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 re connaissance and dendrogeomorphological analysis. The precipitation conditi ons responsible for triggering and reactivating landslides have been deduce d by analysis of rainfall records from two rain gauges located in the area. Two different rainfall patterns relate to landslide occurrence: (i) Withou t antecedent rainfall, high intensity and short duration rains trigger most ly debris flows and shallow slides developed in colluvium and weathered roc ks. A rainfall threshold of around 190 mm in 24 h initiates failures wherea s more than 300 mm in 24-48 h are needed to cause widespread shallow landsl iding; (ii) with antecedent rain, moderate intensity precipitation of at le ast, 40 mm in 24 h reactivates mudslides and both rotational and translatio nal slides affecting clayey and silty-clayey formations. In this case, seve ral weeks and 200 mm of precipitation are needed to cause landslide reactiv ation. Dendrochronological analysis shows that landslide activity in the Ea stern Pyrenees was relatively low between 1926 and 1959 but has since incre ased significantly with reactivation now occurring about every 3 years. (C) 1999 Elsevier Science B.V. All rights reserved.