Michael Lim

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.

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 role of multiple glacier outburst floods in proglacial landscape evolution: The 2010 Eyjafjallajökull eruption, Iceland

The 2010 Eyjafjallajokull eruption in Iceland provided a unique opportunity to quantify the evolution of proglacial geomorphology during a series of volcanogenic jokulhlaups (glacial outburst floods) (>140 events). Time-lapse imagery and repeat terrestrial laser scans before and directly after the eruption show that the jokulhlaup of 14 April 2010 composed 61% of the 57 × 106 m3 total discharge of the combined events, and had the highest peak discharge for the two main flood events, but only deposited 18% of the total volume of sediment in front of Gigjokull glacier. The majority of sediments (67% of a total volume of 17.12 × 106 m3) were deposited by the 15 April 2010 jokulhlaup, and this was followed by extensive reworking by a series of smaller jokulhlaups over the following 29 days that deposited 15% of the total sediment. The geomorphological and sedimentary signatures of the two largest jokulhlaups associated with the onset of the eruption have either been reworked by later floods or are buried by later flood deposits. Consequently, the ice-proximal, posteruption landscape cannot be used to reconstruct the characteristics or magnitudes of either of the two largest jokulhlaups. The findings support a complex-response model in which peak discharge and the bulk of the sediment transported is decoupled by changing routing mechanisms and water:sediment ratios during the eruption.

Cryo-conditioned rocky coast systems: A case study from Wilczekodden, Svalbard

This paper presents the results of an investigation into the processes controlling development of a cryo-conditioned rock coast system in Hornsund, Svalbard. A suite of nested geomorphological and geophysical methods have been applied to characterise the functioning of rock cliffs and shore platforms influenced by lithological control and geomorphic processes driven by polar coast environments. Electrical resistivity tomography (ERT) surveys have been used to investigate permafrost control on rock coast dynamics and reveal the strong interaction with marine processes in High Arctic coastal settings. Schmidt hammer rock tests, demonstrated strong spatial control on the degree of rock weathering (rock strength) along High Arctic rock coasts. Elevation controlled geomorphic zones are identified and linked to distinct processes and mechanisms, transitioning from peak hardness values at the ice foot through the wave and storm dominated scour zones to the lowest values on the cliff tops, where the effects of periglacial weathering dominate. Observations of rock surface change using a traversing micro-erosion meter (TMEM) indicate that significant changes in erosion rates occur at the junction between the shore platform and the cliff toe, where rock erosion is facilitated by frequent wetting and drying and operation of nivation and sea ice processes (formation and melting of snow patches and icefoot complexes). The results are synthesised to propose a new conceptual model of High Arctic rock coast systems, with the aim of contributing towards a unifying concept of cold region landscape evolution and providing direction for future research regarding the state of polar rock coasts.

Characterisation of Recent Debris Flow Activity at the Rest and Be Thankful, Scotland

The Rest and be Thankful (A83) in Scotland has been subject to frequent landslide activity in recent years and the trunk road has gained a reputation as one of the most active landslide sites in the UK. An average of two road closures per annum has been recorded over the last five years. This paper compares the site with other locations in Scotland that are prone to debris flows and explores a range of geomorphological factors using high resolution Terrestrial Laser Scanning data. The site is found to be relatively active, although normalization for mean annual rainfall makes activity at the site comparable to the likes of the Drumochter Pass. Macro-scale slope morphology is found to correspond strongly with the spatial distribution of recent activity. Channelisation is considered to be a significant factor in the overall debris flow hazard by confining flow and enabling entrainment. This was demonstrated during two recent events that mobilized at high elevations and entrained significant volumes of material along long runout paths.