Abstract
Abstract. Over the last decade, many cliff erosion studies have focused on frequency-size statistics using inventories of sea cliff retreat sizes. By comparison, only a few paid attention to quantifying the spatial and temporal organisation of erosion scars over a cliff face. Yet, this spatial organisation carries essential information about the external processes and the environmental conditions that promote or initiate sea-cliff instabilities. In this article, we use summary statistics of spatial point process theory as a tool to examine the spatial and temporal pattern of a rockfall inventory recorded with repeated terrestrial laser scanning surveys at the chalk coastal cliff site of Mesnil-Val (Normandy, France). Results show that: (1) the spatial density of erosion scars is specifically conditioned alongshore by the distance to an engineered concrete groyne, with an exponential-like decreasing trend, and vertically focused both at wave breaker height and on strong lithological contrasts; (2) small erosion scars (10−3 to 10−2 m3) aggregate in clusters within a radius of 5 to 10 m, which suggests some sort of attraction or focused causative process, and disperse above this critical distance; (3) on the contrary, larger erosion scars (10−2 to 101 m3) tend to disperse above a radius of 1 to 5 m, possibly due to the spreading of successive failures across the cliff face; (4) large scars significantly occur albeit moderately, where previous large rockfalls have occurred during preceding winter; (5) this temporal trend is not apparent for small events. In conclusion, this study shows, with a worked example, how spatial point process summary statistics are a tool to test and quantify the significance of geomorphological observation organisation.
Highlights
Though shoreline is mostly rocky worldwide (80 % according to Emery and Kuhn, 1982), our understanding of the physical processes underlying its dynamics remains limited
Surveys based on repeated terrestrial laser scanner (TLS) have demonstrated their capabilities in building dense digital surface models (DSM) of cliff faces, based on which erosion scar inventories can be collated with high accuracy in terms of sizes, and of spatial locations over the cliff face
We advocated that exploratory tools of spatial point process theory could be used as a tool to investigate some questions related to the spatial distribution of erosion scars over a coastal cliff face
Summary
Though shoreline is mostly rocky worldwide (80 % according to Emery and Kuhn, 1982), our understanding of the physical processes underlying its dynamics remains limited. A possible approach to predict the occurrence of sea-cliff instabilities and the induced cliff retreat is to rely on statistical and probabilistic methods These examine frequencysize statistics using inventories of sea-cliff retreat sizes, such as scar surface or retreat volume associated with sea-cliff retreat (Dong and Guzzetti, 2005; Marques, 2008; Brunetti et al, 2009; Lim et al, 2010; Young et al, 2011; Dewez et al, 2013). Surveys based on repeated TLS have demonstrated their capabilities in building dense digital surface models (DSM) of cliff faces (among many others see Rosser et al, 2005, 2007; Lim et al, 2010; Dewez et al, 2007, 2013), based on which erosion scar inventories can be collated with high accuracy in terms of sizes, and of spatial locations over the cliff face
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