High-resolution monitoring of complex coastal morphology changes: cross-efficiency of SfM and TLS-based survey (Vaches-Noires cliffs, Normandy, France)

High-resolution digital elevation models (HRDEMs) on coastal areas, particularly cliffs, are an important asset for analyzing and quantifying the processes affecting their morphology. The Vaches-Noires coastal cliffs, located at the north-western part of the Pays-d'Auge region (Normandy, France), are an ideal site for the study of subaerial and marine erosion processes. Indeed, the coastline evolves through the accumulation of deposits resulting from rotational landslides and/or mudflows at the base of the cliff which are then undermined by the sea. The 3D modeling of a representative sector of the cliffs appears a suitable solution to monitor and quantify this evolution using HRDEMs. This paper presents the whole of a monitoring protocol, with high spatial resolution and high temporal resolution, on an original location characterized by reliefs of Badlands in coastal environments. The first results of these investigations are described with an evaluation of two methods for the creation of 3D models: a heavy method by Terrestrial Laser Scanning (TLS) monitoring, and a light method using Structure from Motion (SfM) terrestrial photogrammetry. The small investment of time in the field and money is an important asset of the SfM method. The textured rendering quality of the SfM model makes it a powerful tool for geomorphological analysis. According to a recurrent monitoring protocol involving both measurements, it was possible to compare the accuracy of the SfM and TLS models. The results show a high accuracy of the SfM models with an overall average error of 0.05 m. The representations of the zones of accumulation and erosion as well as their spatial succession allow to investigate the dynamics of the involved processes with both methods. Preliminary volume calculation results are conclusive for TLS models.