Abstract
AbstractErosion of hard‐rock coastal cliffs is understood to be caused by a combination of both marine and sub‐aerial processes. Beach morphology, tidal elevation and significant wave heights, especially under extreme storm conditions, can lead to variability in wave energy flux to the cliff‐toe. Wave and water level measurements in the nearshore under energetic conditions are difficult to obtain and in situ observations are rare. Here we use monthly cliff‐face volume changes detected using terrestrial laser scanning alongside beach morphological changes and modelled nearshore hydrodynamics to examine how exposed cliffs respond to changes in extreme wave conditions and beach morphology. The measurements cover the North Atlantic storms of 2013 to 2014 and consider two exposed stretches of coastline (Porthleven and Godrevy, UK) with contrasting beach morphology fronting the cliffs; a flat dissipative sandy beach at Godrevy and a steep reflective gravel beach at Porthleven. Beach slope and the elevation of the beach–cliff junction were found to influence the frequency of cliff inundation and the power of wave–cliff impacts. Numerical modelling (XBeach‐G) showed that under highly energetic wave conditions, i.e. those that occurred in the North Atlantic during winter 2013–2014, with Hs = 5.5 m (dissipative site) and 8 m (reflective site), the combination of greater wave height and steeper beach at the reflective site led to amplified wave run‐up, subjecting these cliffs to waves over four times as powerful as those impacting the cliffs at the dissipative site (39 kWm‐1 compared with 9 kWm‐1). This study highlighted the sensitivity of cliff erosion to extreme wave conditions, where the majority (over 90% of the annual value) of cliff‐face erosion ensued during the winter. The significance of these short‐term erosion rates in the context of long‐term retreat illustrates the importance of incorporating short‐term beach and wave dynamics into geomorphological studies of coastal cliff change. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Highlights
Coastal cliff erosion is understood to be due to a complex combination of sub-aerial and marine processes weakening the structural integrity of the cliffs, leading to gradual erosion and episodic mass failure (Pethick, 1984; Trenhaile, 1987; Sunamura, 1992)
Modelling the erosion of cohesive clay cliffs related to shore platform development, Trenhaile (2005, 2009) has shown that nearshore shore platform profile gradient and beach sediment thickness controlled the extent of cliff erosion due to the proximity of breaking waves to the cliff face, where waves break close to the cliff on steeply sloping profiles and farther offshore on wider, more gently sloping profiles
Two key factors that are thought to contribute to the difference in cliff erosion rates between the two sites are the difference between the extreme winter wave conditions at each site and the morphology in modifying wave energy delivery to the cliffs
Summary
Coastal cliff erosion is understood to be due to a complex combination of sub-aerial and marine processes weakening the structural integrity of the cliffs, leading to gradual erosion and episodic mass failure (Pethick, 1984; Trenhaile, 1987; Sunamura, 1992). The variety of cliff profiles and cliff types around the world indicates that there are a wide range of processes involved in shaping cliffed coasts (Emery and Kuhn, 1982). These processes may include erosion due to wave-attack via abrasion, attrition, quarrying and hydraulic action, or physical and chemical weathering of cliff material as a result of rainfall, changes in temperature, biochemical and biophysical erosion (Trenhaile, 1987, 2005, 2016; Sunamura, 1992; Masselink and Hughes, 2003). In terms of cliff response to sea level rise, it is typically understood that a decrease in beach volume or elevation fronting a cliff will lead to an increase in cliff erosion (Walkden and Hall, 2005, 2011; Young and Ashford, 2006; Lee, 2008; Walkden and Dickson, 2008)
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