Estimation of infragravity waves at intermediate water depth (original) (raw)
Related papers
Infragravity-wave dynamics in a barred coastal region, a numerical study
Journal of Geophysical Research: Oceans, 2015
This paper presents a comprehensive numerical study into the infragravity-wave dynamics at a field site, characterised by a gently-sloping barred beach. The non-hydrostatic wave-flow model SWASH was used to simulate the local wave field for a range of wave conditions (including mild and storm conditions). The extensive spatial coverage of the model allowed us to analyse the infragravity-wave dynamics at spatial scales not often covered before. Overall, the model predicted a wave field that was representative of the natural conditions, supporting the model application to analyse the wave dynamics. The infragravity-wave field was typically dominated by leaky waves, except near the outer bar where bar-trapped edge waves were observed. Relative contributions of bar-trapped waves peaked during mild conditions, when they explained up to 50% of the infragravity variance. Near the outer bar, the infragravity wave growth was partly explained by nonlinear energy transfers from short-waves. This growth was strongest for mild conditions, and decreased for more energetic conditions when short-waves were breaking at the outer bar. Further shoreward, infragravity waves lost most of their energy, due to a combination of nonlinear transfers, bottom friction, and infragravitywave breaking. Nonlinear transfers were only effective near the inner bar, whereas near the shoreline (where losses were strongest) the dissipation was caused by the combined effect of bottom friction and breaking. This study demonstrated the model's potential to study wave dynamics at field scales not easily covered by in-situ observations. Second, infragravity waves can break and lose most of their energy in a region close to the shore [van Dongeren et al., 2007; de Bakker et al., 2014 de Bakker et al., , 2015. Third, infragravity waves can lose energy due to bottom friction, although this mechanism is mainly significant in the case of extensive shallow regions such as coral reefs Van Dongeren et al., 2013].
A nearshore long-term infragravity wave analysis for open harbours
Coastal Engineering, 2015
This study presents a comprehensive methodology for the reanalysis, characterization and propagation of wave bound infragravity waves (1/300 Hz to 1/30 Hz) in the nearshore. A deep water short wave reanalysis has been modified analytically to include the energy input associated with infragravity waves (full spectrum), through the decomposition and energy balance of the radiation stress of waves in deep water. A hybrid clustering-numerical wave propagation downscaling technique was used, forced by the modified full wave spectra previously obtained, resulting in hourly series of high spatial resolution nearshore ocean waves, including short and bound long waves. The methodology has been applied to the northern coast of Spain and validated by comparing buoy and pressure gauge records in the coast, for short wave (agitation) and infragravity wave (resonance) responses in open harbours, with very satisfactory results.
In situ observations of infragravity wave directionality at nearshore coastal sites
Ocean Science Discussions
Infragravity waves' is a term used to collectively describe surface gravity waves with periods arbitrarily between 30 and 300 s. In situ observations of infragravity waves at nearshore sites are scarce, and the directionality of the wave field has not received much attention in the past. This paper details a systematic directional analysis of experimental infragravity wave data. Through applying conventional and new directional analysis methods, qualitative and some quantitative characteristics of infragravity wave directions have been resolved. The analysis has found that infragravity waves have a bimodal directional structure with the dominant energy distributed in the propagation sector incident to the coast. It has also been demonstrated that mean infragravity wave directions can be derived, and there is evidence that the directional spreading of infragravity waves is correlated to their wind-generated wave counterparts. Using a numerical model, the qualitative findings were verified; however, contrary to the observations, the dominant direction of the modelled infragravity waves are in the propagation sector outward from the coast. The results provide improved insights into the directionality of infragravity waves, but the disparity between the dominant directions in the model and observations remains to be resolved.
Relative Magnitude of Infragravity Waves at Coastal Dikes with Shallow Foreshores: A Prediction Tool
Journal of Waterway, Port, Coastal, and Ocean Engineering
Despite the widely recognized role of infragravity (IG) waves in many often-hazardous nearshore processes, spectral wave models, which exclude IG-wave dynamics, are often used in the design and assessment of coastal dikes. Consequently, the safety of these structures in environments where IG waves dominate remains uncertain. Here, we combine physical and numerical modeling to: (1) assess the influence of various offshore, foreshore, and dike slope conditions on the dominance of IG waves over those at sea and swell (SS) frequencies; and (2) develop a predictive model for the relative magnitude of IG waves, defined as the ratio of the IG-toSS wave height at the dike toe. Findings show that higher, directionally narrow-banded incident waves; shallower water depths; milder foreshore slopes; reduced vegetated cover; and milder dike slopes promote IG-wave dominance. In addition, the empirical model derived, which captures the combined effect of the varied environmental parameters, allows practitioners to quickly estimate the significance of IG waves at the coast, and may also be combined with spectral wave models to extend their applicability to areas where IG waves contribute significantly.
Infragravity wave generation on shore platforms : bound long wave versus breakpoint 1 forcing 2 3
2020
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 motiongroup bound long waves and breakpoint forcingand 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 subhorizontal 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 subhorizontal 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 > 5 m).
International Journal of coastal and offshore engineering, 2018
In this study, the evolution and dependency of infragravity waves (IGWs) on wind waves for breaking and nonbreaking conditions is separately investigated. The efficiency of two constant cutoff frequencies (0.125 and 0.14 Hz) is compared for wave data measured in the sandy beaches of Nowshahr at the Southern Caspian Sea. It is found that the frequency of 0.125 Hz results higher correlation coefficients between IGWs energy content and two wind wave groups. Two pair different correlation patterns between IGWs in one side and wind waves higher and lower than 0.125 Hz in another side were recognized for breaking and nonbreaking conditions. It can be concluded that the IGWs excitation is controlled by the frequency distribution of wind wave energy. According to 0.125 Hz as more successful option, the correlation of IGWs with swell waves is generally more significant than sea waves. In the nonbreaking wave condition, the IGWs are well correlated with sea waves, whereas no considerable correlation between IGWs and sea waves is found in the breaking condition. It is resulted that IGWs energy is approximately linearly proportional of both swell and sea waves in nonbreaking condition. In the high and moderate conditions of incident wave energy, the density of IGWs energy grows shoreward, while energy attenuation can be detected for IGWs in very low energy waves.
Infragravity wave generation on shore platforms: bound long wave versus breakpoint forcing
Geomorphology
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 (H s > 5 m).
Modelling infragravity motions on a rip-channel beach
Coastal Engineering, 2006
A non-linear shallow water wave model operating on the timescale of wave groups is compared with measurements of infragravity motions on a rip-channel beach to verify the model concepts and assess the model performance. The measurements were obtained during the RIP-current EXperiment (RIPEX) in concert with the Steep Beach Experiment (SBE) performed at Sand City, Monterey Bay, CA, during the spring of 2001. The nearshore bathymetry was made up of shore-connected shoals incised by relatively narrow rip-channels spaced approximately 125 m apart. The comparison considers a 20-day period during which significant changes in both the offshore wave climate and nearshore bathymetry occurred. The temporal variation in infragravity conditions during the experiment is strong, with computational results typically explaining 70% to 80% of the observed infragravity motions within the nearshore. In contrast to the temporal variation, the alongshore spatial variation in infragravity intensity during the experiment is generally weak, even though the underlying bathymetry shows strong depth variations. Model computations suggest preferential coupling between the computed edge wave motions and the quasi-periodic bathymetry is present, a prerequisite for strong spatial variability. However, the infragravity field is dominated by cross-shore infragravity motions, which are only weakly coupled to the quasiperiodic bathymetry, resulting in a weak alongshore variability of the total infragravity motions.
Infragravity Wave Motions and Runup over Shallow Fringing Reefs
Journal of waterway, port, coastal, and ocean engineering, 2010
This paper presents the results of a combined laboratory and numerical investigation into the role of infragravity motions in the wave runup process over fringing coral reefs. Laboratory experiments were performed with a reef profile typical of fringing reef systems along the southeast coast of Guam. Spectral analysis of the measured time histories of surface elevation over the reef face and flats show significant changes to the wave energy spectrum shoreward of the break point. Most of the wave energy in the incident wave frequency band is dissipated within a few wavelengths of the reef face with the wave motions over the reef flat and shoreline dominated by oscillations at infragravity periods ͓O͑100s͒ prototype͔. The infragravity wave energy is minimum at the reef crest and increases as waves propagate shoreward over the reef flat and also with increasing water level on the reef. The dominant infragravity mode is the first reef oscillation mode with a wavelength approximately equal to four times the width of the reef flat. This component is resonantly amplified at the shoreline relative to the incident infragravity energy at the reef crest. A numerical model based on the Boussinesq equations is applied to the laboratory data and is able to describe complex changes to the wave spectrum over the reef flat due to nonlinear wave-wave interactions and wave breaking as well as runup at the shoreline.
Natural Hazards and Earth System Sciences, 2021
Abstract. Many coastlines around the world are protected by coastal dikes fronted by shallow foreshores (e.g. saltmarshes and mudflats) that attenuate storm waves and are expected to reduce the likelihood of waves overtopping the dikes behind them. However, most of the studies to-date that assessed their effectiveness have excluded the influence of infragravity (IG) waves, which often dominate in shallow water. Here, we propose a modular and adaptable framework to estimate the probability of coastal dike failure by overtopping waves (Pf). The influence of IG waves on wave overtopping is included using an empirical approach, which is first validated against observations made during two recent storms (2015 and 2017). The framework is then applied to compare the Pf of the dikes along the Dutch Wadden Sea coast, with and without the influence of IG waves. Findings show that including IG waves results in 1.1 to 1.6 times higher Pf values, suggesting that safety may be overestimated whe...