Hazard of coastal chalk cliff erosion in Upper Normandy: the contribution of terrestrial laser scanning about the understanding of ablation rates, modalities and agents responsible for retreat

International audienceFor coastal cliffs, the erosion monitoring is traditionally performed by studying the rates of retreat of a spatial object tracking over time, often the cliff top (Bird, 2008). This classic approach to coastal geomorphology is used, among others, for risk management. In France, for Risk Prevention Plan (MATE-METL, 1997), the average retreat rates are extrapolated over 100 years to demarcate the zoning of the hazard. However, these average retreat rates are incomplete information because they do not reflect the brutal nature of the hazard. This one results in falls that threaten populations located at the cliff top and at the cliff foot. But the monitoring of these jerks, a priori without warning signs, requires a high spatial resolution. Despite the quality of the methods and techniques used in recent decades (orthophotography, photogrammetry, airborne LIDAR), pluridecimeter or plurimeter margins of error affect quantification. Moreover, the vertical point of view of the data hides some changes (Young et al, 2009), especially scree phenomena, which according to some authors represent at least 10 % of the retreat (May and Heeps, 1985; Hénaff et al., 2002) and may be early signs of instability. In order to follow all changes affecting the cliff face and improve the knowledge of cliff retreat, we have used terrestrial laser scanning instrumentation, in collaboration with the laboratory Domaines Océaniques. It has been deployed along the coastal chalk cliffs of Upper Normandy (France), affected by decimeter annual retreat. The diachronic monitoring was conducted between 2010 and 2013, at high frequency (every 4-5 months) in order to have surveys every season. This study conducted at two sites (the former in a context of active cliff and the latter in a context of abandoned cliff), with similar lithostratigraphic characteristics, provides a quantification of the production of debris with centimeter precision (± 0.01 to 0.04 m) (Letortu, 2013). These surveys allow: - To measure the ablation rates at high resolution with retreat values that, unsurprisingly, prove more important in a context of active cliff than in a context of abandoned cliff; - To identify the significant part of scree production (about 25 %) in the total cliff retreat; - To emphasize that marine actions are not only a transport agent but also a particularly effective agent of erosion; - To visualize one of the modalities of cliff retreat with the creation of a wave notch which propagates instability towards the upper part of the cliff face. This instability generates mass movements coming from the whole cliff face