Event-driven erosion of a glacial till cliff

Abstract

Soft-rock cliff coasts are eroded by various mechanisms. Marine erosion occurs during storm surges and is linked to increased water levels and wave heights while terrestrial erosion by mass movements is affected by precipitation. This study aims at improving the system understanding of soft-rock cliffs by quantifying observed erosion and the boundary conditions necessary for marine erosion and relating terrestrial erosion to certain prerequisites and weather conditions at a glacial till cliff at the German Baltic Sea coast. The changes at the cliff are quantified using digital elevation models obtained from about monthly drone surveys over a period of four years. Marine and terrestrial erosion both occur mostly in winter and set the mutual preconditions. For terrestrial erosion, precipitation is the main enabling factor, a clear quantification is however difficult. For marine erosion, a threshold based on water level and significant wave height is quantified. With a certain increase in water level, a single event is more likely to surpass that threshold than with the same increase in wave height. Moreover, the effects of the exceptionally severe storm Babet in October 2023 are quantified: It eroded more than all other storms during the study period combined and about as much as would be eroded within seven normal years. The findings exemplify a high vulnerability of soft-rock cliffs to sea-level rise and future storm events.