Morphological Response of a Macrotidal Embayed Beach, Porsmilin, France

Abstract

International audienceMorphodynamics of sandy, macrotidal, embayed beaches is complex because of the numerous physical processes interacting at the same location over a wide range of temporal scales. As most of these processes are controlled by beach morphology, dynamic feedbacks are generally observed between hydro- and morphodynamics. Investigating short-term processes is essential in order to improve long term morphological prediction. A key question is to understand how beach slope reacts to forcing conditions, in particular the response time of the beach profile, how long the transient state lasts. This study deals with the spatial and temporal responses of beach morphology to varying incident conditions. Here we report main observations, preliminary results and on-going investigations on the DYNATREZ1 field campaign, which was conducted in the framework of the National Observation Service Dynalit. Beach profiles are shown to adapt rapidly to forcing conditions, within two days, with more intense variations observed in the high tide swash zone. The presence of infragravity waves and their dissipation on the beach is highlighted. It is likely that the very large variability in beach slope observed over a single neapspring cycle is responsible for the accordingly large variability in wave skewness, asymmetry and breaking processes, and thus in sediment fluxes and morphological changes

RTK GNSS-Assisted Terrestrial SfM Photogrammetry without GCP: Application to Coastal Morphodynamics Monitoring

“…Hydrodynamic processes and the morphodynamic response of the beach were evaluated during large-scale field campaigns in 2014 and 2016 [28,29]. Besides, the evolution of the western cliff was monitored using a multi-instrument and multi-parameter campaign during the winter of 2016/2017 [30].…”

A long-term dataset of topography and nearshore bathymetry at the macrotidal pocket beach of Porsmilin, France

“…Field data collected at Porsmilin have been used to investigate the morphodynamic response of macrotidal embayed beaches 31 , 35 , equilibrium modelling of the beach profile 36 , and the conditions controlling beach cusps development 37 . They also contributed to a regional assessment of the shoreline dynamics of the Brittany coast 38 , and erosion and recovery following extreme storm activity during the 2013–2014 winter at both local 39 and European scales 33 , 34 .…”

Diachronic UAV Photogrammetry of a Sandy Beach in Brittany (France) for a Long-Term Coastal Observatory

“…Inland and to the north, Porsmilin Beach is backed by a brackish-water marsh that no longer communicates with the sea. To the east and west, it is flanked by orthogneiss cliffs of about 15 m in height and bounded by headlands extending offshore into rocky reefs that only allow incident waves from the southwest and act as obstacles for longshore sand transport [14].…”

“…The intertidal zone is around 200 m wide in the longshore direction. The median sediment grain size (D50) is 320 µm [18], with significant cross-shore variability, including coarser sediments (D50 ≈ 700 µm) around the crest of intertidal bars and the existence of shingles on the upper beach which become uncovered during episodes of energetic hydrodynamic forcing [14].…”

“…A video imagery system has also been set up, continuously collecting one image per hour (and up to four images per second) since 2014. Intensive field campaigns of hydrodynamic measurements were also carried out in 2014 and 2016 [14,15]. The DYNALIT observatory is compatible with the on-going evolution of methodologies, as well as the diversity of observation approaches.…”