Retreat rates, modalities and agents responsible for erosion along the coastal chalk cliffs of Upper Normandy: The contribution of terrestrial laser scanning

• Two sites with similar lithology but different exposures to marine actions are studied.
• Cliff face changes over time and by location are examined using TLS.
• Marine actions are an effective agent of erosion.
• The proportion of debris fall production is about 25% in the active cliff retreat.
• A basal notch propagates instability towards the upper part of the cliff face.

In order to follow all the changes affecting the coastal chalk cliff face in Upper Normandy and improve knowledge about cliff erosion, repeated terrestrial laser scanning (TLS) surveys were carried out frequently between 2010 and 2013 (every 4–5 months). They were conducted at two sites with similar lithostratigraphic characteristics but different exposures to marine actions (the former being an abandoned cliff and the latter an active cliff). They provide a quantification of the production of debris with centimeter precision (from ± 0.01 to 0.04 m). These surveys provided three major outcomes: 1) cliff retreat rates were measured at high spatial resolution with retreat values, unsurprisingly, 3–4 times higher for an active cliff than for an abandoned cliff. This result highlights that marine actions should be seen as not only a transport agent but also a particularly effective erosion agent; 2) a significant proportion of debris fall production (about 25%) in the total active cliff retreat was identified; and 3) one of the modalities of retreat was visualized as the creation of a basal notch, which propagates instability towards the upper part of the cliff face. Later, this instability generates rock falls coming from the whole cliff face.