Structure‐from‐Motion (SfM) software and unmanned aerial vehicles (UAVs) have been increasingly adopted in the geosciences. The current mainstream generation of 3D models is still expensive as it relies on UAVs, Differential Global Navigation Satellite System (DGNSS) ground control points and commercial software. This paper proposes an end‐to‐end reproducible SfM workflow with minimal legal, financial or field issues. The procedure avoids both UAVs and DGNSS and relies on open‐source software. Interlocked models of a rocky shore in Normandy, France were generated at different scales and point densities, being georeferenced using free spatial data. A spherical target was used for scaling and assessing the relative accuracy, which was better than 1 cm.

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Numerous studies have been devoted to recurved and hooked spits in micro-and meso-tidal environments, but none have really focused on the peculiar right-angle plan shape exhibited by some spit hooks. The morphodynamics of a complex multihooked spit in a macrotidal inlet in Normandy, France, have been investigated using aerial photographs and 12 topographic LiDAR surveys from 2000 to 2017, complemented by hydrodynamic data. Merville Spit is a 2.5 km-long hooked spit, composed of four 'nested' ridges each with a right-angle hook at its tip, that has developed in the vicinity of the sand-rich outlet of the Orne River. Monitoring of the two last ridges shows that ridge formation starts with the attachment of the tip of a swash bar to the fulcrum point of the previous ridge, and ends following its westward elongation and counter-clockwise rotation when its hook impinges on the Orne outlet.The main factor controlling ridge plan-view shape is swash bar dynamics in the vicinity of the spit. The offshore wave direction has no significant impact on ridge and hook behaviour due to the large number of swash bars on the ebb-tidal delta. Over the study period, between 5 and 10 swash bars massively assured sand supply to the upper foreshore, favouring the growth of existing ridges, but also enabling the building of a new ridge. The bars contribute to imparting hook right angles at the tips of the ridges by creating local wave refraction or by providing a sheltering effect. Strong tidal currents in the Orne channel act as a hydrodynamic barrier that stops hook elongation at the edge of the river's outlet. The similar behaviour of the two studied ridges has been synthesized into a new and original conceptual model of complex multi-hooked spit formation and evolution under the influence of swash bars.

Résumé :Les dépôts de sédiments marins pour une gestion à terre sont importants en MancheMer du Nord, ils avoisinent les 240 000 tonnes/année en moyenne sur 2009-2011 excepté les travaux réalisés en 2009 sur Port 2000 au Havre. De plus, les préoccupations environnementales tendent à réduire la quantité de sédiments pouvant être immergée, il est donc nécessaire de trouver de nouvelles filières permettant la valorisation des matériaux non immergeables, tout en garantissant qu'aucune contamination ne soit transmise au milieu récepteur. Considérant de tels volumes pour une valorisation matière, les mélanges doivent intégrer des pourcentages élevés de sédiments fins ce qui rend leur stabilisation difficile à moins d'augmenter le taux de liants. Il est possible de substituer pour partie ces liants par des coproduits industriels ayant des propriétés liantes comme la cendre volante papetière. On parlera de covalorisation de sédiments et de coproduits, technique qui est de loin plus économique et acceptable du point de vue environnemental. Des formulations ont été étudiées tout en limitant le dosage en liants classiques avec un sédiment marin. Pour tous les mélanges, une grande majorité de sédiments (≥ 80%) a été incorporée. Après avoir rappelé les différentes caractéristiques des constituants, les formulations réalisées sont présentées. La détermination des propriétés liantes des formulations (temps de prise, consistance) et de durcissement (résistance, retrait) est faite et les résultats analysés. Les premières conclusions montrent que les sédiments stabilisés ont une résistance à la compression relativement faible pour un matériau routier mais suffisante pour un matériau de remblai. Mots-clés :Sédiments de dragage, Cendre volante papetière, Stabilisation, Matériau.

Abstract. Nature based solutions are growing rapidly in order to mitigate in the near future the effects of climate change and rise of sea level on most anthropogenic coasts. In that frame, the CHERbourg bLOC (CHERLOC) project aims to study new coastal engineering solutions (overtopping, sediment transport) thanks to two new artificial units in two test sites (Normandy, France) considering biodiversity preservation but also societal acceptability. This study details an efficient method to monitor such coastal infrastructure using terrestrial Structure from Motion (SfM). In 2021, surveys were conducted to acquire pictures in April, May, June and November. A time series of 3D photogrammetric models was generated using open source SfM software. The first model was georeferenced using Ground Control Points (GCP) measured by Differential Global Navigation Satellite System (DGNSS) so that it could be used as a reference for the following point clouds using surrounding ripraps assumed to be non-mobile through the period of the study. The georeferencing Root Mean Square Error (RMSE) was found to be 1.8 cm for the April model whereas RMSEs of relative registrations of the following dates were found to be sub-centimetric. These results can be used to observe and measure blocks displacements as well as sand volumes evolution throughout the time series. The biggest displacement was found to be 23 cm between April and June. Sand topographic variation shows a continuous accumulation on selected cross-sections between April and November with an overall height accumulation of about 30 cm. Sand volumes measurements show consistent results with an added volume of 3.67 m3 on the previous areas.