Reconstructing recent landslide activity in relation to rainfall in the Llobregat River basin, Eastern Pyrenees, Spain

A chronology of recent landslides in the upper basin of the Llobregat River, Eastern Pyrenees, has been reconstructed from technical reports, field reconnaissance and dendrogeomorphological analysis. The precipitation conditions responsible for triggering and reactivating landslides have been deduced by analysis of rainfall records from two rain gauges located in the area. Two different rainfall patterns relate to landslide occurrence: (i) Without antecedent rainfall, high intensity and short duration rains trigger mostly debris flows and shallow slides developed in colluvium and weathered rocks. A rainfall threshold of around 190 mm in 24 h initiates failures whereas more than 300 mm in 24–48 h are needed to cause widespread shallow landsliding; (ii) with antecedent rain, moderate intensity precipitation of at least, 40 mm in 24 h reactivates mudslides and both rotational and translational slides affecting clayey and silty–clayey formations. In this case, several weeks and 200 mm of precipitation are needed to cause landslide reactivation. Dendrochronological analysis shows that landslide activity in the Eastern Pyrenees was relatively low between 1926 and 1959 but has since increased significantly with reactivation now occurring about every 3 years.