Abstract
AbstractHigh‐resolution rockfall inventories captured at a regional scale are scarce. This is partly owing to difficulties in measuring the range of possible rockfall volumes with sufficient accuracy and completeness, and at a scale exceeding the influence of localized controls. This paucity of data restricts our ability to abstract patterns of erosion, identify long‐term changes in behaviour and assess how rockfalls respond to changes in rock mass structural and environmental conditions. We have addressed this by developing a workflow that is tailored to monitoring rockfalls and the resulting cliff retreat continuously (in space), in three‐dimensional (3D) and over large spatial scales (>104 m). We tested our approach by analysing rockfall activity along 20.5 km of coastal cliffs in North Yorkshire (UK), in what we understand to be the first multi‐temporal detection of rockfalls at a regional scale. We show that rockfall magnitude–frequency relationships, which often underpin predictive models of erosion, are highly sensitive to the spatial extent of monitoring. Variations in rockfall shape with volume also imply a systemic shift in the underlying mechanisms of detachment with scale, leading us to question the validity of applying a single probabilistic model to the full range of rockfalls observed here. Finally, our data emphasize the importance of cliff retreat as an episodic process. Going forwards, there will a pressing need to understand and model the erosional response of such coastlines to rising global sea levels as well as projected changes to winds, tides, wave climates, precipitation and storm events. The methodologies and data presented here are fundamental to achieving this, marking a step‐change in our ability to understand the competing effects of different processes in determining the magnitude and frequency of rockfall activity and ultimately meaning that we are better placed to investigate relationships between process and form/erosion at critical, regional scales. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd
Highlights
Rockfalls are a frequent process initiated when rock blocks become detached from a rock mass under the influence of gravity (Selby, 1982)
The results presented here provide unique insight into rockfall dynamics and how they vary on a regional scale
Using high-resolution, multi-temporal airborne LiDAR data we have explored regional-scale variations in rockfall magnitude and frequency, to what extent these relations are sensitive to the spatial scale of monitoring, and the implications of our findings for modelling cliff evolution, both in coastal and non-coastal settings
Summary
Rockfalls are a frequent process initiated when rock blocks become detached from a rock mass under the influence of gravity (Selby, 1982). They exert a first-order control on the rate of rock wall retreat on mountain slopes and on rock cliffs (Moore et al, 2009). Their volumes typically range from ~10-2 to 102 m3, but in some cases they have been known to reach 105 m3 (for example, Wieczorek et al, 1998; Stock et al, 2012). In a 10-month dataset captured across an 8500m2 rock face, only 10 of ~1.8 × 105 rockfall events were >1.0m3 and none exceeded 10.0 m3 (Williams et al, 2018), whereas rockfall with volumes up to 2.5 × 103 m3 are known to have occurred at this site in previous years (Rosser et al, 2005a)
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