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

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 km2, i.e., about 10 slope movements/km2. 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).