Coastal areas epitomize the notion of ‘at-risk’ territory in the context of climate change and sea level rise (SLR). Knowledge of the water level changes at the coast resulting from the mean sea level variability, tide, atmospheric surge and wave setup is critical for coastal flooding assessment. This study investigates how coastal water level can be altered by interactions between SLR, tides, storm surges, waves and flooding. The main mechanisms of interaction are identified, mainly by analyzing the shallow water equations. Based on a literature review, the orders of magnitude of these interactions are estimated in different environments. The investigated interactions exhibit a strong spatiotemporal variability. Depending on the type of environments (e.g., morphology, hydrometeorological context), they can reach several tens of centimeters (positive or negative). As a consequence, probabilistic projections of future coastal water levels and flooding should identify whether interaction processes are of leading order, and, where appropriate, projections should account for these interactions through modeling or statistical methods.

Unmanned Aerial Vehicles (UAVs) are being increasingly used to monitor topographic changes in coastal areas. Compared to Light Detection And Ranging (LiDAR) data or Terrestrial Laser Scanning data, this solution is low-cost and easy to use, while allowing the production of a Digital Surface Model (DSM) with a similar accuracy. Three campaigns were carried out within a three-month period at a lagoon-inlet system (Bonne-Anse Bay, La Palmyre, France), with a flying wing (eBee) combined with a digital camera. Ground Control Points (GCPs), surveyed by the Global Navigation Satellite System (GNSS) and post-processed by differential correction, allowed georeferencing DSMs. Using a photogrammetry process (Structure From Motion algorithm), DSMs and orthomosaics were produced. The DSM accuracy was assessed against the ellipsoidal height of a GNSS profile and Independent Control Points (ICPs) and the root mean square discrepancies were about 10 and 17 cm, respectively. Compared to traditional topographic surveys, this solution allows the accurate representation of bedforms with a wavelength of the order of 1 m and a height of 0.1 m. Finally, changes identified between both main campaigns revealed erosion/accretion areas and the progradation of a sandspit. These results open new perspectives to validate detailed morphological predictions or to parameterize bottom friction in coastal numerical models.

This study presents unpublished field observations of infragravity waves, collected at the dissipative beach of Saint‐Trojan (Oléron Island, France) during the storm Kurt (3 February 2017), characterized by incident short waves of significant heights reaching 9.5 m and peak periods reaching 22 s. Data analysis reveals the development of exceptionally large infragravity waves, with significant heights reaching 1.85 m close to shore. Field observations are complemented by numerical modeling with XBeach, which well reproduces the development of such infragravity waves. Model results reveal that infragravity waves were generated mainly through the bound wave mechanism, enhanced by the development of a phase lag with the shortwave energy envelope. Spectral analysis of the free surface elevation shows the generation of superharmonic and subharmonic infragravity waves, the latter dominating the free surface elevation variance close to shore. Modeling results suggest that subharmonic infragravity waves result, at least partly, from infragravity‐wave merging, promoted by the combination of free and bound infragravity waves propagating across a several kilometer‐wide surf zone. Due to the steeper slope of the upper part of the beach profile, observed and modeled reflection coefficients under moderate‐energy show a strong tidal modulation, with a weak reflection at low tide (R²<0.2) and a full reflection at high tide (R²∼1.0). Under storm waves, the observed reflection coefficients remain unusually high for a dissipative beach (R²∼0.5−1.0), which is explained by the development of subharmonic infragravity waves with frequencies around 0.005 Hz, too long to suffer a substantial dissipation.

This study investigates the relevance of infragravity (IG) waves at Albufeira Lagoon Inlet, a shallow wave‐dominated inlet located on the western coast of Portugal. A field experiment carried out in September 2010 revealed the occurrence of low‐frequency oscillations (i.e., 25–300 s) in water levels and current velocities. While these fluctuations were present over the ebb‐tidal delta along the whole tidal cycle, they only appeared between the beginning of the flood and up to 2 h after high tide inside the lagoon. The XBeach modeling system was applied to Albufeira Lagoon Inlet and reproduced the generation and propagation of IG waves and their blocking during the ebb. This behavior was explained by blocking due to opposing tidal currents reaching 2.5 m s⁻¹ in shallow water depths. Numerical results suggest that the breakpoint mechanism and the long bound wave shoaling mechanisms contributed significantly to the generation of IG waves in the inlet. IG waves induced fluctuations in flood currents inside the lagoon reaching temporarily 100% of their magnitude. The fact that these fluctuations occur mostly at flood and not at ebb could promote flood dominance in the lagoon. This hypothesis will have to be verified, namely under storm wave conditions.