Infragravity Waves Research Articles

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.

Beach steepness effects on nonlinear infragravity‐wave interactions: A numerical study

AbstractThe 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.

Numerical Characterization of Harbor Oscillations in the Port of Ensenada, Mexico

A series of numerical experiments from a barotropic configuration of the General Curvilinear Ocean Model (GCOM) was conducted to analyze the response to infragravity (IG) waves of the Port of Ensenada, located within Bahia de Todos Santos (BTS), west coast of Mexico. Experiments with forcing frequencies f = 50?1 min?1, f = 30?1 min?1, f = 25?1 min?1 and f = 16.66?1 min?1 showed the expected increase of energy at the corresponding forcing frequency band and also the appearance of secondary peaks of energy at frequency bands f = 8.33?1 min?1 and f = 4.16?1 min?1 which were identified as modes f1 and f2; being the band at f = 16.66?1 min?1 the zeroth f0 mode. Maximum peak of spectral energy from the numerical experiments was found at frequency band f0 = 16.66?1 min?1 which agreed with the estimated maximum value of the amplification factor and with the T0 mode of oscillation of the port. Distribution of amplitudes inside PE for modes f0, f1 and f2 were also presented. Mode f0 represents a quarter-wave oscillation with amplitudes of the same sign; mode f1 has two nodal lines and mode f2 presents and additional one. Corresponding harbor currents were also calculated, they were in the range 20 - 160 cm?s?1. Finally, in order to elucidate the source of the external signals found in the spectral analysis of this study, the natural oscillation modes of the BTS were estimated. Although more studies are needed, BTS oscillation mode T2 = 16.82 min, was identified as the external forcing that excites larger oscillations within the port.

Assessing Storm Impact on a French Coastal Dune System Using Morphodynamic Modeling

Muller, H.; van Rooijen, A.; Idier, D.; Pedreros, R., and Rohmer, J., 2017. Assessing storm impact on a French coastal dune system using morphodynamic modeling.Natural coastal dune systems provide protection against flooding from the ocean for many coastal communities around the world. An in-depth knowledge of their erosion mechanisms under oceanic stress can help to identify potential weak spots along the coastline and eventually prevent damage to the hinterland. The objective of this study was to assess the effects of hydro-morphodynamic phenomena on the coastal dune morphology during storm events and to evaluate their level of protection. This paper focuses on the physical processes that cause erosion and breaching of sandy dune systems, which are typically a function of the coastal morphology in combination with the hydraulic loading. An XBeach model was set up for the coast of Les Boucholeurs (France), which suffered from severe erosion, dune breaching (at six locations), and inundation of the hinterland during storm Xynthia in February 2010. The model was run for the full storm period, and it accurately reproduced the reported dune breach locations and dimensions with limited calibration. The model results were further utilized to study the temporal and spatial breaching dynamics and to understand the effect of the prestorm morphology on dune breach development. The results show that the dune behavior during the storm was strongly related to the submersion height of the dune crest and anticorrelated with its prestorm geometry (i.e. dune height and width). The governing parameters for the poststorm dune state are, in terms of alongshore-averaged erosion and in order of relative contribution: Submersion height over the dune crest, prestorm dune width, and prestorm dune crest height. Finally, additional simulations showed the limited effects of infragravity waves and the initial bathymetry used in the simulations.

Modeling wave processes over fringing reefs with an excavation pit

The excavation of reef-flat sand and aggregate for engineering projects is a common practice on atoll islands, with attendant coastal management and hazard mitigation issues. To assess the impact of reef-flat excavation pits on wave transformation, a numerical study is carried out based on one-dimensional weakly dispersive, highly nonlinear Boussinesq equations. Model simulations are compared to field observations made at Majuro Atoll, Republic of the Marshall Islands (Ford et al., 2013a), at a 75-m-wide fringing reef flat with a 17-m-wide, 4-m-deep excavation pit and at an adjacent unmodified reef flat. With a calibrated empirical breaking model and uniform friction coefficient, the numerical model is shown to satisfactorily reproduce the observed sea and swell (SS) and infragravity (IG) frequency band wave heights for a moderate amplitude wave event. By removing the lateral morphological differences of the field experiment, the model shows that an excavation pit reduces/increases shoreline IG/SS wave heights compared to numerical tests without the pit, in qualitative agreement with Ford et al. (2013a). An EOF analysis demonstrates that the reduction in IG wave heights is associated with modifications of the ¼ and ¾ wavelength IG standing modes across the reef caused by the pit. Model tests are used to evaluate the effects of cross-reef pit location and width on wave transformation. Shoreline wave heights are least affected by a pit located near the reef crest, and both SS and IG wave heights increase as the pit location moves shoreward. Shoreline wave heights are an increasing function of pit width as the increase in SS wave height exceeds the decrease in IG wave height produced by the pit. The effects of incident wave height and spectral bandwidth are also examined for the reef with and without pit. It is found that both shoreline SS and IG wave heights increase with the increasing incident wave height for both profiles. The numerical model results also demonstrate that the shoreline response is sensitive to the bandwidth of the incident wave spectrum, and larger IG shoreline wave heights are excited for narrower spectral bandwidths until wave reflection limits the forcing.

Hindcasting Long Waves in a Port: An ANN Approach

Infragravity waves play an important role in port operations and many nearshore processes, and therefore their characterization is of major interest for oceanographers and coastal engineers. The lack of proper measurement networks and historic databases makes the development of hindcasting techniques essential. This work presents a fully developed infragravity wave hindcast methodology through Artificial Neural Networks (ANNs) and its application to a case study. The characteristic wave-heights of the low frequency band and the swell band inside a port basin are computed for a period of eight years on the basis of the long-term offshore wave conditions, a short record of searlevel oscillations and the historic tidal harmonic constituents. Based on the results, we construct and analyze the single and the joint probability density functions of the two characteristic wave-heights studied. In addition, we study the relationships between the infragravity energy inside the port and the offshore wave parameters and explore the extreme events during which the low frequency band energy exceeds the swell energy. The findings highlight the potential of the methodology to characterize the infragravity wave conditions inside a port basin and its suitability to study other coastal problems in which these waves are involved.

Block and boulder transport in Eastern Samar (Philippines) during Supertyphoon Haiyan

Abstract. Fields of dislodged boulders and blocks record catastrophic coastal flooding during strong storms or tsunamis and play a pivotal role in coastal hazard assessment. Along the rocky carbonate coast of Eastern Samar (Philippines) we documented longshore transport of a block of 180 t and boulders (up to 23.5 t) shifted upslope to elevations of up to 10 m above mean lower low water level during Supertyphoon Haiyan on 8 November 2013. Initiation-of-motion approaches indicate that boulder dislocation occurred with flow velocities of 8.9–9.6 m s−1, which significantly exceeds depth-averaged flow velocities of a local coupled hydrodynamic and wave model (Delft3D) of the typhoon with a maximum < 1.5 m s−1. These results, in combination with recently published phase-resolving wave models, support the hypothesis that infragravity waves induced by the typhoon were responsible for the remarkable flooding pattern in Eastern Samar, which are not resolved in phase-averaged storm surge models. Our findings show that tsunamis and hydrodynamic conditions induced by tropical cyclones may shift boulders of similar size and, therefore, demand a careful re-evaluation of storm-related transport where it, based on the boulder's sheer size, has previously been ascribed to tsunamis.

Observations of runup and energy flux on a low‐slope beach with high‐energy, long‐period ocean swell

AbstractThe transformation of surface gravity waves from 11 m depth to runup was observed on the low‐sloped (1/80) Agate Beach, Oregon, with a cross‐shore transect of current meters, pressure sensors, and a scanning lidar. Offshore wave heights H0 ranged from calm (0.5 m) to energetic (>7 m). Runup, measured with pressure sensors and a scanning lidar, increases linearly with (H0L0)1/2, with L0 the deep‐water wavelength of the spectral peak. Runup saturation, in which runup oscillations plateau despite further increases in (H0L0)1/2, is not observed. Infragravity wave shoaling and nonlinear energy exchanges with short waves are included in an infragravity wave energy balance. This balance closes for high‐infragravity frequencies (0.025–0.04 Hz) but not lower frequencies (0.003–0.025 Hz), possibly owing to unmodeled infragravity energy losses of wave breaking and/or bottom friction. Dissipative processes limit, but do not entirely damp, increases in runup excursions in response to increased incident wave forcing.

Infragravity‐wave modulation of short‐wave celerity in the surf zone

AbstractThe cross‐shore evolution of individual wave celerity is investigated using two high‐resolution laboratory experiments on bichromatic waves. Individual waves are tracked during their onshore propagation and their characteristics, including celerity, are estimated. The intrawave variability in celerity is low in the shoaling zone but increases strongly after breaking. It is maximum when the infragravity‐wave height to water depth ratio is the largest, that is to say close to the shoreline. There the observed range of individual wave celerity can be as large as the mean celerity value. This variability can be largely explained by the variations in water depth and velocity induced by the infragravity waves. The differences in celerity are such that they lead to the merging of the waves in the inner surf zone for most of the wave conditions considered. Again, the location at which the first waves start merging strongly correlates with the infragravity‐wave height to water depth ratio. The consequences of these findings for celerity‐based depth‐inversion techniques are finally discussed. Surprisingly, accounting for the infragravity‐wave modulation of the velocity field in the celerity estimate does not significantly improve depth estimation in the surf zone. However, it is shown that the occurrence of bore merging decreases significantly the coherence of the wavefield in the surf zone. This loss of coherence could hamper celerity estimation from pixel intensity time series and explain, at least partly, the relatively poor performance of depth‐inversion techniques in the inner surf zone.

Lidar and Pressure Measurements of Inner-Surfzone Waves and Setup

AbstractObservations of waves and setup on a steep, sandy beach are used to identify and assess potential applications of spatially dense lidar measurements for studying inner-surf and swash-zone hydrodynamics. There is good agreement between lidar- and pressure-based estimates of water levels (r2 = 0.98, rmse = 0.05 m), setup (r2 = 0.92, rmse = 0.03 m), infragravity wave heights (r2 = 0.91, rmse = 0.03 m), swell–sea wave heights (r2 = 0.87, rmse = 0.07 m), and energy density spectra. Lidar observations did not degrade with range (up to 65 m offshore of the lidar) when there was sufficient foam present on the water surface to generate returns, suggesting that for narrow-beam 1550-nm light, spatially varying spot size, grazing angle affects, and linear interpolation (to estimate the water surface over areas without returns) are not large sources of error. Consistent with prior studies, the lidar and pressure observations indicate that standing infragravity waves dominate inner-surf and swash energy at low frequencies and progressive swell–sea waves dominate at higher frequencies. The spatially dense lidar measurements enable estimates of reflection coefficients from pairs of locations at a range of spatial lags (thus spanning a wide range of frequencies or wavelengths). Reflection is high at low frequencies, increases with beach slope, and decreases with increasing offshore wave height, consistent with prior studies. Lidar data also indicate that wave asymmetry increases rapidly across the inner surf and swash. The comparisons with pressure measurements and with theory demonstrate that lidar measures inner-surf waves and setup accurately, and can be used for studies of inner-surf and swash-zone hydrodynamics.

Ross ice shelf vibrations

AbstractBroadband seismic stations were deployed across the Ross Ice Shelf (RIS) in November 2014 to study ocean gravity wave‐induced vibrations. Initial data from three stations 100 km from the RIS front and within 10 km of each other show both dispersed infragravity (IG) wave and ocean swell‐generated signals resulting from waves that originate in the North Pacific. Spectral levels from 0.001 to 10 Hz have the highest accelerations in the IG band (0.0025–0.03 Hz). Polarization analyses indicate complex frequency‐dependent particle motions, with energy in several frequency bands having distinctly different propagation characteristics. The dominant IG band signals exhibit predominantly horizontal propagation from the north. Particle motion analyses indicate retrograde elliptical particle motions in the IG band, consistent with these signals propagating as Rayleigh‐Lamb (flexural) waves in the ice shelf/water cavity system that are excited by ocean wave interactions nearer the shelf front.

Source regions and reflection of infragravity waves offshore of the U.S.s Pacific Northwest

AbstractInfragravity waves are oceanic surface gravity waves but with wavelengths (tens of km) and periods (>30 s) much longer than wind waves and swell. Mostly studied in shallow water, knowledge of infragravity waves in deep water has remained limited. Recent interest in deep water infragravity waves has been motivated by the error they may contribute to future high‐resolution satellite radar altimetry measurements of sea level. Here deep water infragravity waves offshore of the Pacific Northwest of the U.S. were studied using differential pressure gauges which were deployed as part of the Cascadia Initiative array from September 2012 to May 2013. Cross correlation of the records revealed direction of infragravity wave propagation across the array, from which source regions were inferred. The dominant source was found to be the coastline to the east, associated with large wind waves and swell incident on the eastern side of the basin. The source shifted southward during northern‐hemisphere summer, and on several days in the record infragravity waves arrived from the western side of the Pacific. Asymmetry of cross‐correlation functions for five of these westerly arrivals was used to calculate the ratio of seaward to shoreward propagating energy, and hence estimate the strength of infragravity wave reflection at periods of 100–200 s. Reflection of these remote arrivals from the west appeared to be strong, with a lower bound estimate of r = 0.49 ± 0.29 (reflection coefficient ± standard error) and an upper bound estimate of r = 0.74 ± 0.06. These results suggest that reflection at ocean boundaries may be an important consideration for infragravity waves in the deep ocean.

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.

Combined infragravity wave and sea‐swell runup over fringing reefs by super typhoon Haiyan

AbstractSuper typhoon Haiyan struck the Philippines on 8 November 2013, marking one of the strongest typhoons at landfall in recorded history. Extreme storm waves attacked the Pacific coast of Eastern Samar where the violent typhoon first made landfall. Our field survey confirmed that storm overwash heights of 6–14 m above mean sea level were distributed along the southeastern coast and extensive inundation occurred in some coastal villages in spite of natural protection by wide fringing reefs. A wave model based on Boussinesq‐type equations is constructed to simulate wave transformation over shallow fringing reefs and validated against existing laboratory data. Wave propagation and runup on the Eastern Samar coast are then reproduced using offshore boundary conditions based on a wave hindcast. The model results suggest that extreme waves on the shore are characterized as a superposition of the infragravity wave and sea‐swell components. The balance of the two components is strongly affected by the reef width and beach slope through wave breaking, frictional dissipation, reef‐flat resonances, and resonant runup amplification. Therefore, flood characteristics significantly differ from site to site due to a large variation of the two topographic parameters on the hilly coast. Strong coupling of infragravity waves and sea swells produces extreme runup on steep beaches fronted by narrow reefs, whereas the infragravity waves become dominant over wide reefs and they evolve into bores on steep beaches.

Infragravity‐wave dynamics in a barred coastal region, a numerical study

AbstractThis paper presents a comprehensive numerical study into the infragravity‐wave dynamics at a field site, characterized by a gently sloping barred beach. The nonhydrostatic wave‐flow model SWASH was used to simulate the local wavefield for a range of wave conditions (including mild and storm conditions). The extensive spatial coverage of the model allowed us to analyze the infragravity‐wave dynamics at spatial scales not often covered before. Overall, the model predicted a wavefield that was representative of the natural conditions, supporting the model application to analyze 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 infragravity‐wave 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.

Boussinesq modeling of spatial variability of infragravity waves on fringing reefs

Spatial variations of infragravity waves on fringing reefs are studied using a fully nonlinear Boussinesq equation model FUNWAVE-TVD. The effects of bottom roughness, forereef slope and tidal water level on significant wave height, wave setup and infragravity wave motion are investigated. Model results show that the cross-shore distributions of significant wave heights and setups are reasonably reproduced using the calibrated bottom friction coefficient. However, the infragravity wave heights over the reef flat are under-predicted. Spatial variations of infragravity waves at different tidal water levels exhibit distinct patterns. Spectral peaks and valleys on the reef flat, indicative of the generation of standing infragravity waves, are observed in the measurements and reproduced by the model. It is demonstrated that the model is capable of simulating the generation and propagation of infragravity motions on fringing reefs. The effects of relative submergence and inverse wave steepness on infragravity waves are investigated through a series of numerical experiments. The results demonstrate that the relative submergence has significant effects on the distributions of infragravity waves on the fringing reef. The maximum infragravity wave height decreases with increasing relative submergence at the reef edge. However, it increases with increasing relative submergence at the inner reef flat.

Acoustic-gravity waves in the atmosphere generated by infragravity waves in the ocean

Abstract Infragravity waves are surface gravity waves in the ocean with periods longer than approximately 30 s. Infragravity waves propagate transoceanic distances and, because of their long wavelengths, provide a mechanism for coupling wave processes in the ocean, atmosphere, and the solid Earth. Here, we present a strict physical justification for the hypothesis that background ocean waves may generate waves in the upper atmosphere. We show that, at frequencies below a certain transition frequency of about 3 mHz, infragravity waves continuously radiate their energy into the upper atmosphere in the form of acoustic-gravity waves. Based on ionospheric observations and estimates of the fluxes of the mechanical energy and momentum from the deep ocean, we conclude that acoustic-gravity waves of oceanic origin may have an observable impact on the upper atmosphere. We anticipate our work to be a starting point for a detailed analysis of global manifestations of the ocean-generated background acoustic-gravity waves.

Generation of low-frequency microseisms by infragravity waves on the southeastern coast of Sakhalin Island

Generation of low-frequency microseisms by infragravity waves on the southeastern coast of Sakhalin Island

Low-Frequency Seiche in a Large Bay

AbstractShort-term observations of sea surface elevations η along the 10-m isobath and long-term observations inside and outside of a large bay (Monterey Bay, CA) were obtained to describe the nodal structure of the modes 0–3 seiches within the bay and the low-frequency (<346 cpd) seiche forcing mechanism. The measured nodal pattern validates previous numerical estimates associated with a northern amplitude bias, though variability exists across the modal frequency band, particularly for modes 0 and 1. Low-frequency oceanic η white noise within seiche frequency bands (24–69 cpd) provides a continuous resonant forcing of the bay seiche with a η2 (variance) amplification of 16–40 for the different modes. The temporal variation of the oceanic η white noise is significantly correlated (R2 = 0.86) at the 95% confidence interval with the bay seiche η that varies seasonally. The oceanic η white noise is hypothesized as being from low-frequency, free, infragravity waves that are forced by short waves.

Processes controlling the seasonal cycle of wave-dominated inlets

This paper reviews the physical processes controlling wave-dominated inlets, based on several studies conducted at two inlets located on the West Coast of Portugal. Once the observed hydrodynamics and morphological changes are reasonably simulated, numerical experiments are performed to explain the development of the inlet during fair weather conditions and its shoaling and closure during winter storms. The former behaviour is explained by a tidal distortion that promotes ebb-dominance while the latter is explained by the combination of several wave-related processes: (1) the -bulldozer effect- due to the shore-normal component of wave forces; (2) the presence of lateral barotropic pressure gradients, accelerating longshore flows towards the inlet; (3) wave blocking during the ebb and (4) a rise in mean sea level, peaking in late autumn. Recent results also suggest that infragravity waves may play a major role.