Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of windgenerated "short waves" (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the shortwave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1-10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the food phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's "hum" (background free oscillations of the solid earth).

Abstract. The English Channel is characterised by strong tidal currents and a wide tidal range, such that their influence on surges is expected to be non-negligible. In order to better assess storm surges in this zone, tide-surge interactions are investigated. A preliminary data analysis on hourly surges indicates some preferential times of occurrence of large storm surges at rising tide, especially in Dunkerque. To examine this further, a numerical modelling approach is chosen, based on the 2DH shallow-water model (MARS). The surges are computed both with and without tide interaction. For the two selected events (the November 2007 North Sea and March 2008 Atlantic storms), it appears that the instantaneous tide-surge interaction is seen to be non-negligible in the eastern half of the English Channel, reaching values of 74 cm (i.e. 50 % of the same event maximal storm surge) in the Dover Strait for the studied cases. This interaction decreases in westerly direction. In the risk-analysis community in France, extreme water levels have been determined assuming skew surges and tide as independent. The same hydrodynamic model is used to investigate this dependence in the English Channel. Simple computations are performed with the same meteorological forcing, while varying the tidal amplitude, and the skew surge differences D SS are analysed. Skew surges appear to be tide-dependent, with negligible values of D SS (<0.05 m) over a large portion of the English Channel, although reaching several tens of centimetres in some locations (e.g. the Isle of Wight and Dover Strait).

In this study, the Lagrangian residual circulation in the Iroise Sea is estimated by a numerical method where the trajectories of the particles released in any given velocity field are calculated by a diagnostic tool. From their knowledge, the residual Lagrangian currents are computed over a whole number of M2 tidal cycles. The Lagrangian residual circulation is mapped from sea surface currents measured by HF radars and from the surface currents computed with the Model for Applications at Regional Scales (MARS), a regional 3D ocean model forced, here, by the Weather Research and Forecasting (WRF) regional meteorological model. In order to overcome inconvenient space-and time-variations in radar coverage, the measured radar data are interpolated, extrapolated and filtered by Open-Boundary Modal Analysis (OMA). The estimated Lagrangian residual currents are compared with real drifts derived from subsurface and surface Lagrangian drifters released in the Iroise Sea in 2005 and 2007. The residual currents are analysed in the light of the physical processes (tides, atmospheric forcing and density-driven currents) known to govern long-term transport in the Iroise Sea. The similarities between drifter trajectories and the Lagrangian residual circulation inferred from either HF radar surface current measurements or modelled velocities confirm the interest of the methodological approach and make it a reasonable candidate for adaptation to the operational forecast of long-term transport.