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Numerical and physical modelling of coastal cliff retreat processes between La Hève and Antifer capes, Normandy (NW France)

Abstract : The study area is located along the French eastern Channel coastline, in Upper Normandy, between the La Hève and Antifer capes. The sedimentary series dip North-Eastward by 0.7° and comprise clay layers (Kimmeridgian and Albian clays) located at varying vertical elevation along the cliff (about 90 m high). This coastal cliff section is undergoing slow gravitational deformation, assumed to take place along the clay layers. Using numerical and experimental models, we investigated the role of clay layers, acting as potential detachment layers, on the development of gravitational instabilities. In order to properly design our models, we characterized the rock mechanical parameters using triaxial shear tests on in-situ samples dated from Kimmeridgian to Cenomanian. We ran a series of numerical simulations using a finite-element code applied to 2-D cliff cross section having varying lithological and mechanical layering. Under the sole effect of gravity forces, the plastic strains are mainly localised along the two clay layers, and the predicted displacements evidence a seaward sliding of the cliff. We further investigated gravitational deformation in the cliff using experimental models subjected to high pore-fluid pressure using by injecting compressed air at the base of the model. The presence of fluid overpressure can trigger spontaneous dismemberment of a cliff that would otherwise remain stable. The region of the model located near the initial cliff spread seaward, which generated a set of normal faults that propagated landward and create a frontal toe bulge. We modelled a more complex initial geometry closer that observed in the field by simulating the northward progressive deepening of a potential décollement (the Gault clay). In the segment of the model where the potential décollement layer lay above or at the cliff's base, the absence of frontal buttress allowed for a large part of the cliff to slide seaward. The compressional toe of the deformed wedge progressively grew, thus acting as a buttress preventing further sliding. Removal of this toe by erosion lead to renewed gliding. Going northward, the vertical position of the potential décollement layer deepened, below the base of the cliff. There, the chalky shore platform acts as a buttress and the cliff deformation and seaward translation stops. Results lead us to hypothesize that cliff retreat would be faster where the potential décollement layer is located higher in the cliff along the southern segment. Therefore, the overall cliff trend would progressively rotate counterclockwise with ongoing deformation and erosion. This modelling works illustrates why the coastline trend changes drastically between the north (N60E) and the south segment (N25E). This progressive coastline re-orientation through time is confirmed by the fan-shape morphology of the bathymetry offshore the southern segment whereas the northern one presents a slight and regular slope.
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Jérôme Le Cossec, Anne Duperret, Bruno Vendeville, Said Taibi. Numerical and physical modelling of coastal cliff retreat processes between La Hève and Antifer capes, Normandy (NW France). Tectonophysics, Elsevier, 2012, 510, pp.104-123. ⟨10.1016/j.tecto.2011.06.021⟩. ⟨insu-00798021⟩

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