Alongshore variability in wave energy transfer to coastal cliffs

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

What controls the geometry of rocky coasts at the local scale?

There is a need to understand the controls on rocky coastal form in order to predict the likely response to climate changes and sea-level rise. Spatial variations in coastal geometry result from inheritance and contemporary processes, notably erosive wave intensity and rock resistance. We studied a 4.2 km long section of coastline (Staithes, North Yorkshire, UK) using LiDAR point cloud data and ortho-photographs. We represented the coast as a series of densely-spaced (25 m) and resampled (0.2 m) 2D cross-sections. GIS-based statistical analysis allowed us to identify relationships between coastal morphology, geology (lithology and rock structure) and wave intensity. We found the following statistically-significant relationships: 1) more intensive waves and weaker rocks are associated with steeper shore platforms, 2) higher platforms and cliff toes are associated with weaker and more variable rocks, and 3) surface roughness increases with greater wave intensity, decreased density of discontinuities and decreased variability of intact rock hardness. However, these relationships are weak, which suggests the potential role of coastal inheritance and/or the need to better represent rock resistance in coastal models

Forensic rockfall scar analysis: Development of a mechanically correct model of rockfall failure

The mechanical controls on small (< 10 m3), individual rockfall in jointed rock masses are not well constrained. We use forensic analysis of rockfall detachment surfaces (scars) which display fractured surfaces broken through intact rock, termed rock bridges as well as pre-existing discontinuities, to understand failure mechanisms. The relative significance of intact rock fracture versus release along pre-existing surfaces in stability has not been thoroughly investigated using field data. The relative role of each of these components determines where weakening, is important in controlling the nature and timing of rockfall. This is vital for defining mechanically accurate models of failure. An initial inventory of rockfall scars from coastal rock cliffs was captured using high-resolution gigapixel imaging and terrestrial laser scanning to determine these relationships. Fracture mapping, planar surface identification, and weathering classification were undertaken to identify similarities in the mechanical controls on failure. Preliminary analysis reveals that even small rockfall display a multi-stage failure history, whereby final failure occurs through fracture of a single unweathered rock-bridge. Intact rock breakage accounts for 22 ±12% of the full scar surface. The rock bridges are commonly clustered at the scar crest or base, while planar pre-existing joint surfaces dominate the scar center. This suggests that although cantilevered, most rockfalls in this inventory are more likely to fail through tension. We consider volumetric and lithologic controls on failure mode, and consider the wider potential of this approach

Quantifying the environmental controls on erosion of a hard rock cliff

Linking hard rock coastal cliff erosion to environmental drivers is challenging, with weak relationships commonly observed in comparisons of marine and subaerial conditions to the timing and character of erosion. The aim of this paper is to bring together datasets to explore how best to represent conditions at the coast and to test relationships with erosion, which on this coast is primarily achieved via rockfalls. On the N. Yorkshire coast in the UK we compare a continuously monitored microseismic dataset, regionally monitored coastal environmental conditions, modelled at-cliff conditions and periodic high-resolution 3D monitoring of changes to the cliff face over a 2-year period. Cliff-top microseismic ground motions are generated by a range of offshore, nearshore and at-cliff sources. We consider such ground motions as proxies for those conditions that promote the occurrence of rockfalls and erosion. Both these data and modelled at-cliff water levels provide improved insight into conditions at, and wave energy transfer to, the cliff. The variability in microseismic, modelled and regionally-monitored environmental data derives statistically significant relationships with increases in the occurrence of rockfalls. The results demonstrate a marine control on the total volume and size characteristics of rockfalls. The strongest relationships found are with rockfalls sourced from across the entire cliff, rather than just at the toe, indicating that the marine influence, albeit indirectly, extends above and beyond the area inundated. These results identify failure mechanisms driving erosion, where a range of processes unique to the coast trigger failure, but in a manner beyond purely wave action at the cliff toe. Greater erosion occurs at the cliff toe. However, comparing water level inundation frequency, microseismic energy transfer and erosion, we observe that heights up the cliff that correspond with water levels associated with low frequency, high energy storms, or more frequent inundation, do not experience increased erosion. Our results describe the relationship between inundation duration, energy transfer and erosion of hard rock cliffs, and illustrate the relative intensity of erosion response to variations in these conditions. Implicitly our data suggests that in future, cliffed rocky coasts may be relatively quick to respond to changes in environmental forcing