The temporal and spatial scales of rocky coast geomorphology: a commentary

Kennedy, David M.; Coombes, Martin A.; Mottershead, D. N.

doi:10.1002/esp.4150

Cited by 21 publications

(8 citation statements)

References 14 publications

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“…L >10 3 m). Given the emerging need and capability to address this scale of investigation (Kennedy et al, 2017), we considered the effect of using a variable length scale, or here the area of coastal cliff monitored, to calculate β. In order to ensure that the monitored frequency density of large events was not influenced by a sampling bias associated with short-term monitoring of a stochastic process (an infrequent large event captured, or missed, by chance within a short time window), we considered the size of the longest rockfall axis across the three inventories (Figure 3a).…”

Section: Spatial Variation In Scaling Parametersmentioning

confidence: 99%

Emergent characteristics of rockfall inventories captured at a regional scale

Benjamin

Rosser

Brain

2020

Earth Surf Processes Landf

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High‐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

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Section: Spatial Variation In Scaling Parametersmentioning

confidence: 99%

Emergent characteristics of rockfall inventories captured at a regional scale

Benjamin

Rosser

Brain

2020

Earth Surf Processes Landf

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“…Before the 1990s, the vast majority of coastal geomorphology work focused on accumulation coasts, due to the presence of many economic challenges threatened by rapid regressive changes, or potentially accelerated by contemporary mean sea level rise (Trenhaile, 2000;Woodroffe, 2002;Kennedy et al, 2017). However, rocky and cliffy coasts are said to represent more than 75% of the world's coastline (Emery and Kuhn, 1982;Davis and Fitzgerald, 2003), and are now one of the few sources of sediment for beaches.…”

Section: Introductionmentioning

confidence: 99%

Sedimentary Coastal Cliffs of Normandy: Modalities and Quantification of Retreat

Costa

Maquaire

Letortu

et al. 2019

Journal of Coastal Research

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The Normandy cliffs studied in this paper correspond to the north-western termination of the Paris sedimentary basin. The latter is characterized by the existence of more or less high, undulating and sometimes faulted plateaus, explaining the lithostratigraphic diversity of the outcrops, and the variety of types of cliff falls and gravitational landslides encountered. These plateaus are carved into cliffs with much faster retreat due to the outcropping of sedimentary formations (from the Jurassic to the Upper Cretaceous) favorable to weathering. Spatial and temporal variations of Norman sedimentary cliff retreat rates over multi-temporal data are examined. Data are derived from historical maps and air photographs but also from recent lasergrammetric and photogrammetric monitoring. These latter measures are on specific sites monitored at high frequency and resolution. The diachronic analysis of all these data gives retreat rates in line with the international literature, 0.2-0.3 m/yr. The spatial variations of the cliff retreat rates, at the Normandy scale, can be explained by geological structure, especially at the cliff foot, but also by the influence of cliff collapses or anthropogenic obstacles that disrupt the longshore drift (rates can be multiplied by 2). The contribution of the recent lasergrammetric and photogrammetric techniques shows along the Norman cliffs (1) the spatial distribution of the retreat or evolution rates on the cliff face. For chalk cliff of Seine Maritime, the ablation rate evaluated by TLS (Terrestrial Laser Scan) on active cliffs over a period of 7 years corroborates that established by photo-interpretation (observed over nearly 50 years), i.e. around 36 cm/year for the cap d'Ailly and almost zero for the abandoned cliffs of Dieppe; scree movements (debris falls) represent 100% of the evolution of the abandoned cliff faces while they account for 2% of the total retreat of the active cliff of cap d'Ailly. About the rhythms, multi-temporal data shows that the temporalities evolution of cliff extends from 1 to 7 decades according to the lithology. The high resolution and frequency monitoring provide also some information about the factors responsible for triggering gravitational landslides (rockfall, slide, debris fall). For the cliff characterized by landslides (Villerville et Vaches Noires), which are under the dominating influence of the rainfall and the groundwater level evolution, the study proposed a regional warning, especially in cases in which the piezometer of the site exceed a depth of 11m (Groundwater Level) and the effective rain on a 4months-period is over 250 mm. In this respect, and for chalk and limestone cliffs, the monitoring is inconclusive, because the origin of evolutions is more multifactorial.

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“…Driven by marine and subaerial processes, the development of shore platforms is caused mainly by erosion mechanisms such as joint block removal occurring at the meso-scale (cm to m) (Trenhaile, 1972;Naylor and Stephenson, 2010;Stephenson and Naylor, 2011a, b) and granular disintegration at the micro-scale (mm to cm) (Stephenson and Kirk, 2000;Porter et al, 2010). When averaged over centennial to millennial scales intertidal shore platforms hundreds of metres wide can develop (Kennedy et al, 2017). The balance between marine and subaerial processes in driving this erosion is highly varied and site specific (Kennedy et al, 2011;Stephenson et al, 2013) with the relative importance of subaerial processes increasing with distance from the sea (Kanyaya and Trenhaile, 2005).…”

Section: Introductionmentioning

confidence: 99%

Hourly to daily‐scale microtopographic fluctuations of supratidal sandstone

Yuan

Kennedy

Stephenson

2018

Earth Surf Processes Landf

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A traversing micro‐erosion meter (TMEM) was used to measure short‐term microtopographic changes on a supratidal rock surface at Marengo, Australia. In order to describe the characteristics of rock surface behaviour at different temporal scales, the TMEM site was monitored at 2‐hourly, daily and multiday (3.5 days) periods. The rock surface was highly dynamic at 2‐hourly scale, repeatedly falling and rising by up to 0.644 mm. Two‐hourly surface change was also characterised by spatial heterogeneity, with contraction and expansion occurring concurrently at centimetre scale across the rock surface. Two‐hourly microtopographic change was linked to the microclimate with significant relationships between surface movement and relative humidity (R2 = 0.27) and air temperature (R2 = 0.24). Expansion was observed across the rock surface when there was a fluctuation of 18% in relative humidity, and when cloudy the rock surface remained active during the day. Temporal variability in surface movement was also observed over a 24 h period with three distinct periods observed: falling (06:00–12:00), rising (12:00–20:00) and stable (20:00–06:00). The daily surface change underwent no net microtopographic change (no loss of material). In contrast, an overall contraction was observed at multiday scale. Rock surface behaviour in coastal environments is therefore complex with many cycles of expansion and contraction acting concurrently albeit at different temporal scales from hours to multiple days. © 2018 John Wiley & Sons, Ltd.

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The temporal and spatial scales of rocky coast geomorphology: a commentary

Cited by 21 publications

References 14 publications

Emergent characteristics of rockfall inventories captured at a regional scale

Emergent characteristics of rockfall inventories captured at a regional scale

Sedimentary Coastal Cliffs of Normandy: Modalities and Quantification of Retreat

Hourly to daily‐scale microtopographic fluctuations of supratidal sandstone

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