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Analyzing the spatial structure of sea cliff instabilities through repeated high resolution terrestrial laser scanning surveys and point processes statistics

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Résumé

On rocky coasts, slope failures are spatially discontinuous and temporally intermittent. Many studies have addressed frequency-size statistics by means of collapse scar inventories, but only a few have addressed their spatial distribution. Yet, spatial patterns carry information about the external processes and the predisposing factors underlying sea-cliff retreat. Over the last decade, terrestrial laser scanning (TLS) has enabled high accuracy surveys of collapse sizes and locations, which opens the way for addressing sea-cliff instabilities within the theoretical statistical background of "spatial point process". We use an inventory of >8500 sea cliff failures collated from 6 repeated TLS surveys over 2.5yr along a coastal chalk cliff in Normandy (France). We first show, through the analysis of residuals, that the spatial process underlying the occurrence of instabilities is inhomogeneous, which means that the spatial density (number of events per unit area) spatially varies along the coastline and along the cliff height. Based on the computation of the inhomogeneous second-order spatial characteristics (e.g., Ripley's K- function) and Monte-Carlo tests, we highlight several trends: 1. Small instabilities (volume <10-2 m3) cluster in patches over a spatial radius of less than 5-10m, and disperse above this distance; 2. Larger instabilities present a regular spatial pattern whatever the distance between events; 3. Assuming that a unique process both generates small and large instabilities, we show that the probability of occurrence of a given event, whether "small" or "large", is not influenced by its neighbourhoods; 4. Conversely, assuming that two distinct processes generate small and large events, we show that they are not independent (for a given epoch); 5. The occurrence of small instabilities seems not to be influenced by past events, but a temporal dependence may exist for larger instabilities.
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Dates et versions

hal-00857378 , version 1 (03-09-2013)

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  • HAL Id : hal-00857378 , version 1

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Jeremy Rohmer, Thomas Dewez. Analyzing the spatial structure of sea cliff instabilities through repeated high resolution terrestrial laser scanning surveys and point processes statistics. International Conference on Geomorphology - IAG 2013, Aug 2013, La Villette, France. ⟨hal-00857378⟩

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