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Durham, United Kingdom

Lim M.,Institute of Hazard and Risk Research | Rosser N.J.,Institute of Hazard and Risk Research | Petley D.N.,Institute of Hazard and Risk Research | Keen M.,Boulby Mine
Journal of Coastal Research

The influence of waves and tides on the development of coastal cliffs has long been recognised as an important contributor to long-term coastline evolution. However, the relationship between the assailing force of waves and the resistance afforded by foreshore and cliff material that governs the processes through which cliff change occurs remains inadequately quantified and poorly understood. This is further confounded by a limited appreciation of the interplay between the coastal landforms and the range of processes that control their evolution. To explore this, we compare microseismic ground movements resulting from wave impacts to the occurrence of rockfalls from a section of cliffs on the North Yorkshire, United Kingdom, coastline. The results indicate that critical tide levels exist at which waves, in combination with wind directions coinciding with the greatest fetch, generate notably higher levels of energy delivery to the cliff face and that these levels, in turn, correspond to increased levels of material detachment from both within and above the cliff toe. Foreshore microtopography is shown to have a significant influence on wave energy flux and impact timing at the cliff face. The link between relative sea level and geomorphological work done by wave action is both spatially heterogeneous and tightly constrained by foreshore topography, yet local scale topographic controls are rarely considered in scenarios of future coastal change. The timing of relative increases in rockfall activity is also shown to correlate with preceding seismic events, which may indicate a lag or threshold in the geomorphic response of the cliff. Finally, the article uses modelled increases in inundation to explore the influence of topography on the distribution of changes to the tidal regime under future sea-level rise scenarios. These data highlight the need for a greater understanding of cliff behaviour if, in the context of sea-level rise, future coastal evolution is to be predicted. © Coastal Education & Research Foundation 2011. Source

Lim M.,Institute of Hazard and Risk Research | Rosser N.J.,Institute of Hazard and Risk Research | Allison R.J.,University of Sussex | Petley D.N.,Institute of Hazard and Risk Research

The processes of change on near-vertical coastal cliffs have been quantified using terrestrial photogrammetry and laser scanning. The approach allows discrete rockfall geometry to be measured, and source located in three dimensions. This paper presents the analysis of an inventory of over 100 000 discrete rockfalls, recorded from the Liassic coast at Staithes, North Yorkshire (UK), where a rock face area of over 16 000 m2 has been monitored over a 20 month period. The data obtained on three-dimensional scar geometry and source position give an insight into rockfall characteristics from a range of rock types, cliff heights and geometries. Multiple failure mechanisms such as overhang collapse, constant spalling, fragmentation and large scale, coherent rockfalls have been observed and related to rock-type controlled processes on the rock face. The spatially referenced rockfall scar data are used to assess the influence of environmental controls on variable rock mass properties, such as rock type, structure and cliff geometry. Analysis of rockfall magnitude-frequency reveals notable similarities between coastal rockfalls and inventories from non-coastal environments. The resolution of the monitoring data allows quantification of rockfalls down to volumes of 1.25 × 10- 4 m3 to be consistently sampled and measured. This complete magnitude-frequency relationship suggests that rather than evolving exclusively through isolated, sporadic losses, coastal cliff geomorphology reflects interconnected processes in which each rockfall is part of a continuum of change to the rock face. Further detailed assessment of the rock face reveals the control of the pre-failure morphology on subsequent failure patterns, for example, the quantity of rock protrusion from the cliff is positively correlated with subsequent failure volume. The continuum of activity and the controls on failure identified within these data suggest that the episodic behaviour of coastal cliffs previously assumed may have been overstated by coarser resolution monitoring data. The findings improve our understanding of the evolution of coastal cliffs and highlight areas for further research into both cliff processes and the character of rock slope failures in general. © 2009 Elsevier B.V. All rights reserved. Source

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