Coastal cliff-top ground motions as proxies for environmental processes
Emma C. Norman, Nick J. Rosser, Matthew J. Brain, David N. Petley and Michael Lim
Journal of Geophysical Research: Oceans, 2013. doi:10.1002/2013JC008963 v. 118, no. 12, p. 6807 – 6823
Abstract
A two-year dataset of coastal cliff microseismic ground motions is used to explore energy transfer to a cliff. The long-term dataset enables us to characterise cliff motion responses to a wide range of environmental processes and examine whether short-term characteristics are representative of the long-term. We examine whether cliff-top motions are reliable proxies for environmental processes to inform future investigations into the drivers of erosion. The study is based at an actively eroding, macrotidal, hard rock cliffed coast where considerable intra-annual variability in wave, tide and storm conditions permit the examination of a full range of environmental permutations. Three frequency bands of ground motion are identified that represent wind and wave processes that transfer energy to the cliff. Examining mean energy transfer by aggregating the frequency bands by sea water elevation reveals a notable departure from tidal inundation duration alone, of relevance to understanding the timing, duration and intensity of effective processes of erosion. Peak energy transfer to the cliff face occurs during the largest storms where water levels significantly exceed those of tidal inundation rather than at locations most frequently inundated by tides. We anticipate it is therefore these conditions that are likely to be most effective in eroding hard rock coasts, rather than periods which accrue energy transfer associated with still or calm waters, and hence tidally modulated inundation may not relate well to coastal erosion. Promisingly, despite signal overlap and noise, cliff-top motions can be used as proxies for the processes that transfer energy to the coast.
