Alongshore variability in wave energy transfer to coastal cliffs

Abstract

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. 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.

Storm-Induced Boulder Displacements: Inferences from Field Surveys and Hydrodynamic Equations

“…In fact, the effects of the local bathymetry may be larger than the effects of the regional-scale coastal morphology, which are in general assumed dominant. For example, Reference [49] found significant variations in wave energy transmission over short distances (about 100 m) due to the foreshore features, thus suggesting a similar scale of variation for the coastal recession rate.…”

Hard-Rock Coastal Modelling: Past Practice and Future Prospects in a Changing World

“…Most models have concentrated on the sectional shape of rock coasts rather than the plan form, and there has been little attempt to integrate changes occurring in the horizontal and vertical planes. The dominant feedbacks in the two planes are quite different [8]. Whereas the sectional shape is generally assumed to be controlled by the relationship between rates of wave attenuation and the gradient of the bottom, the plan shape is thought to reflect the effect of headland-bay morphology on wave refraction and the protection afforded by accumulating beach material in the bays.…”

“…The most effective wave erosional processes therefore operate at or near to the waterline which, according to most hydraulic models, also experiences the highest wave-generated pressures [1] (pp. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], [2] (pp. [29][30][31][32][33][34][35][36][37].…”

Wave impacts on coastal cliffs: Do bigger waves drive greater ground motion?

“…The amount of wave energy that reaches the nearshore depends on a range of factors, including wave climate (height and direction), nearshore bathymetric influences on wave refraction (Limber et al, 2014;Vann Jones et al, 2018) and local changes in water depth that influence wave breaking (Trenhaile, 1987;Kennedy, 2016). As well as the amount of energy that is available, how this energy is subsequently transferred to the cliff via wave impacts is also important.…”

“…Multiple studies show coastal ground motion is tidally modulated with elevated levels of high-frequency (>0.3 Hz) ground motion when the tidal stage allows wave-cliff interaction (e.g. Adams, 2005;Young et al, 2011;Earlie et al, 2015;Vann Jones et al, 2015, 2018. In contrast, low-frequency (0.01-0.1 Hz) ground flexing is generated through pressure fluctuations associated with wave loading of the foreshore from sea swell and infragravity waves (Adams, 2005;Young et al, 2011Young et al, , 2013Young et al, , 2016 and gravitational attraction (Agnew and Berger, 1978).…”