Nicholas J. Rosser

Terrestrial laser scanning for monitoring the process of hard rock coastal cliff erosion

Hard rock cliffs represent approximately 75% of the worlds coastline. The rate and nature of the mechanisms that govern the retreat of these cliffs remain poorly constrained, primarily because conventional approaches employed to monitor these processes are generally inadequate for describing cliff erosion processes directly. These techniques are usually centred upon the interpretation of data collected periodically from aerial sensors, including stereographic aerial photographs and more recently air-borne LIDAR. These methods are generally not capable of assessing the pattern of erosion on the cliff face due to the oblique viewing angles, and hence tend to concentrate upon the resultant recession of the cliff top rather than change on the cliff face. Thus, processes of undercutting and small scale iterative failures of localized sections of the cliff face are generally not recorded. It is only when a failure affects the cliff top that any retreat is recorded. It is therefore unsurprising that cliff erosion is commonly deemed to be episodic. This paper presents a new approach to detailed cliff process monitoring using terrestrial laser scanning (TLS), which directly monitors changes on coastal cliff faces. The method allows the quantification of failures ranging in scale from the detachment of blocks of a few centimetres in dimension through to large rock, debris or soil, falls, slides and flows over 1000 m3. The collection of data is on-site and rapid and hence cost effective, providing a detailed description of the nature of coastal cliff erosion. This paper describes the methodological approach and demonstrates the range of results which can be generated, here shown for 16 months of monitoring data collected for a near-vertical cliff section on the coast of North Yorkshire, UK. The results demonstrate that terrestrial laser scanning can be used to quantify cliff failures to a previously unobtainable precision. The results reveal a strong spatial and temporal pattern of cliff collapse which contradicts commonly held perceptions of the nature of coastal cliff development.

Patterns of precursory rockfall prior to slope failure

In this paper we examine data generated using high-resolution three-dimensional laser scanning monitoring of coastal rock cliffs. These data are used to identify spatial and temporal patterns in rockfall activity behavior prior to slope failure. Analysis of the data suggests that given sufficient measurement precision precursory behavior, here manifest as the rate of rockfall activity prior to failure, can be detected, measured, and monitored. Environmental conditions appear to have a diminishing influence on the occurrence of increasingly large slope failures. The monitoring data implies a time-dependent sequence in the occurrence of smaller rockfalls in the period leading to the largest failures recorded. This behavior is attributed to the mechanisms of strain accumulation in the rock mass resulting from brittle failure of the slope. The implication is that combining these data with models of failure mechanisms may allow failure time to be forecast from wide-area monitoring of precursory behavior. These findings have implications for the management of potentially unstable slopes, the understanding of slope failure mechanisms, and the generation of a new type of slope failure warning systems.

Quantifying the Controls and Influence of Tide and Wave Impacts on Coastal Rock Cliff Erosion

Abstract 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.

Dynamic controls on erosion and deposition on debris-flow fans

Debris flows are among the most hazardous and unpredictable of surface processes in mountainous areas. This is partly because debris-flow erosion and deposition are poorly understood, resulting in major uncertainties in flow behavior, channel stability, and sequential effects of multiple flows. Here we apply terrestrial laser scanning and flow hydrograph analysis to quantify erosion and deposition in a series of debris flows at Illgraben, Switzerland. We identify flow depth as an important control on the pattern and magnitude of erosion, whereas deposition is governed more by the geometry of flow margins. The relationship between flow depth and erosion is visible both at the reach scale and at the scale of the entire fan. Maximum flow depth is a function of debris-flow front discharge and pre-flow channel cross-section geometry, and this dual control gives rise to complex interactions with implications for long-term channel stability, the use of fan stratigraphy for reconstruction of past debris-flow regimes, and the predictability of debris-flow hazards.

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.

Spatial distributions of earthquake-induced landslides and hillslope preconditioning in northwest South Island, New Zealand

Current models to explain regional-scale landslide events are not able to account for the possible effects of the legacy of previous earthquakes, which have triggered landslides in the past and are known to drive damage accumulation in brittle hillslope materials. This paper tests the hypothesis that spatial distributions of earthquake-induced landslides are determined by both the conditions at the time of the triggering earthquake (time-independent factors) and the legacy of past events (time-dependent factors). To explore this, we undertake an analysis of failures triggered by the 1929 Buller and 1968 Inangahua earthquakes, in the northwest South Island of New Zealand. The spatial extents of landslides triggered by these events were in part coincident. Spatial distributions of earthquake-triggered landslides are determined by a combination of earthquake and local characteristics, which influence the dynamic response of hillslopes. To identify the influence of a legacy from past events, we first use logistic regression to control for the effects of time-independent variables. Through this analysis we find that seismic ground motion, hillslope gradient, lithology, and the effects of topographic amplification caused by ridgeand slope-scale topography exhibit a consistent influence on the spatial distribution of landslides in both earthquakes. We then assess whether variability unexplained by these variables may be attributed to the legacy of past events. Our results suggest that hillslopes in regions that experienced strong ground motions in 1929 were more likely to fail in 1968 than would be expected on the basis of time-independent factors alone. This effect is consistent with our hypothesis that unfailed hillslopes in the 1929 earthquake were weakened by damage accumulated during this earthquake and its associated aftershock sequence, which influenced the behaviour of the landscape in the 1968 earthquake. While our results are tentative, they suggest that the damage legacy of large earthquakes may persist in parts of the landscape for much longer than observed subdecadal periods of post-seismic landslide activity and sediment evacuation. Consequently, a lack of knowledge of the damage state of hillslopes in a landscape potentially represents an important source of uncertainty when assessing landslide susceptibility. Constraining the damage history of hillslopes, through analysis of historical events, therefore provides a potential means of reducing this uncertainty. Published by Copernicus Publications on behalf of the European Geosciences Union. 502 R. N. Parker et al.: Earthquake-induced landslides and hillslope preconditioning

The integration of terrestrial laser scanning and numerical modelling in landslide investigations

Abstract Terrestrial laser scanning (TLS) can be used to either complement or replace traditional methods of characterizing both the geometry and structural geology of unstable slopes. TLS data collected from a failed bedrock slope threatening the main east–west highway in the Bhutan Himalaya are presented and interrogated for structural information. The structural data, along with TLS-derived slope geometry and cross-sectional profiles, are suitable for use within commercially available slope stability packages to derive solutions for the causes of instability, likely geometry of failure, and future activity under varied scenarios. The methods also allow the possibility of future model verification and calibration though TLS monitoring. The results of TLS-based numerical modelling utilizing a commercially available code are presented and the implications for slope surveying, numerical modelling, monitoring and management are discussed.

Molecular Characterization and Geological Microenvironment of a Microbial Community Inhabiting Weathered Receding Shale Cliffs

Shales play an important role in many earth system processes including coastal erosion, and they form the foundations of many engineering structures. The geobiology of the interior of pyrite-containing receding shale cliffs on the coast of northeast England was examined. The surface of the weathered shales was characterised by a thin layer of disordered authigenic iron oxyhydroxides and localised acicular, platy and aggregated gypsum, which was characterised by Raman spectroscopy, XAS and SEM. These chemical changes are likely to play an important role in causing rock weakening along fractures at the micron scale, which ultimately lead to coastal retreat at the larger scale. The surface of the shale hosts a novel, low-diversity microbial community. The bacterial community was dominated by Proteobacteria, with phylotypes closely associating with Methylocella and other members of the γ-subdivision. The second largest phylogenetic group corresponded to Nitrospira. The archaeal 16S rRNA phylotypes were dominated by a single group of sequences that matched phylotypes reported from South African gold mines and possessed ammonia monooxygenase (amoA) genes. Both the phylogenetic and the mineral data show that acidic microenvironments play an important role in shale weathering, but the shale has a higher microbial diversity than previously described pyritic acid mine drainage sites. The presence of a potentially biogeochemically active microbial population on the rock surface suggests that microorganisms may contribute to early events of shale degradation and coastal erosion.

Rapid sequestration of rock avalanche deposits within glaciers

Topographic development in mountainous landscapes is a complex interplay between tectonics, climate and denudation. Glaciers erode valleys to generate headwall relief, and hillslope processes control the height and retreat of the peaks. The magnitude–frequency of these landslides and their long-term ability to lower mountains above glaciers is poorly understood; however, small, frequent rockfalls are currently thought to dominate. The preservation of rarer, larger, landslide deposits is exceptionally short-lived, as they are rapidly reworked. The 2013 Mount Haast rock avalanche that failed from the slopes of Aoraki/Mount Cook, New Zealand, onto the glacier accumulation zone below was invisible to conventional remote sensing after just 3 months. Here we use sub-surface data to reveal the now-buried landslide deposit, and suggest that large landslides are the primary hillslope erosion mechanism here. These data show how past large landslides can be identified in accumulation zones, providing an untapped archive of erosive events in mountainous landscapes.

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.