The Effect of Rock Collapse on Coastal Cliff Retreat along the Chalk Cliffs of Northern France

El Khattabi, J.; Carlier, E., and Louche, B., 2018. The effect of rock collapse on coastal cliff retreat along the chalk cliffs of northern France. Coastal retreat is a topic that concerns all countries located on the coastal edge. This is the case for the chalky cliffs located in the north of France. Here, the cliff base is characterized by chalk rock rich in clay with low mechanical characteristics. To understand the appearance of rock collapses causing cliff retreat, the methodology applied in this report is based on an analytical study and numerical modeling. The model developed with CESAR (i.e. finite element software dedicated to stability) shows deformation affecting gray and blue chalk with separations reaching 3 cm. Simulations carried out with UDEC (i.e. Universal Distinct Element Code) show that constraints are organized around joints when the blocks rearrange themselves, and the effects are mainly seen at the base of the cliff. This study demonstrates the appearance of irregular conjugate fractures within the marly levels of the blue chalk. This type of fracture appears as a result of pressure from the overlying layers and the influence of lateral stress induced by the “Grand Blanc Nez” formation. These fractures propagate towards the gray chalk and set the limits of different blocks. The displacement of these blocks leads to the obstruction of conduits, which increases the water pressure. The destabilization of the cliff is reflected in the appearance of release cracks, which are strongly influenced by variations in groundwater level. These findings were verified by numerical modeling. This example demonstrates that rock collapses are responsible for coastline retreat in this area. The combination of unpredictable natural phenomena associated with a concentrated spatial area creates a high-risk coastal area retreating at around 0.25 m/y.