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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 sea-cliffs, cliff retreat under intermittent and discontinuous series of slope failures. Many studies have focused on frequency-size statistics using inventories of sea cliff retreat sizes but, only a few have addressed the statistics of their spatial structures (patterns), i.e. the manner the surface scars (associated with sea-cliff retreat) occur in space on the cliff face. These can be of great interest because they carry information about the external processes and the predisposing factors underlying the sea-cliff retreat. Over the last decade, the use of terrestrial laser scanning (TLS) has allowed great achievements to collate such inventories with high accuracy not only in terms of sizes, but also of spatial locations. This opens the way to formally treat sea-cliff instabilities in the framework of "spatial point process". We use an inventory of >8500 sea cliff failures collated from repeated TLS surveys (6 measurement epochs over 2.5y) along a coastal cliff in Normandy (France). We first show, through the analysis of residuals, that the spatial process underlying the occurrence of instabilities is inhomogeneous, i.e. the 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 characteristics (e.g., Ripley's K- function) and Monte-Carlo tests, we highlight several trends: 1. Small instabilities (volume<10-2 m3) aggregate over a radius of < 7.5-10m, and disperse above this distance; 2. Larger instabilities present a regular spatial pattern; 3. Assuming that two distinct processes generate small and large events, we show that they are not independent (for a given epoch); 4. Assuming a unique process, we show that the probability of occurrence of a given event, whether "small" or "large", is not influenced by its neighbourhoods; 5. The occurrence of small instabilities is not influenced by past events, but a temporal dependence may exist for larger instabilities.
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Dates et versions

hal-00844574 , version 1 (15-07-2013)

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  • HAL Id : hal-00844574 , 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. 8th Internationale Conference on Geomorphology - IAG 2013, Aug 2013, La Vilette, France. ⟨hal-00844574⟩
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