Abstract
The alongshore distribution of wave energy is believed to be an important control on the spatial variability of coastal erosion. There is, however, a lack of field data quantifying the alongshore variability in wave energy on rock coasts, whereby the relative control of coastline geometry versus foreshore characteristics on wave energy delivery remains unclear. A number of studies have identified high-frequency cliff-top ground shaking to be generated by wave impacts at the cliff toe during high tides (HT). To capture the variability of wave-cliff impact energy along-coast, we installed an array of cliff-top seismometers along a 1 km stretch of coastline in North Yorkshire, UK. Our aim is to constrain how wave energy transfer to the cliff toe varies, and to examine the relative energy transfer around typical coastline features, including a bay and headlands. Whilst the greatest HT ground motion energy is recorded at a headland and the lowest at the centre of the bay (5% of that observed at the headland), we identify no systematic alongshore variation in the HT ground motion energy that can be related to coastline morphology. We also note considerable variation between features of similar form: the total HT ground motion energy at one headland is only 49% of the next headland 1 km alongshore. Between neighbouring sites within the bay, separated by only 100 m, we observe up to an order of magnitude difference in ground motion energy transfer. Our results demonstrate the importance of the foreshore in driving the variations in energy delivery that we observe. Local alterations in water depth and foreshore topography control the alongshore distribution of wave energy available to generate cliff HT ground motions. Importantly, this apparently local effect overrides the influence of macroscale coastal planform morphology, which has previously been assumed to be the dominant control. The results show that foreshore characteristics that hold influence over wave energy transfer vary significantly over short (~100 m) distances, and so we expect erosion controlled by wave impacts to vary over similar scales.
Highlights
The distribution of wave energy along coastlines is an important control on the spatial variability of erosion (e.g. Sallenger et al, 2002; Murray and Ashton, 2013)
Using high tides (HT) ground motion energy as a proxy for wave energy transfer to the cliff toe, we demonstrate that at this hard rock, low-sediment coast, the relative energy observed at different positions alongshore is determined more by foreshore characteristics than the macro-scale coastline planform morphology
Using an array of seven cliff-top seismometers placed at ca. 100 m intervals along a 1 km stretch of coastline, we quantified the alongshore distribution of relative wave energy transfer
Summary
The distribution of wave energy along coastlines is an important control on the spatial variability of erosion (e.g. Sallenger et al, 2002; Murray and Ashton, 2013). The focussing of wave energy and erosion around rock coastlines is controlled by prevailing wave and current directions (Carter et al, 1990), the availability and distribution of sediment (Sunamura, 1976; 1982; Limber and Murray, 2011), coastline planform and foreshore geometry (e.g. Klein and Menezes, 2001; Bowman et al, 2009; Hapke et al, 2009; Limber and Murray, 2011; Limber et al, 2014), coastal bathymetry (e.g. Trenhaile, 1987; Komar, 1997), and relative rock strength and the resistance to wave energy transfer (Sunamura, 1977; 1992). Studies that examine the role of coastline geometry in determining wave energy and erosion focussing are largely based on field monitoring on softer rock coasts where both sediments and beach material play a dominant role in controlling erosion (e.g. Klein and Menezes, 2001; Sallenger et al, 2002; Bowman et al, 2009), which is observed in numerical modelling studies of coastal change (e.g. Limber and Murray, 2011; Limber et al, 2014). Wave energy transfer to the coastline is determined by the degree to which the coastline projects seaward and is more exposed to incoming waves and greater energies (Carter et al., 1990; Limber et al, 2014)
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