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).

Many low-elevation, coral reef-lined, tropical coasts are vulnerable to the effects of climate change, sea level rise, and wave-induced flooding. The considerable morphological diversity of these coasts and the variability of the hydrodynamic forcing that they are exposed to make predicting wave-induced flooding a challenge. A process-based wave-resolving hydrodynamic model (XBeach Non-Hydrostatic, “XBNH”) was used to create a large synthetic database for use in a “Bayesian Estimator for Wave Attack in Reef Environments” (BEWARE), relating incident hydrodynamics and coral reef geomorphology to coastal flooding hazards on reef-lined coasts. Building on previous work, BEWARE improves system understanding of reef hydrodynamics by examining the intrinsic reef and extrinsic forcing factors controlling runup and flooding on reef-lined coasts. The Bayesian estimator has high predictive skill for the XBNH model outputs that are flooding indicators, and was validated for a number of available field cases. It was found that, in order to accurately predict flooding hazards, water depth over the reef flat, incident wave conditions, and reef flat width are the most essential factors, whereas other factors such as beach slope and bed friction due to the presence or absence of corals are less important. BEWARE is a potentially powerful tool for use in early warning systems or risk assessment studies, and can be used to make projections about how wave-induced flooding on coral reef-lined coasts may change due to climate change.

The numerical model SWASH is used to investigate nonlinear energy transfers between waves for a diverse set of beach profiles and wave conditions, with a specific focus on infragravity waves. We use bispectral analysis to study the nonlinear triad interactions, and estimate energy transfers to determine energy flows within the spectra. The energy transfers are divided into four types of triad interactions, with triads including either one, two or three infragravity-frequency components, and triad interactions solely between sea-swell wave frequencies. The SWASH model is validated with a high-resolution laboratory data set on a gently sloping beach, which shows that SWASH is capable of modeling the detailed nonlinear interactions. From the simulations, we observe that especially the beach slope affects nonlinear infragravity-wave interactions. On a low-sloping beach, infragravity-wave energy dominates the water motion close to shore. Here infragravity-infragravity interactions dominate and generate higher harmonics that lead to the steepening of the infragravity wave and eventually breaking, causing large infragravity energy dissipation. On the contrary, on a steep-sloping beach, sea-swell wave energy dominates the water motion everywhere. Here infragravity frequencies interact with the spectral peak and spread energy to a wide range of higher frequencies, with relatively less infragravity energy dissipation. Although both beach types have different nonlinear interaction patterns during infragravity-wave dissipation, the amount of infragravity-wave reflection can be estimated by a single parameter, the normalized bed slope.

Two field data sets of near-bed velocity, pressure, and sediment concentration are analyzed to study the influence of infragravity waves on sand suspension and cross-shore transport. On the moderately sloping Sand Motor beach (≈1:35), the local ratio of infragravity wave height to sea-swell wave height is relatively small (H_IG/H_SW0. When the largest sea-swell waves are present during negative infragravity velocities (bound wave, negative correlation r₀), most sand is suspended here, and the infragravity sand flux q_IG is offshore. When r₀ is positive, the largest sea-swell waves are present during positive infragravity velocities (free wave), and q_IG is onshore directed. For both cases, the infragravity contribution to the total sand flux is, however, relatively small (IG/H_SW>0.4), most sand is suspended during negative infragravity velocities, and q_IG is offshore directed. The infragravity contribution to the total sand flux is considerably larger and reaches up to ≈60% during energetic conditions. On the whole, H_IG/H_SW is a good indicator for the infragravity-related sand suspension mechanism and the resulting infragravity sand transport direction and relative importance.