Processes and rate of retreat of the clay and sandstone sea cliffs of the northern Boulonnais (France)

Retreat of the clay and sandstone cliffs of the northern Boulonnais (France) has been quantified using stereophotogrammetry. The low retreat rate of this coastal strip - 0.08 m/yr between 1939 and 2003 - is far less than that encountered on chalk and clay-chalk cliffs of either side of the Channel, and even less than a previous estimate of 0.17 m/yr regularly quoted in management studies. The retreat rate is closely related to shore platform morphology and dynamics. The shore platform presents 1) a steeply sloping ramp due to the accumulation of flat calcareous megaclasts that reduce marine erosion; 2) upstanding bare platform surfaces, related to tectonic deformation; and 3) thick platform-beaches trapped in troughs. In all three cases, the reflective behaviour of the nearshore protects the cliff foot from the incoming waves. Two critical eroding segments are the result of changes in the platform sedimentary budget. Around Cran Poulet, and between Plage de la Sirène and Pointe de la Courte Dune, the retreat rate is up to 0.25 and 0.15 m/yr, respectively. At Cran Poulet, recession has been facilitated by the extraction of pebble for more than half a century, whereas erosion of the beach at la Sirène is probably linked to severe erosion of the coastline in the adjacent Wissant Bay. Mass movements on the cliff face are essentially shallow-seated translational slides along with small debris falls and mudflows. The instability of the Argiles de Châtillon is greatly diminished by their sandy and silty texture and by the presence of interstratified solid shelly limestone beds that allow steep slopes to develop in rather weak material. The 'vertical erosion antecedent' is the erosional mode of the cliff, and its reduced efficiency explains the slow recession of the cliff. This study will help to determine the long term evolution of the Boulonnais coast.