The rainfall-triggered landslide event of November 1983 in the Lisbon Region: a contribution for the knowledge of regional slope instability

On November 18, 1983, a heavy and short-duration rainfall event (164 mm/24 hours ) occurred, particularly, in the northern Lisbon metropolitan area, associated with an atmospheric  depression, localised southwest of the Azores islands, which evolved into a cut-off depression. This rainfall event triggered a large number of landslides, with evident regional importance, which remained little explored until now.

Event-based landslide inventories identify the location of landslides caused by a single trigger, which is crucial to understand the predisposition factors and to assess landslide susceptibility for different types of landsides occurring in the study area. The purpose of this study is to elaborate a complete event-based landslide inventory for this rainfall-triggered event, which is used  to model landslide susceptibility at the regional scale, helping to understand the combined action of the trigger and the landslide predisposing factors.

The landslide inventory is built based on the interpretation of high-resolution analogic aerial photographs (1:15,000 scale), obtained after the event, between the 8^th and the 13^th December 1983. Recent orthophotomaps covering the complete study area were used as the baseline image for the process of geo-referencing the aerial photos. In addition, it was necessary to perform local correction of deformations, resulting from the conic projections of aerial photographs, to improve the geometry of landslide limits. The landslide inventory allows to identify the landslide event spatial extension as well as to characterize the landslide typology, landslide morphometric parameters and event magnitude.

The landslide susceptibility is assessed using the event-based landslide inventory and a set of geo-environmental landslide predisposing factors that are modelled with the statistical method of the Information Value. The obtained results are interpreted taking in account the combination of landslide predisposing factors, but also of the spatial distribution of the rainfall that triggered the landslide event.

 

Acknowledgements

This work is part of the project Riskcoast - Development of tools to prevent and manage geological risks on the coast linked to climate change (SOE3/P4/EO868, Interreg Sudoe). Sérgio C. Oliveira is funded by FCT - Portuguese Foundation for Science and Technology, I.P., through the project BeSafeSlide - Landslide Early Warning soft technology prototype to improve community resilience and adaptation to environmental change (PTDC/GES-AMB/30052/2017) and by the Research Unit UIDB/00295/2020, UIDP/00295/2020.