Quantifying the controls and influence of tide and wave impacts on coastal rock cliff erosion

The influence of waves and tides on the development of coastal cliffs has long been recognised as an important contributor to long-term coastline evolution. However, the relationship between the assailing force of waves and the resistance afforded by foreshore and cliff material that governs the processes through which cliff change occurs remains inadequately quantified and poorly understood. This is further confounded by a limited appreciation of the interplay between the coastal landforms and the range of processes that control their evolution. To explore this, we compare microseismic ground movements resulting from wave impacts to the occurrence of rockfalls from a section of cliffs on the North Yorkshire, United Kingdom, coastline. The results indicate that critical tide levels exist at which waves, in combination with wind directions coinciding with the greatest fetch, generate notably higher levels of energy delivery to the cliff face and that these levels, in turn, correspond to increased levels of material detachment from both within and above the cliff toe. Foreshore microtopography is shown to have a significant influence on wave energy flux and impact timing at the cliff face. The link between relative sea level and geomorphological work done by wave action is both spatially heterogeneous and tightly constrained by foreshore topography, yet local scale topographic controls are rarely considered in scenarios of future coastal change. The timing of relative increases in rockfall activity is also shown to correlate with preceding seismic events, which may indicate a lag or threshold in the geomorphic response of the cliff. Finally, the article uses modelled increases in inundation to explore the influence of topography on the distribution of changes to the tidal regime under future sea-level rise scenarios. These data highlight the need for a greater understanding of cliff behaviour if, in the context of sea-level rise, future coastal evolution is to be predicted.