Coastal cliff‐top ground motions as proxies for environmental processes

Abstract

[1] 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.

Alongshore variability in wave energy transfer to coastal cliffs

“…Previous studies have demonstrated that high-frequency cliff shaking, with the frequency range varying slightly between study locations, produces higher correlation coefficients with modelled and measured cliff toe wave conditions than microseism (MS) and long-period (LP) frequencies that represent wave loading in the nearshore and offshore (Norman et al, 2013, Vann Jones et al, 2015Young et al, 2016). HT ground motion signals have also produced higher correlation coefficients with observed rockfall (Vann Jones et al, 2015).…”

“…Wave model. In the absence of monitored cliff toe wave conditions, a linear wave model based on that of Battjes and Stive (1985) (see Norman et al, 2013) was used to transform wave heights measured at the wave buoy. Wave heights were transformed by shoaling and energy dissipation via turbulence as they break in decreasing water depths (calculated from the tide gauge time-series for the monitoring period) across the nearshore and foreshore profile normal to the cliff seaward of each seismometer ( Fig.…”

“…Adams et al, 2005;Young et al, 2011Young et al, , 2012Young et al, , 2013Young et al, , 2016Dickson and Pentney, 2012;Norman et al, 2013;Earlie et al, 2015;Vann Jones et al, 2015), which typically include long-period ground motions (b0.05 Hz/N20 s) generated by infragravity waves on the foreshore/ beach (e.g. Young et al, 2011Young et al, , 2012; double frequency microseisms (0.1-0.2 Hz/5-10 s) generated by the superposition of waves either in deep water, or following reflection from the coast which have a period half of that of the ocean waves (e.g.…”

“…Hedlin and Orcutt, 1989;Friedrich et al, 1998), where the frequency band is determined by shallow water wave periods, typically 0.1-0.05 Hz/10-20 s, although this has been shown to vary with local wave periods (e.g. Norman et al, 2013;Young et al, 2013); high frequency cliff shaking (N1 Hz) is generated by wave impacts at the cliff toe (e.g. Vann Jones et al, 2015;Young et al, 2016) or foreshore edge (Dickson and Pentney, 2012) when tide heights allow.…”

“…Vann Jones et al, 2015;Young et al, 2016) or foreshore edge (Dickson and Pentney, 2012) when tide heights allow. High frequency cliff shaking generated by wave-cliff impacts, have been found to provide a valuable proxy measurement of wave energy transfer directly to the cliff (Norman et al, 2013;Young et al, 2016), with statistically-significant relationships observed with monitored cliff erosion (Vann Jones et al, 2015). To date, cliff-top ground motion studies have largely focussed on single seismometers or on shore-normal transects of multiple seismometers (Dickson and Pentney, 2012;Young et al, 2012Young et al, , 2013), yet given the sensitivity of data to nearshore wave conditions and cliff toe impacts, it is reasonable to assume that alongshore variations in wave impacts and loading will be reflected in commensurate variations in microseismic response.…”