were examined in relation to standard models. The highest incidence of contemporary landsliding appeared to be related to the proximity of a major river capture site, a geomorphological event that had been dated at 100 000 BP. This produced localized rapid incision, a ten fold increase in sediment removal and the creation of oversteepened slopes that were only recently degrading to their long-term angle of stability. In addition to contemporary landslides, field mapping identified anomalous geological structures that proved to be degraded erosional remnants of ancient landslides. Relating these remnants to the river terrace sequence in the region provided some control on the relative ages of these ancient or 'fossil' landslides. It was concluded that any assessment of landslide risk in the study area would need to take into account not only geological materials and the contemporary geomorphological environment but also the geomorphological history of the region.