Seasonal activity quantification of coast badlands by TLS monitoring over five years at the “Vaches Noires” cliffs (Normandy, France)

The study focuses on the evolution of the coastal badlands of the Vaches Noires cliffs (Normandy, France). The study aims to provide a better understanding of the functioning of these cliffs with badland morphology and to verify the potential of the TLS (Terrestrial Laser Scanner) tool to quantify the volumes moved and the rates of change. Thus, three related gullies (G2 to G4) with contrasting dynamics were monitored three to four times per year between September 26, 2014 and November 19, 2019. Each gully was divided into nine “Functional Morphological Units” (FMU) in order to identify the respective weight of erosion, transport and deposition of materials related to seasonal subaerial and marine forcing. A global assessment of the evolution over five years shows mainly an ablation in the upper part of the slopes, but also on the flanks and fronts of the crests. The middle and lower parts of the gullies alternate between erosion and deposition of materials while the basal scarp (foot of the cliff) is in progradation. By successively analysing the TLS models, seasonal patterns common to each gully are identified. We observe a pronounced hydro-gravitational dynamics in autumn/winter for all FMUs, a transition period in spring, then a period of relative stability in summer. Quantification of surface volumes transiting from upstream to downstream slopes shows that gully hydro-gravitational dynamics are unequal with periods of low, moderate and high activity between each gully. The volumes of materials produced during high or low activity (“High global activity” (Hga) and “Low global activity” (Lga)) are strongly correlated with the curve of cumulative rainfall.A conceptual model synthesizing the modalities of initiation of the hydro-gravitational processes in the different FMUs is proposed at the end of the article. For each season, the model provides ranges of quantified material volumes in each FMUs and specifies the role of meteorological configurations on the hydric state of marl-limestone slopes, put in perspective with other sites with badlands morphologies. Despite some limitations, the TLS tool remains relevant to monitor and spatialize the evolutionary processes of cliffs with complex topography and rapid morphological changes.