Landslides in the North of Lisbon Region (Portugal): Conditioning and triggering factors |
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| Institution: | 1. Department of Civil and Environmental Engineering, Imperial College London, London, SW7 2AZ, UK;2. Geohazards and Earth Processes Research Group, Faculty of Engineering, University of Nottingham, Nottingham, NG7 2RD, UK;1. Unit of Hydraulic Engineering, University of Innsbruck, Technikerstraße 13, 6020 Innsbruck, Austria;2. Institute of Geology, University of Innsbruck, Innrain 52f, 6020 Innsbruck, Austria;3. Unit of Geotechnical and Tunnel Engineering, University of Innsbruck, Technikerstraße 13, 6020 Innsbruck, Austria;1. Department of Civil and Structure Engineering, Kyushu University, Fukuoka 8190395, Japan;2. Department of Geotechnical Engineering, School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China |
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| Abstract: | 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. Human's 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. |
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