Maria Luísa Rodrigues

The role of conditioning and triggering factors in the occurrence of landslides : a case study in the area north of Lisbon (Portugal)

Abstract This study of the recent landslide activity in the region north of Lisbon is based on detailed field mapping (1:2000 scale) in five sample areas. A total of 574 landslides were identified in an area of 61.6 km 2 , i.e., about 10 slope movements/km 2 . Thirty-six percent of the slope movements were shallow translational slides, affecting Upper Pleistocene and Holocene unconsolidated deposits. The deeper and larger translational and rotational slides developed in marls and clays of different lithological units. About 20% of the total number of landslides were triggered by human activity (slope cutting, artificial fills and river channel diversion) as a consequence of urban development around Lisbon. The most important triggering factor was rainfall. Although moderate-intensity rainfall episodes may be responsible for a few small landslides on river banks and artificial cuts (return period of about 3 years), two main situations were identified: (1) high-intensity rainfall episodes, which caused flash floods and gave rise to numerous slides and falls, due to bank erosion (return period of over 55 years); and (2) long-lasting rainfall periods, which caused the deeper translational, rotational and complex slides (return period of about 25 years).

Landslides in the North of Lisbon Region (Portugal): Conditioning and triggering factors

Detailed geomorphological mapping carried out in 5 sample areas in the North of Lisbon Region allowed us to collect a set of geological and geomorphological data and to correlate them with the spatial occurrence of landslide. A total of 597 slope movements were identified in a total area of 61.7 km2, which represents about 10 landslides per km2. The main landslide conditioning factors are: lithology and geological structure, slope angle and slope morphology, land use, presence of old landslides, and human activity. The highest landslide density occurs in Cretaceous marls and marly limestones, but the largest movements are in Jurassic clays, marls and limestones. The landslide density is higher on slopes with gradients above 20 °, but the largest unstable area is found on slopes of 10 ° to 15 °, thus reflecting the presence of the biggest slope movements. There is a correlation between landslides and topographical concavities, a fact that can be interpreted as reflecting the significance of the hydrological regime in slope instability. Concerning land use, the highest density of landslides is found on slopes covered with shrub and undergrowth vegetation. About 26% of the total number of landslides are reactivation events. The presence of old landslides is particularly important in the occurrence of translational slides and complex and composite slope movements. 20% of the landslide events were conditioned by anthropomorphic activity. Humans intervention manifests itself in ill-consolidated fills, cuts in potentially unstable slopes and, in a few cases, in the changing of river channels. Most slope movements in the study area exhibit a clear climatic signal. The analysis of rainfall distribution in periods of recognised slope instability allows the distinction of three situations: 1) moderate intensity rainfall episodes, responsible for minor slope movements on the bank of rivers and shallow translational slides, particularly in artificial trenches; 2) high intensity rainfall episodes, originating flash floods and most landslides triggered by bank erosion; 3) long-lasting rainfall periods, responsible for the rise of the groundwater table and triggering of landslides with deeper slip surfaces.