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International audienceBetween December 2013 and March 2014, a cluster of about 12 storm events hit the coast of Brittany with an exceptional frequency. It was in February that these storm events were the most frequent and particularly virulent. The significant wave heights measured off Finistère reached respectively 12.3 m and 12.4 m during Petra and Ulla storms on February 5 and 14. However, analysis of hydrodynamic conditions shows that only three episodes promoted extreme morphogenetic conditions because they were combined with high spring tide level. The first one occurred on January 1rst to 4, it was followed by events from February 1rst to 3, and March 2-3. During these three extreme events observed tide levels were above highest astronomical tide level (HAT). Maximum surge level (0.97m) was reached during Ulla storm of February 14-15. For comparison, we must go back in the winter of 1989-90 to find such extreme storm frequency. High frequency topomorphological measurements were achieved on more than ten coastal zones distributed around Brittany peninsula to assess the effects of these storms on shoreline erosion. They show that during the first phase (December-January), meeting it's climax from 1rst to 4th January 2014, shoreline erosion has been limited, with the exception of southern Brittany. This is due to the SW orientation of waves. For all monitoring sites, it has averaged -2.7m, the averaged minimum equal to 0.6m, and the averaged maximum at -6.20m. During the second phase from mid-January to mid-February, reaching it's climax on 1-2 of February storm corresponding to the most morphogenetic event of the winter, the average of shoreline retreat reached -4.2m, the averaged minimum reached approximately -1.5m, the averaged maximum -9.5m. It is essentially the Northern and Western coast of Brittany that experienced largest shoreline retreat due to W-NW storm wave orientation. During the third and last phase, running from mid-February to mid-March, and characterized by the March 2-3 extreme morphogenetic event, shoreline retreat was very low. It reaches -1m on average, for an averaged minimum of -0.6m and an averaged minimum of -1.9m. Considering the whole winter 2013-14 period, shoreline erosion for all monitoring sites reached -6.3m on average, with a minimum of about -0.2 m and a maximum of -30.1m. Depending on the type of environment, it appears that the dunes have retreated the most, followed by gravel or sandy barriers; the lowest erosion rates concern beaches backed by low cliffs cut in highly consistent materials such as periglacial deposits (head). Considering the three morphogenous episodes, the morphological response in terms of shoreline retreat of beaches and barriers was different. Storm occurring at the beginning of February induced the largest erosive rates partly explained by the large morphological sensitivity of beaches and barriers which were weakened by the previous storm events in the beginning of January. Conversely, the storm of March induced very few impacts. These elements show...

In the dual context of coastal hazard intensification and the growing number of stakes exposed to these hazards, coastal observatories are in demand to provide a structured framework dedicated to long-term monitoring. This article describes the drone-based photogrammetry monitoring performed since 2006 on Porsmilin Beach (Brittany, France) in the framework of the DYNALIT (Littoral and Coastline Dynamics) observatory, focusing on data quality and the consistency of long-term time series under the influence of multiple technological evolutions: Unmanned Aerial Vehicles (UAV) platforms with the arrival of electric multirotor drones, processing tools with the development of structure-from-motion (SfM) photogrammetry, and operational modes of survey. A study case is presented to show the potential of UAV monitoring to study storm impacts and beach resilience. The relevance of high-accuracy monitoring is also highlighted. With the current method, an accuracy of 3 cm can be achieved on the digital elevation model (DEM) and the orthophotograph. The question of the representativity and frequency of DEM time points is raised.

Long-term datasets documenting the evolution of coastal forms and processes, through the provision of recurring beach as well as shoreface morphological observations and accompanying time-series of environmental controls, remain difficult to collect and are rarely made available. However, they are increasingly needed to further our understanding of coastal change and to improve the models that will help planning what our future coast will be. This data descriptor presents the results of topographic and bathymetric surveys at Porsmilin, a macrotidal embayed beach situated in Brittany, northwest France. The Porsmilin beach survey program was launched in January 2003 by the Institut Universitaire Européen de la Mer (IUEM/Univ. Brest) and is continuing today in the framework of the French coastal observation service SNO-DYNALIT. The dataset contains over 16 years of monthly beach profile surveys and a large collection of repeated high-resolution subtidal and subaerial digital elevation models (DEMs). The dataset is accompanied by time-series of inshore waves and water levels, and enriched metadata, that will facilitate its future reuse in coastal research.

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International audienceThe georeferencing process is crucial to the accuracy of Terrestrial Laser Scanner data, in particular in the context of diachronic studies relying on multi-temporal surveys. The use of Ground Control Points in the georeferencing process can however be complex when confronted with the practical constraints of coastal surveying.A simple and quick alternative method called “pseudo-direct georeferencing” is proposed in the present paper. This method involves internal inclinometers to measure roll and pitch angles and a centimetric GPS to measure the position of the TLS center and the position of one backsight target. When assessing the transformational uncertainty by using a set of independent ground validation points for both classical indirect and proposed pseudo-direct methods, we respectively obtain Root Mean Square errors of 4.4 cm for the indirect method and 3.8 cm for the pseudo-direct metho

Intertidal DEM have a growing interest for ecological, geological and land development purposes. However, due to the characteristics of this environment, DEM are not easy to obtain and frequently not good enough in resolution to be interpreted. In this study, waterline method has been used to build a high-resolution DEM. This technique is basically based on the accurate localization of the boundary between water and sand. This limit being an iso-height line, multitemporal detection can be used to derive a DEM. Because of the distinct physical characteristics of both sand and water, this limit is clearly visible on thermal infrared images. We tested this method on a beach of Aber Benoit (France) by recording the shoreline movements during a whole tide rise with a thermal infrared camera with simultaneous check of the water height (DGPS on board a boat) to provide an absolute calibration. The reconstructed DEM, validated by an independent DGPS profile, has a mean resolution of 2 cm and accounts for details smaller than a centimeter.

In order to gain new insights into the agents and processes controlling hard rock coast erosion and retreat, a high frequency and multi-parameter monitoring field experiment was carried out from February until May 2017 in Porsmilin cliffs (Plougonvelin, Finistère, Brittany). This survey aimed to discriminate the relative contributions of subaerial (groundwater infiltration, watertable variations…) and marine (infragravity wave loading, wave impact...) agents and processes in the evolution of rock microfracturing. A better knowledge of their contributions will allow to improve the forecasting/ prevention of cliff failure.