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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). [ABSTRACT FROM AUTHOR]

More than three decades ago, it was noted that the ocean infragravity bound wave increasingly lags behind the forcing short-wave groups when propagating towards the shore. To date, the most recent theoretical prediction of this so-called phase lag remained a first-order approximation in terms of depth variations. Here, a new semi-analytical solution is proposed which does not rely on this approximation. Strong agreement is obtained when the new solution is compared with high-resolution laboratory data involving both bichromatic and random wave conditions. This newly proposed theoretical phase lag is then extensively compared with the former one, highlighting an increasing discrepancy between the two solutions as the relative bottom slope increases. The four influencing parameters, namely the bottom slope, the water depth, the incident short-wave peak period and the incident group period, are shown to impact, each in a specific way, the bound wave phase lag. While the latter is seen to increase with lower water depths and/or with higher short-wave peak periods, both the bottom slope and the group period can affect the phase lag in a different manner. Indeed, steeper bed slopes induce lower phase lags in shallow water but higher ones in deep water, while higher group periods induce higher phase lags for gentle slopes but lower ones for steep slopes.

On the basis of a time-domain higher-order boundary element method, a two-dimensional fully nonlinear numerical wave flume is developed to investigate the nonlinear interactions between a regular wave and a submerged horizontal plate in the presence of uniform currents. A two-point method is used to discriminate bound (i.e., nonlinearly forced by and coupled to free waves) and free harmonic waves propagating upstream and downstream from the structure. The proposed model is verified against experimental and other numerical data for wave-current interaction without obstacles and nonlinear wave scattering by a submerged plate in the absence of currents. A first-order analysis shows that the reflection coefficient increases in the following current (i.e., current in the same direction as the incident wave) and decreases in the opposing current (i.e., current in the opposite direction to the incident wave). Moreover, the plate length for the maximum reflection to occur is not sensitive to the current. A second-order analysis indicates that downstream from the plate, the current has a stronger influence on the secondary free mode than on the first free mode. The energy transfer between the fundamental wave and the higher harmonics is intensified by a following current but weakened by an opposing current. The second free harmonic wave amplitude is affected more by the opposing current than it is by the following current. [ABSTRACT FROM AUTHOR]

The propagation of bound and free infragravity wave energy under short-wave breaking conditions induced by a longshore bar is investigated through field observations. Near-bottom pressure records deployed in a cross-shore array at S. Jacinto barred beach (Portugal) were analysed. In these observations, short-wave breaking induced by the longshore bar occurred mainly in intermediate water depths. The ratio between bound and total (bound plus free) infragravity wave energy was estimated with the methodology of Herbers et al. (1994). This ratio had a maximum value of 0.6 at the breaking onset, which is consistent with values presented elsewhere. A decrease of the bound infragravity wave energy was observed after short-wave breaking near the bar crest. This is in part due to nonlinear energy transfers from infragravity to short-wave frequencies as detected on the imaginary part of the bispectrum, in a weakly dispersive Boussinesq approximation. The other part is due to the process of release/transform bound into free infragravity wave energy that was assessed with a simplified energy balance equation. A conceptual model for the exchange of bound infragravity wave energy during short-wave breaking in intermediate water depths is proposed. This conceptual model involves both the kh and the ga/C 2 generalized nonlinearity parameter. • Field observations of bound infragravity wave energy during breaking and non-breaking short-wave conditions over a bar. • Nonlinear energy transfers from infragravity to short-wave frequencies increase with generalized nonlinearity parameter ga/C 2 . • A conceptual model for bound infragravity wave energy during short-wave breaking in intermediate water depths is proposed. [ABSTRACT FROM AUTHOR]