A decade of erosion monitoring of coastal chalk cliff faces at a seasonal frequency (2010−2021) using terrestrial lasergrammetry and photogrammetry (Varengeville-sur-Mer and Dieppe, Normandy, France): Technical evolution and main results

Abstract

Every four to five months between October 2010 and July 2021, cliff face topography was monitored with centimeter-scale precision at two sites in Normandy, each with a chalk cliff linear length of several tens to hundreds of meters. These sites (6.5 km apart as the crow flies) have relatively similar lithological and meteorological contexts (the latter is affected by seasonal variations of oceanic temperate climate) but are affected differently by marine agents: Varengeville-sur-Mer is an active cliff (influenced by both marine and subaerial factors), while Dieppe is an abandoned cliff (affected only by subaerial factors). Thanks to our seasonal monitoring using terrestrial lasergrammetry and then terrestrial photogrammetry, our main outcomes are: 1) the cliff face erosion rate (2010–2021) is approximately 50 times higher in the active cliff context than in the abandoned one (0.51 ± 0.02 m/year and 0.00 ± 0.01 m/year, respectively), reinforcing the importance of marine actions in the erosion of Norman chalk cliffs; 2) unsurprisingly, along the active cliff, erosion peaks generally occur in winter, with periods of stability preferentially in summer; 3) in Varengeville-sur-Mer, of the 103 gravity movements identified on the cliff face, rock falls accounted for 68 % of total erosion, compared with 32 % for debris falls, the latter value being higher than reported in the scientific literature (from 2 to 25 %); 4) nine cases of cliff foot departure generating a basal notch, which later triggered rock falls over the entire cliff face by instability propagation in Varengeville-sur-Mer, have been identified. This monitoring continues, providing a valuable database for understanding cliff erosion (rates, rhythms, and modalities).