Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “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 short-wave 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 flood 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).@en

Sous, D.; Tissier, M.; Bouchette, F.; Dodet, G., and Rey, V., 2020. Extreme wave events on barrier reefs: A driver for critical regime? In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 654-658. Coconut Creek (Florida), ISSN 0749-0208. This paper describes the hydrodynamic regimes reached over barrier reefs during extreme wave events. The study is based on a combined approach relying, on one hand, on an extensive in-situ experiment over the Ouano reef-lagoon system, New Caledonia and, on the other hand, on wave-resolving numerical modeling.@en

The present paper reports on a field experiment performed over a shallow, roughness-varying barrier reef at Maupiti island, French Polynesia. The spectral wave energy balance is examined, outside the breaking zone and accounting for non-linear transfers and mean current, to estimate the wave friction factor. This latter varies from 0.05 to 4, with dependence on the ratio between near-bed orbital amplitude and roughness height well predicted by an adjusted parameterization from Madsen (1995). The present results are discussed with respect to other field data recovered on coral and rocky grounds.@en

A complete annual cycle of the dynamics of fine-grained sediment supplied by the Omo and smaller rivers is simulated for Lake Turkana, one of the world’s large lakes, with the hydrodynamic, wave and sediment transport model Delft3D. The model is forced with river liquid and solid discharge and wind data in order to simulate cohesive sediment transport and resuspension. It simulates stratification due to salinity, wave generation and dissipation, and sediment advection and resuspension by waves and currents, with multiple cohesive sediment fractions. A comparison of the simulation results with remotely-sensed imagery and with available in-situ sediment deposition rates validates the model. By devising simulation scenarios in which certain processes were switched on or off, we investigated the contribution of waves, wind-induced surface and bottom currents, salinity-induced stratification and river jet, in resuspending and transporting fine sediments in the lake basin. With only the wind or river influence, most of the sediment deposition occurs in the first 10 km off the Omo River mouth and at a depth  30 m. This study sheds new light on sediment transport in Lake Turkana and in great lakes in general, favouring the view that wind-waves can be the main agent that transports sediment away from river mouths and to deeper areas, as opposed to river-plume or gravity-driven transport.@en

This paper reports on a combined experimental and numerical study dedicated to barrier reefs hydrodynamics. A network of pressure sensors and velocity profilers has been deployed for more than 2 months over the Ouano reef barrier, New Caledonia. The primary aim of the study is to assess the relevance of the classical depth-averaged momentum balance in such a complex and poorly documented environment. The combined analysis of experimental and numerical measurements reveals a specific hydrodynamic behavior contrasting with sandy beaches and fringing reefs. The cross-reef current induced by wave breaking over the barrier reef plays an important role in the momentum budget, in particular through friction processes. The hydrodynamic behavior over the barrier reef is thus characterized by the progressive transition from a nearly classical beach type behavior on the forereef, where the gradient of radiation stress is balanced by a barotropic pressure gradient associated to the wave setup, to an open-channel type regime, dominated by frictional head loss. The reef top wave setup shows a clear depth dependency mainly attributed to the forereef curvature. During extreme wave events, the measurements tend to indicate a transition toward a critical hydraulic regime above the reef top. The numerical simulations, involving a non-hydrostatic wave-resolving model coupled to a (Formula presented.) turbulence model, highlight the vertical structure of the flow. Over the reef flat, a classical log-layer profile is observed, in agreement with measurements, while above the forereef an anticlockwise circulation develops under the breaking zone.@en

This paper reports a combined observational and numerical study of wave transformation over barrier reefs. The field instrumentation, which consists in a cross-shore network of pressure sensors and one high resolution velocity profiler, has been deployed for more than two months over the Ouano reef barrier, New Caledonia. The combined analysis of observations and numerical simulations shows that the reef barrier acts as an efficient, but depth-dependent, filter for incoming wave energy. The data analysis reveals the presence of very low frequency standing wave patterns due to reflection either at the reef barrier inner boundary or at the lagoon shore. Another striking feature is the development of undular bore trains over the reef flat.@en