Rainfall and weather conditions inducing intense landslide activity in northern Spain (Deba, Guipúzcoa)

Abstract

The Deba area is intensely affected by frequent shallow landslides triggered by rainfall. This contribution explores the role of rainfall in landslide activity during a quite long time span (60 years), from a large network of rainfall gauges and a complete inventory of landslides. Out of 1,180 landslides inventoried, more than 50% occurred simultaneously in 6 known dates, corresponding to 6 episodes triggering multiple landslides; 3,241 rainfall episodes have been automatically recognized and characterized in terms of rainfall amount and duration, providing a representative dataset that covers a wide range of movement types and behaviors. Relationship between rainfall episodes driving multiple movements simultaneously has not been explored in depth so far in northern Spain. The analysis provides different results. The extraordinary character of the triggering rainfall has been assessed and empirical rainfall thresholds (total amount, and mean intensity), producing multiple landslides, has been found and compared with others described in literature. Also, the meteorological conditions associated to those extreme events have been recognized: multiple landslide occurrences are triggered by extreme convective rainfall: intense, short and with limited horizontal extent, as well as a marked summer-autumn seasonality. This weather pattern is more characteristic of Mediterranean areas than of mild marine west-coast climates. The definition of the conditions of the multiple landslide occurrence events, qualitative and quantitative, makes it possible to better understand the behaviour of slopes, which is essential for better predictability of landslide occurrence.