Exploring climate change induced geomorphological tipping points on soft-cliffed coasts

Coastal systems evolve through a wide variety of physical, ecological and human processes, operating over multiple timescales. One coastal type of interest is an unmanaged, soft-cliffed coast, where hydrodynamic, erosive and avalanching processes interact to create a dynamic and often rapidly receding coast. Anthropogenic sea level rise is expected to accelerate recession and cause cliff submergence, a transition in coastal typology, impacting local communities, habitats and infrastructure. 

In this presentation, we explore the long-term (centuries and longer) geomorphological behaviour of a soft-cliffed coast forced by relative sea level rise. We describe continuous erosive processes by a generalised set of time-averaged hydrodynamic and erosion governing equations, driving smooth deformation of coastal morphology. This description is general enough to encompass many existing hydrodynamic and erosion models, meaning that results derived in this work hold for a large family of model parameters and parametrisations. 

A key physical process on soft-cliffed coasts is collapsing of the cliff face. The timescale of collapsing is shorter than the time-averaged hydrodynamic and erosion timescales and can be treated as an instantaneous process. This jump in state means that the mathematical framework of non-smooth (or hybrid) dynamical systems must be used to explore the evolution of these coasts. 

We identify two geomorphological states toward which the system converges: a repeatedly collapsing receding cliff system, approached when sea level is static, and a transgressing rocky platform without a cliff, approached for high rates of sea level rise. Our analysis focuses on the transitions between these attracting states over anthropogenic sea level rise scenarios. We find that cliff submergence can be characterised as a “tipping point” behaviour, reframing changes in coastal type as potentially irreversible impacts of anthropogenic climate change. This is an underexplored geomorphological phenomenon and may help us interpret the history of the Earth’s coastal systems, as well as explore future scenarios. The description of time-averaged hydrodynamic and erosion processes is general, strengthening the statement that the tipping point behaviour discussed is a realistic phenomenon, rather than a mechanism only seen for specific model parametrisations.  

This work also impacts the modelling of human-coastal coupled systems, since some management decisions, e.g. beach nourishments and the erection of coastal defences may be treated as instantaneous processes, and the framework of non-smooth dynamical systems is one avenue towards understanding long-term system behaviour.