Tidal inlets connect the ocean to inner water bodies and are present worldwide. Shallow inlets display fast morphological changes, due to complex interactions between tides, waves, and shallow depths. Their closure is commonly observed under storm waves, but the underlying processes remain only partly understood. Here, we present new field evidence that infragravity waves contribute to the closure of shallow inlets. The analysis of new field data collected at a shallow inlet under storm waves reveals that infragravity waves up to 0.4 m high can propagate inside the lagoon during flood but are blocked by opposing currents during ebb. At the passage of an infragravity wave crest, currents peak over 2.5 m/s and increase instantaneous sand fluxes by 2 orders of magnitude. Large accumulations of sand at the lagoon entrance damp tidal propagation until full inlet closure. This mechanism provides a new explanation for the closure of shallow inlets observed worldwide.

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This study investigates the contribution of shortwave breaking to storm surges through a high-resolution hindcast of the sea state and storm surge associated with the extra-tropical storm Klaus. This storm made landfall in January 2009 in the Southern Bay of Biscay and produced the largest storm surges observed in this region over the last 20 years, with 1.70 m in the Arcachon Lagoon and 1.10 m in the Adour Estuary. A fully-coupled 3D modelling system, which uses a vortex force formalism to represent wavecurrent interactions, is applied with a spatial resolution down to 35 m in the surf zones in order to properly compute the wave-induced setup. Modelling results reveal that the wave setup contributes by up to 40 % and 23 % to the storm surge peak in the Adour Estuary and the Arcachon Lagoon respectively. Accounting for wave forces in the circulation model improves storm surge predictions by 50 to 60 %. This is explained by the dominant role played by wave forces in the momentum balance at the inlets under storm waves. Numerical experiments further reveal that the wave-induced setup can be tidally-modulated, although this phenomenon seems to be sitespecific. Finally, a sensitivity analysis highlights the importance of the model grid resolution in the surf zones to correctly resolve the wave setup along open-ocean coasts. Inside the lagoon, the storm surge and wave setup are

This study explores the spatial distribution and the driving mechanisms of the wave‐induced cross‐shore flow within the shoreface and surf zone of a dissipative beach. Unpublished results from a field campaign carried out in early 2021 under storm wave conditions are presented and compared with the predictions from a state‐of‐the‐art phase‐averaged three‐dimensional circulation modeling system based on the vortex force formalism. Under storm wave conditions, the cross‐shore flow is dominated by a strong seaward‐directed current in the lower part of the water column. The largest current velocities of this return current are located in the surf zone, where the dissipation by depth‐induced breaking is most intense, but offshore‐directed velocities up to 0.25 m/s are observed as far as 4 km from the shoreline (≃12 m‐depth). Numerical experiments further highlight the key control exerted by non‐conservative wave forces and wave‐enhanced mixing on the cross‐shore flow across a transition zone, where depth‐induced breaking, whitecapping, and bottom friction all significantly contribute to the wave energy dissipation. Under storm conditions, this transition zone extended almost 6 km offshore and the cross‐shore Lagrangian circulation shows a strong seaward‐directed jet in the lower part of the water column, whose intensity progressively decreases offshore. In contrast, the surf zone edge appears clearly delimited under fair weather conditions and the seaward‐directed current is weakened by a near bottom shoreward‐directed current associated with wave bottom streaming in the shoaling region, such that the clockwise Lagrangian overturning circulation is constrained by an additional anti‐clockwise overturning cell at the surf zone edge.