Understanding the role of wave impact in driving coastal rock cliff evolution

Abstract

The energy delivered to hard rock cliffs from the sea is acknowledged to be a key control on cliff erosion, via hydraulic and mechanical processes. Although direct wave attack amplified by shoaling, foreshore wave set-up and reflection is widely acknowledged to be the dominant driver of coastal cliff erosion, the energy delivered and its variation through time and in relation to cliff change have not yet been quantified. Previous studies have relied upon modeling the transformation of offshore wave-buoy data, which overlooks near-shore and foreshore modification of the wave climate. The study of microseismic ground motion of coastal cliffs potentially provides a valuable method to directly examine energy delivery to the coast. NERC GEF loan #879, and its extension, provided 10 broadband seismometers, which were installed in an array on the top of a coastal cliff, which has an existing extensive array of monitoring equipment including weather, 3D cliff change, laser-based water-surface measurement and a rock-face monitoring system. Using ground motion velocity as a proxy for energy delivery has enabled the interactions between the cliff change and environmental conditions to be explored. Data has allowed us to question existing models of cliff erosion, in particular to assumptions associated with using far-field data, or simplified models such as cliff inundation durations, as adequate ways of describing the changes observed.