INSIGHTS INTO CONTROLS ON CHALK CLIFF COAST FAILURES FROM COMBINED SEISMIC AND UAV MONITORING

Rocky cliff coasts are a unique environment for the study of mass wasting processes, as these settings are affected by both terrestrial and marine processes, either or both of which may drive failures. In order to gain insight into the processes of coastal chalk cliff failure, we monitored small landslides and rockfalls along a chalk cliff coast on the island of Rugen, on the Baltic coast of Germany. Our records span 25 months and combine continuous seismic monitoring and repeat UAV surveys. The UAV data were processed with a structure from motion workflow that we term co-alignment, which enables the creation of 3D models with high comparative accuracy, despite the difficulties of the field site and the lack of ground control. This enabled us to accurately detect failures and measure their locations and volumes. The seismic monitoring network allowed us to detect the seismic signals of failures as small as 4 m³, providing extremely precise timing for the events. Using this precise timing information, we explored the links between failures and a range of potential triggers, including precipitation, wind, temperature, and marine processes. We find that over our study period, marine processes were negligible as a trigger of cliff failure. Instead, failure of the chalk was controlled by moisture over several timescales. Precipitation had a direct impact on the cliff, while water in the shallow subsurface had an impact that varied seasonally due to plant uptake during the summer and over multiple years due to the occurrence of wetter and drier than average years.