Infragravity Waves Research Articles

A Note on the Second-Order Contribution to Extreme Waves Generated During Hurricanes

High wind speeds generated during hurricanes result in the formation of extreme waves. Extreme waves by nature are steep meaning that linear wave theory alone is insufficient in understanding and predicting their occurrence. The complex, highly transient nature of the direction of wind and hence of waves generated during hurricanes affects this nonlinear behavior. Herein, we examine how this directionality can affect the second-order nonlinearity of extreme waves generated during hurricanes. This is achieved through both deterministic calculations and experiments based on the observations of Young (2006, “Directional Spectra of Hurricane Wind Waves,” J. Geophys. Res. Oceans, 111(C8), epub). Our calculations show that interactions between the tail and peak of the spectrum can become significant when they travel in different directions, resulting in second-order difference components that exist in the linear range of frequencies. These calculations are generally supported by experimental observations, but we note the difficulty of generating and focusing the high-frequency tail of the spectrum experimentally. Bound second-order difference components or subharmonics typically exist as low frequency infra-gravity waves. Components that exist in the linear range of frequencies may be missed by conventional methods of processing field data where low-pass filtering is used and hence overlooked. In this note, we show that in idealized directional spreading conditions representative of a hurricane, failing to account for second-order difference components may lead to underestimation of extreme wave height.

A novel machine learning algorithm for tracking remotely sensed waves in the surf zone

This paper describes a novel image processing technique that detects wave breaking and tracks waves in the surf zone using machine learning procedures. Using time-space images (timestacks), the algorithm detects white pixel intensity peaks generated by breaking waves, confirms these peaks as true wave breaking events by learning from the data's true colour representation, clusters individual waves, and obtains optimal wave paths. The method was developed and tested using data from four sandy Australian beaches under different incident wave and light conditions. Results are a representation of the position of the wave front through time, i.e., space-time data, which when shown overlaid on the original timestack shows the high degree of accuracy of the method developed here. The utility of the method is demonstrated in two ways: 1) through a comparison between the instantaneous wave speed calculated from the wave paths with the theoretical shallow water wave speed, and 2) by obtaining optical intensities that could be translated into wave roller lengths. The algorithm developed here has the potential to improve understanding numerous nearshore process such as bore propagation and capture in the surf zone, surf zone energy dissipation, surf beat and infragravity waves, and as a direct speed input for depth inversion methods.

Seismic Observation of Tsunami at Island Broadband Stations

AbstractPrevious studies have reported seismic observations of tsunami recorded at island broadband stations. Coastal loading by the tsunami can explain them. For further quantification, we model tsunami propagation assuming an axisymmetric structure: a conical island with a flat ocean floor. The total tsunami wavefield can be represented by superposition between an incident tsunami wave and the scattering. The ground deformation due to the total tsunami wavefield at the center is calculated using static Green's functions for elastic half‐space with a first‐order correction for bathymetry. By fitting the modeled displacement to observed seismic data, we can infer the incident tsunami wave, which can be interpreted as the virtual tsunami amplitude without the conical island. First, we apply this new method to three components of seismic data at a volcano island, Aogashima, for the 2015 Torishima‐Oki tsunami earthquake. The estimated tsunami amplitude from the vertical component is consistent with the offshore array observation of absolute pressure gauges close to the island (1.5–20 mHz). The estimated incident azimuth from the three components is also consistent with ray theory. Second, we apply this method to seismic data at four island broadband stations in the Indian ocean for the 2010 Mentawai tsunami earthquake in Indonesia. Despite the limited observed frequency range from 0.5–2.0 mHz, the amplitudes and incident azimuths are consistent with past studies. These observations can complement offshore tsunami observations. Moreover, this method is applicable not only for a tsunami but also for background ocean infragravity wave activity.

Study of irregular wave run-up over fringing reefs based on a shock-capturing Boussinesq model

This paper presents the results of a parametric study of irregular wave run-up over fringing reefs using the shock-capturing Boussinesq wave model Funwave-TVD to better understand the role of fringing reefs in the mitigation of wave-driven flooding. Laboratory experiments were newly performed with a typical fringing reef profile and typical hydrodynamic conditions to validate the model. Experimental data shows irregular wave run-ups are dominated by the low-frequency motions and confirms the run-up resonant phenomenon over the back-reef slope, which has been revealed in previous numerical studies. It is demonstrated that irregular wave evolution and run-up over fringing reefs are reasonably reproduced by the present model with a proper grid size. However, the infragravity run-up height and highest 2% run-up height over the back-reef slope are under-predicted due to the underestimation of the infragravity wave height over the reef flat. The validated model was then utilized to model irregular wave transformations and run-ups under different conditions. Through a series of numerical experiments, the effects of key hydrodynamic and reef geometry parameters, including the reef flat width, water depth over the reef flat, fore-reef slope angle and back-reef slope angle, on the irregular wave run-up were investigated. Variations of spectral components of irregular wave run-ups were examined to better understand the physical process underlying the effect of each parameter.

Propagation of bichromatic-bidirectional waves

ABSTRACT This study aims to investigate the transformations experienced by the mean water level and radiation stress tensor during the propagation of Bichromatic-Bidirectional (Bi-Bi) waves on a slope of 1:22 and water depth varying from 55 cm to 26 cm, simulating laboratory conditions. A computer program written in Python was used to compute those quantities at different combinations of wave angles and periods. The setup and setdown of the mean water level are strongly dependent on the combination of periods and direction of the primary waves, as they propagate along the slope, modifying the bound infragravity wave. Mohr’s circles for the radiation stress tensor showed significant changes of diameter and center at different points along the basin. The radiation stress components for the Bi-Bi waves are the sum of the stresses associated with each primary wave and a nonlinear term which results from the interference between primary waves. Disregarding these nonlinear terms may significantly affect the nearshore hydrodynamics prediction.

ESTIMATION OF BOUND AND RELEASED INFRAGRAVITY WAVES BASED ON WAVE OBSERVATION AND NUMERICAL SIMULATION IN SHALLOW WATER

It is mentioned that observed infragravity waves consist of bound waves propagating with short-wave groups, released waves due to reduction of short crest waves and free waves existing in a field. Though it is difficult to distinguish among them, a standard spectrum for infragravity waves is defined by using a relation to a wind wave spectrum. In this study, a comprehensive definition of standard spectrum is newly proposed to estimate infragravity wave heights with the relation between the ratio of wave height and the ursell number of observed wave property, represented by selected data of wave observation in shallow water. Moreover, the release process of bound waves at a harbor entrance is reproduced in numerical simulation using a Boussinesq model for short-wave transformation. These results are verified by comparison to infragravity waves observed at outside/inside of a harbor for a month.

UNDULAR BORE DEVELOPMENT OVER A LABORATORY FRINGING REEF

Several studies have reported the development of
 undular bores over fringing coral reefs (e.g, Gallagher,
 1976; Nwogu and Demirbilek, 2010) but the importance
 of this phenomenon for reef hydrodynamics has never
 been studied. Yet, the transformation of a long wave
 (e.g., swell or infragravity wave) into an undular bore
 leads to significant modifications of the wave field. The
 formation of undulations is for example associated to a
 significant increase of the leading bore height. Moreover,
 if the undulations have enough time to develop (i.e. if the
 reef flat is wide enough), the initial long wave will
 ultimately split into a series of solitons (e.g., Grue et al.,
 2008). All this is likely to affect wave run-up. As reeffronted
 coastlines are particularly vulnerable to flooding,
 a good understanding of long wave transformation over
 the reef flat, including their possible transformation into
 undular bores, is crucial.
 In this study, we investigate undular bore development
 over reef-type profiles based on a series of laboratory
 experiments. More specifically, we aim to characterize
 the conditions under which undular bores develop, and
 analyse how their development affect the hydrodynamics
 at the toe of the reef-lined beach and the resulting wave
 run-up.

OBSERVATION OF NEARSHORE WAVE-WAVE INTERACTION USING UAV

Infragravity waves, generated by nearshore wave-wave interaction, potentially increase the coastal hazard. Lack of detailed observation of nearshore wave fields however makes it difficult to fully understand the behavior of infragravity waves under wave-wave interactions. These days, UAVs (Unmanned Aerial Vehicles) have enabled us to easily capture the top-view images of the dynamic nearshore behavior with sufficiently high spatial and temporal resolutions. In this study, we conducted UAV-based observations of cross-shore variations of wave spectral characteristics to clarify the nearshore wave-wave interactions.

INFRAGRAVITY PERIOD OSCILLATIONS IN A CHANNEL HARBOR NEAR A RIVER MOUTH

Port of Bayonne, located in SW France, is a channel harbor situated near the river mouth of the Adour. Long-period oscillations have repeatedly caused snapping of mooring lines of berthed ships and have led to wave resonances in an adjacent marina (seiche). To investigate mechanisms for generation of theses oscillations, a field campaign was carried out during a one-year return-period storm (Hs = 6 m and Tp = 15 s): four pressure sensors were deployed inside the port. To complement the data and to better understand the governing processes that lead to the wave transformations in Port of Bayonne, the storm event was computed with the Boussinesq-type model, BOSZ. The data confirm the model results, which show generation of long infragravity (IG) waves by the incident swell around the harbor entrance and free propagation of these waves without amplification over far distances inside Port of Bayonne. Excited by these long waves, resonance oscillations are only noticeable in a small enclosed marina. Though the IG-waves are not causing substantial changes to the water level along the harbor channel, they are suspected to excite the ships’ eigen modes, which consequently results in mooring problems.

OPTIMUM FREQUENCY DISCRETIZATION AND ITS IMPLICATIONS ON EXTREME INFRA-GRAVITY WAVE RUNUP

Throughout history “sneaker” waves or “king” waves (in Australia), also referred in this study as extreme infragravity (EIG) waves, have been responsible for killing several people throughout the world. Particularly on the coast of Oregon, more than 21 people have died since the year 1990 due to this phenomenon (http://www.oregonlive.com). People that are usually walking by the beach on a “nice” day are suddenly surprised and washed away by what appears to be a “tsunami-like” wave which in reality is an EIG wave.

2D OVERTOPPING AND IMPACT EXPERIMENTS IN SHALLOW FORESHORE CONDITIONS

This paper introduces the 2D experiments conducted for the CREST project in the wave flume of Ghent University. The experiments focus on wave interactions with low-crested sea dikes fronted by a shallow foreshore and mildly to steeply sloping beaches, which is a very typical situation along the Belgian coast. Foreshore slopes of 1/20, 1/35, 1/50 and 1/80 were tested for a range of low to high energy wave conditions, a variation in wave steepness and two water levels. The main goal was to obtain a dataset in which the effects of the infragravity waves on the wave-structure interactions (i.e. wave overtopping and impact forces) can be studied. The tests included high spatial resolution surface elevation measurement tests, which is new for beaches including a dike in the inner surf zone. From the first results it became clear that the foreshore slope influences the wave transformation up to the dike toe. The influence is apparent comparing to existing (semi-) empirical models for prediction of the spectral wave period at the dike toe and wave overtopping at the dike crest. The high spatial resolution data show a steep increase in infragravity significant wave height in the very shallow area in front of the dike.

FINDING BICHROMATIC-BIDIRECTIONAL WAVES WITH ADVs

The aim of this study is to investigate Bichromatic-Bidirectional waves to characterize the subtractive wave-wave nonlinear interactions, using adaptive techniques rather than traditional spectral techniques. A physical model test in a 3D-wave basin was conducted and measurements were made with two arrays of ultrasonic sensors of free surface and one array of ADVs. The Hilbert-Huang transform, aided by the Multivariate Empirical Mode Decomposition, was applied to the orbital velocity data and the main characteristics of the infragravity wave (velocity amplitude, period and direction) were extracted with a good precision.

A study of long wave run-ups on a bi-linear beach slope induced by solitary and transient-focused wave group

ABSTRACT This paper presents long wave run-ups and rundowns on a bi-linear slope beach detected by using an image processing technique. For long wave run-ups, solitary wave run-up is obtained through experimental tests by Hydraulic-Control Wave-maker (HCW) and the dataset are validated by comparing Open-Source field Operation and Manipulation (OpenFOAM) with Reynolds Averaged Navier-Stokes (RANS)-type model and COrnell University Long and intermediate WAVE modelling package (COULWAVE) with Boussinesq-type model. For infragravity wave run-up, furthermore, a transient-focused wave group are generated and validated by the comparison of the numerical simulations. The results present not only that a low-frequency waves are enhanced by shoaling and breaking processes due to the bi-linear slope beach compare to the analytical solution on plane beach but that the series of wave run-ups are dominant by the low-frequency wave induced by the transient-focused wave groups.

Tsunami versus Infragravity Surge: Comparison of the Physical Character of Extreme Runup

AbstractRecent observations of energetic infragravity (IG) flooding events, such as those in the Philippines during Typhoon Haiyan, suggest that IG surges may approach the coast as breaking bores with periods of minutes: a very tsunami‐like characteristic. Energetic IG waves have been observed in various locations around the world and have led to loss of lives and damages to property. In this study, a comparison of overland flow characteristics between tsunamis and energetic IG wave events is presented. In general, whenever the tsunamis and energetic IG waves have similar runup, tsunamis tend to generate greater flow depths and longer flood durations than IG. However, flow velocities and Froude number are larger for IG primarily due to bore‐bore capture. This study provides a statistical and physical discriminant between tsunami and IG, such that in areas exposed to both, a proper interpretation of overland transport, deposition, and damage is possible.

Coherence between infragravity waves and ambient water motions over cape-associated shoals

Cross-shelf water motions influence fluxes of nutrients, larvae and sediments, which in turn affect nearshore morphodynamics. Among these motions are infragravity waves, which typically exhibit periodicities from 20 to 200 s, and originate from multiple sources including the superposition of incident wave fields. To examine how infragravity waves co-vary with water motions in an area of complex bathymetry, we analyzed measurements from acoustic Doppler current profilers around cape-associated shoals near Cape Canaveral, on the Florida (USA) Atlantic coast. Observations of water motions and infragravity wave heights at the outer and inner swales of two isolated shoals (Shoal E and Shoal D), located approximately 15 km offshore in ∼ 14 m water depth, were subjected to squared coherency and wavelet coherence analyses. Coherences between infragravity wave heights and flow conditions were unsteady, i.e. variable over time, and exhibited differences between outer and inner swales. Subtidal flows (periods > 0.5 days) were sporadically coherent with the total and bound infragravity wave heights at both inner and outer swales. Tidal flows (∼ 2 cycles/day) were coherent with the total infragravity wave heights only at outer swale locations. These results indicate that tidal motions may modulate the generation of free (directed both oppositely to and in the direction of short-wave propagation) infragravity waves by short-wave groups shoaling and by refractive trapping at shoals. Subtidal flows, on the other hand, may influence triad interactions that generate infragravity motions.

Sand Suspension and Transport During Inundation of a Dutch Barrier Island

Overwash and inundation of barrier islands transport large amounts of sediment landward, which could potentially increase the aggradation of these islands in times of sea level rise. However, not much is known about the detailed processes of sediment suspension and transport during inundation. Here we analyze field data of suspended sediment, water levels, waves, and currents which were collected during five inundation events on a barrier island in the Netherlands. We found that depth‐integrated suspended sand concentrations and cross‐shore sand transport showed high variability during and between inundation events at our location, where 80% of the combined transport from all inundation events was completed before high tide. This is primarily caused by variations in cross‐shore flow velocities which were strongest (up to 1.2 m/s onshore) before high tide. However, episodically high depth‐integrated suspended sand concentrations (defined as >2kg/m2) were observed on infragravity time scales (∼20–200 s), suggesting that the contribution of infragravity waves to the combined bed shear stresses of waves and currents was important. High contributions of infragravity waves to the transport coincided with observed bore‐like wave shapes, which might partly be attributed to higher short waves riding and suspending sediment at the position of the crest. Two transport regimes were thus found to govern the transport during inundation: a flow‐driven regime when flow velocities were high (>0.5 m/s) and the ratio of infragravity wave and current related shields numbers was below 0.11 and an episodic regime when this ratio exceeded 0.11.

Effects of reef morphology variations on wave processes over fringing reefs

Many low-lying tropical and sub-tropical atolls fringed by coral reefs are susceptible to coastal inundation during extreme wave events. Previous studies have shown that the infragravity (IG) wave is the dominant component of shoreline run-up compared to the sea and swell (SS) wave and the wave-induced setup. To better understand both the SS and IG wave dynamics over a fringing reef with various morphologies, a series of laboratory experiments were conducted in a wave flume based on an idealized fringing reef profile. The shoreline responses of waves to different reef morphologies with/without the reef crest, the lagoon and the reef surface roughness were examined. IG wave resonance on the reef flat was identified by a spectral analysis of the shoreline wave records. Subsequently, a numerical model based on the Boussinesq equations was validated by the experimental data. The model was then applied to investigate the impacts of varying reef morphologic features (fore-reef slope, reef-crest width, lagoon width, and reef roughness coefficient) on the shoreline wave motions.

Observing the Laboratory Interaction of Undertow and Nonlinear Wave Motion over Barred and Nonbarred Beaches to Determine Beach Profile Evolution in the Surf Zone

ABSTRACT Choi, J.; Roh, M., and Hwang, H., 2018. Observing the laboratory interaction of undertow and nonlinear wave motion over barred and nonbarred beaches to determine beach profile evolution in the surf zone. A vertically two-dimensional laboratory experiment was performed with a movable bed to investigate the interaction of undertow and sawtooth-shaped nonlinear wave motion over the cross-shore evolution of barred (storm) and nonbarred (normal) beach profiles. The bottom elevations, free-surface motion, and flow velocity were measured under regular wave conditions, excluding infragravity waves, over a movable bed. The observations were conducted in the surf zone during alternate repeats of the two wave conditions until each quasiequilibrium state was reached. From the net sediment transport of the observation area, it was shown that the storm case was in an erosion state and that the normal case was in an accretion state. The measurements confirmed that the strong undertow, which is a dominant factor...

The offshore boundary condition in surf zone modeling

Numerical models predicting surfzone waves and shoreline runup in field situations are often initialized with shoreward propagating (sea-swell, and infragravity) waves at an offshore boundary in 10–30 m water depth. We develop an offshore boundary condition, based on Fourier analysis of observations with co-located current and pressure sensors, that accounts for reflection and includes nonlinear phase-coupling. The performance of additional boundary conditions derived with limited or no infragravity observations are explored with the wave resolving, nonlinear model SWASH 1D. In some cases errors in the reduced boundary conditions (applied in 11 m depth) propagate shoreward, whereas in other cases errors are localized near the offshore boundary. Boundary conditions that can be implemented without infragravity observations (e.g. bound waves) do not accurately simulate infragravity waves across the surfzone, and could corrupt predictions of morphologic change. However, the bulk properties of infragravity waves in the inner surfzone and runup are predicted to be largely independent of ig offshore boundary conditions, and dominated by ig generation and dissipation.

Hydrodynamics and Sediment Mobility Processes Over a Degraded Senile Coral Reef

AbstractCoral reefs can influence hydrodynamics and morphodynamics by dissipating and refracting incident wave energy, modifying circulation patterns, and altering sediment transport pathways. In this study, the sediment and hydrodynamic response of a senile (dead) barrier reef (Crocker Reef, located in the upper portion of the Florida Reef Tract) to storms and quiescent conditions was evaluated using field observations and the Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport model. Waves, circulation, and resultant sediment mobility were modeled across different reef zones. Sediment mobility during quiescent periods and the passage of far‐field storms are driven by nonbreaking waves and, to a lesser degree, regional circulation. Spatial variability in these processes produces the present‐day distribution of sediment grain size at Crocker Reef, wherein finer‐grain material along a shallow central ridge is frequently mobilized (43% to 62% of the time), winnowed away, and deposited along the lower‐energy flanks and in the fore reef where sand mobility occurs less frequently (32% to 43% and 1% to 22% of the time, respectively). Analysis of wave conditions for the period of 2006–2014 supports that wave heights rarely exceed the threshold for breaking (0.1% and 0.3% at the reef crest and at the reef flat, respectively), predominantly during the passage of tropical storms. There is a shift to a wave‐breaking regime during near‐field storms, creating the potential for mobilization of larger material and enhanced reef degradation. Sediment mobility can be enhanced due to wave skewness or the generation of free infragravity waves during periods of depth‐induced wave breaking.