Matthew J. Brain

Coastline retreat via progressive failure of rocky coastal cliffs

Despite much research on the myriad processes that erode rocky coastal cliffs, accurately predicting the nature, location, and timing of coastline retreat remains challenging, and is confounded by the apparently episodic nature of cliff failure. The dominant drivers of coastal erosion, marine and subaerial forcing, are anticipated to increase in the future, so understanding their present and combined efficacy is fundamental to improving predictions of coastline retreat. We captured change using repeat laser scanning across 2.7 × 104 m2 of near-vertical rock cliffs on the UK North Sea coast over 7 yr to determine the controls on the rates, patterns, and mechanisms of erosion. For the first time we document that progressive upward propagation of failure dictates the mode and defines the rate at which marine erosion of the toe can accrue retreat of coastline above; this is a failure mechanism not conventionally considered in cliff stability models. Propagation of instability and failure operates at these sites at 10 yr time scales and is moderated by local rock mass strength and the time dependence of rock fracture. We suggest that once initiated, failure propagation can operate ostensibly independently to external environmental forcing, and so may not be tightly coupled to prevailing subaerial and oceanographic conditions. Our observations apply to coasts of both uniform and complex lithology, where failure geometry is defined by rock mass strength and structure, and not intact rock strength alone, and where retreat occurs via any mode other than full cliff collapse.

The effects of normal and shear stress wave phasing on coseismic landslide displacement.

Predictive models used to assess the magnitude of coseismic landslide strain accumulation in response to earthquake ground shaking typically consider slope-parallel ground accelerations only and ignore both the influence of coseismic slope-normal ground accelerations and the phase relationship between dynamic slope-normal and slope-parallel accelerations. We present results of a laboratory study designed to assess the significance of the phase offset between slope-normal and slope-parallel cyclic stresses on the generation of coseismic landslide displacements. Using a dynamic back-pressured shearbox that is capable of simulating variably phased slope-normal and slope-parallel dynamic loads, we subjected sediment samples to a range of dynamic loading scenarios indicative of earthquake-induced ground shaking. We detail the variations in strain accumulation observed when slope-normal and slope-parallel stresses occur independently and simultaneously, both in and out of phase, using a range of dynamic stress amplitudes. Our results show that the instantaneous phasing of dynamic stresses is critical in determining the amount of coseismic landslide displacement, which may vary by up to an order of magnitude based solely on wave-phasing effects. Instantaneous strain rate is an exponential function of the distance normal to the Mohr Coulomb failure envelope in plots of shear stress against normal effective stress. This distance is strongly controlled by the phase offset between dynamic normal and shear stresses. Our results demonstrate that conditions considered by conventional coseismic slope stability models can either overestimate or underestimate earthquake-induced landslide displacement by up to an order of magnitude. This has important implications for accurate assessment of coseismic landslide hazard.

Progressive Failure Cycles and Distributions of Earthquake-Triggered Landslides

Advances in the collection and analysis of landslide inventory data have allowed for greater understanding of spatial distributions of landslides triggered by earthquakes. However, current approaches to analysing and modelling these phenomena do not account for the response of the individual potential landslide masses and their temporally evolving stability. This stems, in part, from the lack of a conceptual model describing the effect of seismic waves on the strength and stability of hillslopes, which can be applied at the regional scale and over long (multiple earthquake) time scales. Here we present such a conceptual model linking weakening via progressive failure, inertial displacements driven by seismic ground accelerations, and the repeating failure of sections of hillslopes through time. We explore the implications of the model for how various characteristics of earthquake-triggered landslide distributions are interpreted and understood. These include the apparently stochastic nature of spatial landslide occurrence, spatial patterns of landsliding, landslide magnitude-frequency distributions, global variability in numbers of landslides triggered by earthquakes, and in particular why in any earthquake smaller areas of hillslope fail than do not, even in regions of apparently high landslide susceptibility. Finally, we also propose means of testing the validity of this model relative to alternative hypotheses.

Past, Present and Future Perspectives of Sediment Compaction as a Driver of Relative Sea Level and Coastal Change

Compaction describes a range of natural syn- and post-depositional processes that reduce the volume of sediments deposited in low-lying coastal areas, causing land-level lowering and a distortion of stratigraphic sequences. Compaction affects our reconstructions and understanding of historic sea levels, influences how relative sea level changes in the future and can act as a catalyst for rapid, widespread changes in coastal geomorphology. Rates of compaction-induced relative sea-level rise vary across space and through time in response to a range of natural and anthropogenically accelerated processes and conditions. This paper provides a summary of our understanding of the causes and effects of compaction, considering findings from key palaeoenvironmental and stratigraphic studies, sea-level reconstructions and recent observational data. It then considers the implications of these findings for our ability to project compaction-induced relative sea-level and associated coastal changes into the future.

The control of earthquake sequences on hillslope stability

Earthquakes trigger landslides in mountainous regions. Recent research suggests that the stability of hillslopes during and after a large earthquake is influenced by legacy effects of previous seismic activity. However, the mechanisms that control hillslope stability in response to ground shaking are poorly constrained in ductile hillslope materials, inhibiting our ability to fully explain the nature of earthquake-triggered landslides. We used geotechnical laboratory testing to simulate earthquake loading of hillslopes and to assess how different sequences of ground shaking influence hillslope stability prior to, during and following an earthquake ‘mainshock’. Ground-shaking events prior to a mainshock that do not result in high landslide strain accumulation can increase bulk density and interparticle friction. This strengthens a hillslope, reducing landslide displacement during subsequent seismicity. By implication, landscapes in different tectonic settings will likely demonstrate different short- and long-term responses to single earthquakes due to differences in the magnitude, frequency and sequencing of earthquakes.

What controls the Geometry of Rocky Coasts at the Local Scale

ABSTRACT Swirad Z.M.; Rosser, N.J.; Brain, M.J., and Vann Jones, E.C., 2016. What controls the geometry of rocky coasts at the local scale? In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 612–616. Coconut Creek (Florida), ISSN 0749-0208. 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.