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Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 2 | Copyright
Nat. Hazards Earth Syst. Sci., 15, 349-362, 2015
https://doi.org/10.5194/nhess-15-349-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 27 Feb 2015

Research article | 27 Feb 2015

Analysing the spatial patterns of erosion scars using point process theory at the coastal chalk cliff of Mesnil-Val, Normandy, northern France

J. Rohmer and T. Dewez J. Rohmer and T. Dewez
  • BRGM, 3 av. C. Guillemin, B.P. 36009, 45060 Orléans Cédex 2, France

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.

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This article uses summary statistics of spatial point process theory to study the spatio-temporal pattern of a rockfall inventory recorded with repeated terrestrial laser scanning surveys at a chalk coastal cliff site in Normandy, France. This allows testing and quantifying the significance of geomorphological observations. From a spatial distribution perspective, behaviours of small and large scars cannot be considered equivalent, suggesting that erosion processes and triggering factors differ.
This article uses summary statistics of spatial point process theory to study the...
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