Mass movement hazard assessment of active coastal cliffs (Northeast Brazil)

Abstract

Cliffs are steep coastal features that are created by structural factors and erosion. Rainfall, gravity, the energetic action of waves, and mineral dissolution are the primary forces controlling these features. Inherent to their evolution and natural dynamics is the frequent occurrence of mass movements of varying magnitudes. The retreat of the top and base of cliffs is an active natural process. In addition, this is a unique landscape in which tourism and summer vacations occur throughout the year. Consequently, there is a risk to visitors and residents who often get too close to the slopes or sit at the base of cliffs, exposing themselves to natural hazards. This study aimed to assess the hazards faced by beach visitors with respect to the occurrence of mass movements. Two methodologies were combined: CoastSnap NE participatory monitoring and photogrammetry using unmanned aerial vehicles (UAV), which made it possible to identify and account for cliff movement events. The results indicated that between May 2021 and January 2022, there were 106 mass movements, including falling blocks, landslides, and debris flows, which moved up to 18 m from the slope. In addition, it was evident that the greatest number of events occurred during the rainy months in the region, highlighting a greater hazard during this time of the year. The results also made it possible to delineate hazard zones, indicating areas where beach visitors were more or less likely to be struck by rock displacements from cliffs. Therefore, it was concluded that the monitoring methodologies presented can be reproduced cartographically, helping to mitigate and prevent accident risk caused by mass movements and contributing to land-use planning strategies in coastal zones.