Geomorphological consequences of frequent and infrequent pluviometric and hydrological events in a Mediterranean mountain area

Abstract

Intense rainfall events are a major trigger for flooding and landslides throughout regions with a Mediterranean climate. They create problems for settlements and infrastructures built across their paths. Changes in landuse have also been important in increasing or decreasing the intensity of the flood problem and the mobility of land surface materials, especially changes in the intensity of grazing on mountain pastures or historical deforestation and recent reforestation. This paper focuses on the rainfall thresholds that distinguish common events from rare, hazardous events. During the last few years, rainstorms of different intensities have occurred in the Central Spanish Pyrenees, including one of exceptional character. Large, historical debris flows have been studied, as well as the actual sediment transport in small experimental catchments. This study shows that during the most frequent events suspended sediment transport is the common geomorphic process. River bedload is mobilized in river channels several times per year, while small rock avalanches and channelized debris flows have a return period of at least 5 years. Hillslope debris flows are triggered by rainfall events with a 25-30 year return period. Reactivation of large, deep mass movements is linked to rainfalls of around 100 year return period (between 130 and 160 mm in 24 hours). Catastrophic geomorphic processes occur when precipitation exceeds a 100 year return period, as was the case of the Biescas campsite disaster. Geomorphic processes triggered by intense rainfall events have caused major damages and human disasters but the hazards have been reduced by the introduction of several control measures, including reforestation, the construction of check-dams, canalization of river segments and improved flood forecasting.