Generation Of Infragravity Waves Research Articles

Infragravity wave generation on shore platforms: Bound long wave versus breakpoint forcing

Shore platforms are ubiquitous morphological features along rocky coastlines and display a spectrum of forms from gently-sloping to sub-horizontal with a low tide cliff. They generally front eroding coastal cliffs and play an important natural coastal protection role by dissipating wave energy, especially during energetic wave conditions. Sea-swell wave energy dissipates during wave breaking, but the transfer of incident wave energy to lower frequencies, resulting in infragravity waves, can enable significant amounts of wave energy to persist up to the shoreline. This residual wave motion at the shoreline can carry out geomorphic work, for example by directly impacting the cliff face, but also for removing cliff-toe debris. There are two main mechanisms for generating infragravity wave motion – group bound long waves and breakpoint forcing – and it is not known which of these mechanisms operate on shore platforms. Here we show, using field data collected at a sloping platform in England and a sub-horizontal platform in New Zealand, and supported by numerical modelling, that the group bound long wave mechanism is most important on sloping platforms, whereas breakpoint forcing dominates on sub-horizontal platforms. Our results also suggest that the infragravity wave motion on the sloping platform is somewhat more energetic than that on the sub-horizontal platform, implying that the latter type of platform may provide better protection to coastal cliffs. However, site-specific factors, especially platform elevation with respect to tidal level and platform gradient, play a key role in wave transformation processes on shore platforms and more field data and modelling efforts are required to enhance our understanding of these processes, especially collected under extreme wave conditions (Hs > 5m).

Infragravity Wave Generation by Wind Gusts

AbstractThrough the analysis of propagation times of infragravity wave packets along ray paths, reanalysis data, and our field measurements in the East Mediterranean, we find evidence of deep water infragravity wave generation by offshore storms. We confirmed the results also using deep water pressure cell measurements in the Pacific. The known nearshore generation mechanism showed large discrepancies with the observed infragravity energy near Aogashima, Japan, during winter. A new model of deep water infragravity wave generation is developed, based on nonlinear interactions of wind wave triads with submesoscale wind oscillations. The observed underprediction of infragravity waves is resolved using this new gustiness‐based model. The new source term is found to be of importance during strong storms in the open ocean and underlines the importance of accounting for submesoscale wind oscillations in wind wave models.

Physical and Numerical Modeling of Infragravity Wave Generation and Transformation on Coral Reef Platforms

AbstractWave transformation across reef platforms strongly controls sediment transport processes and coral reef island morphodynamics with infragravity (IG) waves playing an important contributing role. A small‐scale (1:50) laboratory experiment and prototype numerical modeling are used to explore the characteristics of IG wave motion on coral reefs. The slope of the fore reef is the key factor controlling the mechanism of IG wave generation. Steep slopes (>1/10) are dominated by landward and seaward propagating breakpoint‐forced long waves, whereas incoming and then released bound long waves become increasingly important for slopes <1/20. The breakpoint‐forced long wave mechanism is the more effective generator of IG energy, and the most energetic IG motion (normalized by incident wave motion) is generated on reef platforms with a fore reef slope > 1/6. The water level relative to the reef platform hreef is also a key factor, and the largest IG waves are generated for a ratio between hreef and offshore significant wave height Hs,o of −0.25 to 0.75, that is, when most waves break across the reef slope and a fully saturated surf zone extends across the reef platform. An island on the reef platform substantially increases the contribution of IG waves to the total wave spectrum, but increased reef surface roughness reduces IG importance. Under the most optimal conditions, the IG wave height averaged across the platform is 20–30% of the incident offshore wave height. The geomorphic influence of IG waves is considered most significant for reef platforms with energetic waves breaking on the steepest fore reefs.

Peculiarities of Generation of Infragravity Waves

The peculiarities of the generation of infragravity sea waves with the periods from 30 s to 5 minutes are identified by the processing and analysis of experimental data obtained with the laser meter of hydrosphere pressure variations in the Vityaz’ Bay (the Sea of Japan).

Wind Waves and Cliff Shaking on Macrotidal Shore Platforms: A Case-study from North Yorkshire, U.K.

ABSTRACT Kennedy, D.M.; Vann Jones, E.C.; Dickson, M.E., and Rosser, N.J., 2018. Wind waves and cliff shaking on macrotidal shore platforms: A case-study from North Yorkshire, U.K. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 436–440. Coconut Creek (Florida), ISSN 0749-0208. Wind waves are a major erosive agent on the rocky coast. The physical impact of waves is also important in driving erosion above the splash zone. This occurs through the transferral of energy into the bedrock by the action of wave breaking causing microseismic shaking. In this study we concurrently measure wave energy and resultant microseismic shaking on a macrotidal shore platform at Staithes, North Yorkshire, U.K. It is found that infragravity wave generation is limited when compared to microtidal coasts and where it does occur it is confined to the wave breaking zone. Cliff shaking ...

New Evidence of Breakpoint Forced Long Waves: Laboratory, Numerical, and Field Observations

AbstractNumerical modeling and analysis of laboratory and field data are combined to show new evidence of breakpoint forced long waves. The laboratory data and numerical modeling show that the breakpoint can oscillate at frequencies not directly linked to the wave group envelope for specific bichromatic wave groups. The breakpoint therefore oscillates at frequencies where incident bound long waves are not present, enabling direct identification of breakpoint forced waves. Spectral analysis and cross‐shore wave propagation patterns indicate that the breakpoint forced waves occur at subharmonic wave groups frequencies, the repeat frequency of the short waves within the group, and at frequencies associated with triad interactions. The results indicate that in this case the long waves are generated by breakpoint forcing rather than by energy transfer during shoaling of the incident waves. The slowly varying shape of the short waves in the groups and wave merging at the breakpoint controls the modulation of the breakpoint in these cases. The spectral analysis is applied to two field data sets where the breakpoint oscillation, incident waves and shoreline oscillation were measured, and infragravity wave generation is again identified at discrete frequencies that are not present in the incident wave group envelope, with merging of waves at the breakpoint again evident. The magnitudes of the generated infragravity waves outside the surf zone are also dependent on the normalized surf zone width. Both sets of observations provide further evidence for the generation of infragravity waves by the breakpoint forcing mechanism in the field.

Consistent nonlinear stochastic evolution equations for deep to shallow water wave shoaling

Nonlinear interactions between sea waves and the sea bottom are a major mechanism for energy transfer between the different wave frequencies in the near-shore region. Nevertheless, it is difficult to account for this phenomenon in stochastic wave forecasting models due to its mathematical complexity, which mostly consists of computing either the bispectral evolution or non-local shoaling coefficients. In this work, quasi-two-dimensional stochastic energy evolution equations are derived for dispersive water waves up to quadratic nonlinearity. The bispectral evolution equations are formulated using stochastic closure. They are solved analytically and substituted into the energy evolution equations to construct a stochastic model with non-local shoaling coefficients, which includes nonlinear dissipative effects and slow time evolution. The nonlinear shoaling mechanism is investigated and shown to present two different behaviour types. The first consists of a rapidly oscillating behaviour transferring energy back and forth between wave harmonics in deep water. Owing to the contribution of bottom components for closing the class III Bragg resonance conditions, this behaviour includes mean energy transfer when waves reach intermediate water depths. The second behaviour relates to one-dimensional shoaling effects in shallow water depths. In contrast to the behaviour in intermediate water depths, it is shown that the nonlinear shoaling coefficients refrain from their oscillatory nature while presenting an exponential energy transfer. This is explained through the one-dimensional satisfaction of the Bragg resonance conditions by wave triads due to the non-dispersive propagation in this region even without depth changes. The energy evolution model is localized using a matching approach to account for both these behaviour types. The model is evaluated with respect to deterministic ensembles, field measurements and laboratory experiments while performing well in modelling monochromatic superharmonic self-interactions and infra-gravity wave generation from bichromatic waves and a realistic wave spectrum evolution. This lays physical and mathematical grounds for the validity of unexplained simplifications in former works and the capability to construct a formulation that consistently accounts for nonlinear energy transfers from deep to shallow water.

Infragravity waves and horizontal seafloor compliance

AbstractWe report the first consistent observation of horizontal seafloor compliance induced by infragravity (IG) waves. Long‐period IG ocean waves manifest themselves as broad, dominant features in ocean bottom pressure and vertical deformation spectra, but signals are rarely (if ever) identified on the horizontal components of traditional ocean bottom seismometers (OBS) due to low signal level and high current‐induced tilt noise at long periods. We examine two OBS stations with shallow‐buried seismometers: the Monterey Ocean Bottom Broadband site offshore California and the Ocean Seismic Network (OSN) pilot site OSN1B near Hawaii. We use nearby weather buoys to investigate the relationship between the presence of infragravity waves and environmental conditions. We find strong evidence that infragravity wave generation is primarily confined to the near‐coastal environment. Additional IG source information is found by examining the directionality of passing IG waves as a function of frequency, which we analyze using the coherence between pressure and the two horizontal components. Finally, we evaluate the implications for a joint vertical and horizontal compliance inversion.

The sources of deep ocean infragravity waves observed in the North Atlantic Ocean

Infragravity waves are long-period (25–250 s) ocean surface gravity waves generated in coastal zones through wave-wave interactions or oscillation of the breaking point. Most of the infragravity wave energy is trapped or dissipated near coastlines, but a small percentage escapes into the open oceans. The source of deep ocean infragravity waves is debated, specifically whether they come mostly from regions with strong source waves or from sites with particular morphologies/orientations. We correlate measurements of infragravity waves in the deep North Atlantic Ocean with infragravity wave generation parameters throughout the Atlantic Ocean to find the dominant sources of deep ocean infragravity wave energy in the North Atlantic Ocean. The deep ocean infragravity wave data are from a 5 year deployment of absolute pressure gauges west of the Azores islands (37°N, 35°W) and shorter data sets from seafloor tsunami gauges (DART buoys). Two main sources are identified: one off of the west coast of southern Europe and northern Africa (25°N–40°N) in northern hemisphere winter and the other off the west coast of equatorial Africa (the Gulf of Guinea) in southern hemisphere winter. These regions have relatively weak source waves and weak infragravity wave propagation paths to the main measurement site, indicating that that the site morphology/orientation dominates the creation of deep ocean infragravity waves. Both regions have also been identified as potential sources of global seismological noise, suggesting that the same mechanisms may be behind the generation of deep ocean infragravity waves and global seismological noise in the frequency band from 0.001 to 0.04 Hz.

Infragravity response to variable wave forcing in the nearshore

Analysis of field observations of sea‐surface elevation reveals the role of time‐varying incident forcing and bound wave release mechanisms on the generation of infragravity waves on a barred beach. Observations of infragravity response were obtained during sea breeze and swell periods with wave height varying from 0.2 to 1.2 m and wave period from 3 to 16 s. The observations show a stronger infragravity response to longer period incident swell than to short period wind‐sea. During the peak of the sea breeze, short period wind‐sea is conducive to long wave generation by breakpoint forcing. Under swell‐dominated conditions, the frequency and wave number of incoming bound waves associated with wave groups may satisfy the linear dispersion relationship, leading to resonant forcing of a free wave that appears to be released from the wave group. The transition between long wave generation due to bound wave release and time‐varying breakpoint forcing agrees with the surf beat similarity parameter.

Infra-gravity wave generation by the shoaling wave groups over beaches

A physical parameter, µb, which was used to meet the forcing of primary short waves to be off-resonant before wave breaking, has been considered as an applicable parameter in the infra-gravity wave generation. Since a series of modulating wave groups for different wave conditions are performed to proceed with the resonant mechanism of infra-gravity waves prior to wave breaking, the amplitude growth of incident bound long wave is assumed to be simply controlled by the normalized bed slope, βb. The results appear a large dependence of the growth rate, α, of incident bound long wave, separated by the three-array method, on the normalized bed slope, βb. High spatial resolution of wave records enables identification of the cross-correlation between squared short-wave envelopes and infra-gravity waves. The cross-shore structure of infra-gravity waves over beaches presents the mechanics of incident bound- and outgoing free long waves with the formation of free standing long waves in the nearshore region. The wave run-up and amplification of infra-gravity waves in the swash zone appear that the additional long waves generated by the breaking process would modify the cross-shore structure of free standing long waves. Finally, this paper would further discuss the contribution of long wave breaking and bottom friction to the energy dissipation of infra-gravity waves based on different slope conditions.

Infragravity wave source regions determined from ambient noise correlation

We use a backprojection method to determine dominant directions of ocean infragravity waves from 60–200 s period from cross correlations between 5 ocean bottom differential pressure gauges located off the coast of Sumatra. We observe infragravity waves arriving from all directions, but there is a dominant source direction that represents coherent propagation along the coast from the southeast or from the south. This study demonstrates the effectiveness of our projection technique, which may be applied to past and future seismic data to improve models of ocean infragravity wave generation and tsunami propagation.

NUMERICAL COMPUTATION OF INFRAGRAVITY WAVE DYNAMICS AND VELOCITY PROFILES USING A FULLY NONLINEAR BOUSSINESQ MODEL

We present experimental and numerical analysis of nonlinear processes responsible for generating infragravity waves
 in the nearshore. We provide new experimental data on random wave propagation and associated velocity profiles in
 the shoaling and surf zones of a very mild slope beach. We analyze low frequency wave generation mechanisms and
 dynamics along the beach and examine in detail the ability of the fully nonlinear Boussinesq- type model SERR1D
 (Cienfuegos et al., 2010) to reproduce the complex dynamics of high frequency wave propagation and energy transfer
 mechanisms that enhance infragravity wave generation in the laboratory.

Oblique sub- and super-harmonic Bragg resonance of surface waves by bottom ripples

We consider a class of higher order (quartet) Bragg resonance involving two incident wave components and a bottom ripple component (so called class III Bragg resonance). In this case, unlike class I/II Bragg resonance involving a single incident wave and one/two bottom ripple components, the frequency of the resonant wave, which can be reflected or transmitted, is a sum or difference of the incident wave frequencies. In addition to transferring energy across the spectrum leading to potentially significant spectral transformation, such resonances may generate long (infragravity) waves of special importance to coastal processes and engineering applications. Of particular interest here is the case where the incident waves are oblique to the bottom undulations (or to each other) which leads to new and unexpected wave configurations. We elucidate the general conditions for such resonances, offering a simple geometric construction for obtaining these. Perturbation analysis results are obtained for these resonances predicting the evolutions of the resonant and incident wave amplitudes. We investigate special cases using numerical simulations (applying a high-order spectral method) and compare the results to perturbation theory: infragravity wave generation by co- and counter-propagating incident waves normal to bottom undulations; longshore long waves generated by (bottom) oblique incident waves; and propagating–standing resonant waves due to (bottom) parallel incident waves. Finally, we consider a case of multiple resonance due to oblique incident waves on bottom ripples which leads to complex wave creation and transformations not easily tractable with perturbation theory. These new wave resonance mechanisms can be of potential importance on continental shelves and in littoral zones, contributing to wave spectral evolution and bottom processes such as sandbar formation.

Investigation of the generation of infra-gravity waves

Three different types of wave profiles including mono-chromatic, bi-chromatic and tri-chromatic waves, were generated on bottom slopes of 1/10, 1/20 and 1/40 in a super wave flume at the Tainan Hydraulics Laboratory (THL) to investigate the generation of infra-gravity waves. The wave steepness and bottom slopes were shown to determine the generation mechanism of infra-gravity waves, which are usually the principal dynamics in the swash zone. These results show that the varied infra-gravity waves would be locally forced by nonlinear wave groups, especially at the outer breakpoint. In order to understand the physical nature of infra-gravity waves, the characteristics of the energy transport of each chosen wave is examined by means of wavelet and auto-bispectral analysis. Finally, a detailed analysis of the hydrodynamics of the shoaling process is also discussed in this paper.

Linear modeling of infragravity waves during Delilah

The generation of infragravity waves by directionally spread shortwaves incident on an alongshore uniform beach is investigated. Two mechanisms responsible for the generation of infragravity waves are considered: the release of the bound infragravity waves associated with changes in the spatial variation of the incident shortwave energy and the forcing of trapped waves by obliquely incident directionally spread shortwaves. The infragravity wave response is examined using linear shallow water equations, taking into account the presence of bottom friction, setup of the mean water level, rollers, and longshore current. Summing all infragravity contributions due to difference interactions between pairs of shortwave spectral components with the same frequency difference results in infragravity energy density spectra. Calculated spectra are compared with measured spectra obtained during the Delilah field experiment for 11 consecutive days, including calm, mild, and storm conditions. In the comparison, attention is focused on the frequency distribution of the infragravity energy density throughout the surf zone and the transformation of the root mean square infragravity wave height. The measured infragravity response in the frequency range from 0.01 to 0.06 Hz is reproduced by the computations, with relatively broad spectra offshore and a clear nodal structure closer to shore. Typically 80% of the infragravity wave height variability is explained by the model of which 30% or less is due to bound infragravity waves. Differences occur mainly during storm events in which the infragravity energy is underpredicted in the outer surf zone and overpredicted in the inner surf zone.

Long–wave forcing by the breaking of random gravity waves on a beach

This paper presents new laboratory data on long-wave (surf-beat) forcing by the random breaking of shorter gravity water waves on a plane beach. The data include incident and outgoing wave amplitudes, together with shoreline oscillation amplitudes at long-wave frequencies, from which the correlation between forced long waves and short-wave groups is examined. A detailed analysis of the cross-shore structure of the long-wave motion is presented, and the observations are critically compared with existing theories for two-dimensional surf-beat generation. The surf beat shows a strong dependency on normalized surf-zone width, consistent with long-wave forcing by a time-varying breakpoint, with little evidence of the release and reflection of incident bound long waves for the random-wave simulations considered. The seaward-propagating long waves show a positive correlation with incident short-wave groups and are linearly dependent on short-wave amplitude. The phase relationship between the incident bound long waves and radiated free long waves is also consistent with breakpoint forcing. In combination with previous work, the present data suggest that the breakpoint variability may be the dominant forcing mechanism during conditions with steep incident short waves.