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

Abstract

Coastal cliff erosion is caused by a combination of marine forcing and sub‐aerial processes, but linking cliff erosion to the environmental drivers remains challenging. One key component of these drivers is energy transfer from wave–cliff interaction. The aim of this study is to directly observe cliff ground motion in response to wave impacts at an individual wave scale. Measurements are described from two coastal cliff sites: a 45‐minute pilot study in southern California, USA and a 30‐day deployment in Taranaki, New Zealand. Seismometers, pressure sensors and video are used to compare cliff‐top ground motions with water depth, significant wave height (Hs) and wave impact types to examine cliff ground motion response. Analyses of the dataset demonstrate that individual impact events can be discriminated as discrete events in the seismic signal. Hourly mean ground motion increases with incident Hs, but the largest hourly peak ground motions occurred across a broad range of incident Hs (0.9–3.7 m), including during relatively calm conditions. Mean hourly metrics therefore smooth the short‐term dynamics of wave–cliff interaction; hence, to fully assess wave impact energy transfer to cliffs, it is important also to consider peak ground motion. Video analyses showed that the dominant control on peak ground motion magnitude was wave impact type rather than incident Hs. Wave–cliff impacts where breaking occurs directly onto the cliff face consistently produced greater ground motion compared to broken or unbroken wave impacts: breaking, broken and unbroken impacts averaged peak ground motion of 287, 59 and 38 μm s−1, respectively. The results illustrate a novel link between wave impact forcing and cliff ground motion response using individual wave field measurements, and highlight the influence of wave impact type on peak energy transfer to coastal cliffs. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

Multi-objective optimisation of a rock coast evolution model with cosmogenic <sup>10</sup>Be analysis for the quantification of long-term cliff retreat rates

“…In turn, delivery of wave energy is mediated by the configuration of the shore platform, beach width, and wave climate (Sunamura, 1992). Thus, the processes that effect the weathering, erosion, and transport of shore platform, intact cliff, failed cliff, and other beach material are an important part of the whole process of "cliff erosion" (Coombes, 2014;Hurst et al, 2016;Limber and Murray, 2011;Masteller et al, 2020;Naylor and Stephenson, 2010;Prémaillon et al, 2018;Thompson et al, 2019). These complex and varied processes make predicting long-term cliff erosion rates difficult.…”

Conceptual model of fracture‐limited sea cliff erosion: Erosion of the seaward tilted flyschs of Socoa, Basque Country, France

“…Incident waves exert landward pressure, but cliff-reflected waves also have the possibility to exert pressure seaward from under the slab, particularly with specific wave shape that leads to high-energy impacts (e.g. Thompson et al, 2019). With these two horizontal force vectors, joint set J1 resists erosion with its landward dipping surface.…”

Seismic signatures of individual wave impacts on a coastal cliff

“…These studies demonstrate that it is difficult to directly link cliff response to wave forcing, as wave impacts can be highly variable due to dynamic wave breaking processes in front of the cliff. Thompson et al (2019) have recently shown, using observations from California and Taranaki, that the break point of a wave, relative to the cliff face, is fundamental for understanding the amount of energy the wave transmits into bedrock. Small variations in wave height and water depth between waves can modify the breakpoint location, and even small changes in breakpoint location can significantly alter the type of impact that the cliff receives.…”