Infragravity Band Research Articles

Unstructured swan modelling of free infragravity waves over the Southern North Sea

Infragravity (IG) waves are relatively long waves with typical periods of several tens of seconds to several minutes. The energy at the IG band plays an important role in nearshore areas. For example, IG waves can significantly contribute to dune erosion and sediment transport (e.g., Roelvink et al., 2009), and may excite harbor oscillations (e.g., Bowers, 1977). Furthermore, IG waves may result in destructive inundation events (e.g., Roeber and Bricker, 2015). These documentations of IG waves' impacts emphasise the necessity to account for IG contributions as part of coastal hazard assessments, especially under storm conditions.

Bed level changes in the surf zone during post-storm beach recovery

The study of post-storm beach recovery is important for economic development and the protection of life in coastal areas. In this study, field observations were conducted for 21 days in the surf zone of Dongdao Beach, Hailing Island, China, after tropical storm “Cempaka”. Data on depth, wave, Eulerian velocity, sediment, three-dimensional topography of the beach, and high-frequency variations in bed-level elevation were collected. The results showed that the beach experienced medium- to low- to medium-energy waves during field observations and covered two complete astronomical tide cycles. Contrary to the effect of wave energy conditions on beaches under normal wave conditions, a higher wave energy during beach recovery can promote silting and accelerate beach recovery. Tidal water level is an important factor affecting beach restoration, and a smaller tidal range is conducive to beach accretion. In a mixed semidiurnal tide, beach erosion and accretion occurred in the “highest tide” and “sub-highest tide” tidal cycles, respectively, and the combined effect of the two affected the change in the bed level in a mixed semidiurnal tide. After the storm, the hydrodynamic forcing mechanism and self-organization process of the sand bar jointly drove the formation of the topography of the bar channel in the surf zone. After the storm stopped, the spectral energy in free surface elevation was mainly distributed in the very low frequency and decayed rapidly at the infragravity band. The very low-frequency pulsation of the surf zone during recovery is a prominent feature of bed-level elevation, depth, and velocity. This study provides a good case for the study of hydrodynamic and bed level changes in the post-storm surf zone, as well as a reference for future studies of the intrinsic mechanisms post-storm beach recovery processes around the world.

Development and Application of an Infragravity Wave (InWave) Driver to Simulate Nearshore Processes

AbstractInfragravity waves are key components of the hydro‐sedimentary processes in coastal areas, especially during extreme storms. Accurate modeling of coastal erosion and breaching requires consideration of the effects of infragravity waves. Here, we present InWave, a new infragravity wave driver of the Coupled Ocean‐Atmopshere‐Waves‐Sediment Transport (COAWST) modeling system. InWave computes the spatial and temporal variation of wave energy at the wave group scale and the associated incoming bound infragravity wave. Wave group‐varying forces drive free infragravity wave growth and propagation within the hydrodynamic model of the coupled modeling system, which is the Regional Ocean Modeling System (ROMS) in this work. Since ROMS is a three‐dimensional model, this coupling allows for the combined formation of undertow currents and infragravity waves. We verified the coupled InWave‐ROMS with one idealized test case, one laboratory experiment, and one field experiment. The coupled modeling system correctly reproduced the propagation of gravity wave energy with acceptable numerical dissipation. It also captured the transfer of energy from the gravity band to the infragravity band, and within the different infragravity bands in the surf zone, the measured three‐dimensional flow structure, and dune morphological evolution satisfactorily. The idealized case demonstrated that the infragravity wave variance depends on the directional resolution and horizontal grid resolution, which are known challenges with the approach taken here. The addition of InWave to COAWST enables novel investigation of nearshore hydro‐sedimentary dynamics driven by infragravity waves using the strengths of the other modeling components, namely the three‐dimensional nature of ROMS and the sediment transport routines.

Wave runup and inundation dynamics on a perched beach

Sandy beaches perched over rocky shore platforms are common globally, yet their mixed sand and rocky morphology present challenges for quantifying and predicting wave runup and inundation. For typical linear beach profiles, simple relationships can be made between vertical runup and horizontal inundation based on beach slope. However, as topographic irregularities increase, substantial deviations from these relationships can occur. Shore platforms often experience a range of beach states, from full sand coverage (accreted) to complete shore platform erosion (exposed). As a result, existing methods for quantifying runup and inundation on purely sandy beaches are generally not directly applicable to these coastlines. To advance our understanding of runup and inundation on perched beaches, the aim of this work is three-fold: (1) to provide a method for quantifying beach slope and runup across a range of perched beach profiles, (2) to assess the relationship between vertical runup and horizontal inundation observations, and (3) to understand how different hydrodynamic mechanisms (i.e., setup, swash) contribute to runup during different beach states. To achieve this, we conducted an 8-month field study along a perched beach in southwestern Australia that experiences large seasonal variations in beach state and wave climate. A method was developed to measure runup and beach slope when only the topography shoreward of the shore platform edge was known. Using this method, an approximately linear relationship between inundation and runup was identified by incorporating beach slope. Our observations suggest that the components that dominate runup on perched beaches were primarily dependent on beach state (i.e., accreted versus exposed). Runup was dominated by swash in the infragravity band when in an accreted beach state, and setup when in an exposed beach state. These results aid our understanding of coastal processes on perched beaches, while providing new methods applicable to a range of perched beach profiles.

River Plume Modulation by Infragravity Wave Forcing

AbstractWe present measurements at the mouth of Río Maipo, a small river that discharges into an energetic surfzone on the coast of Chile, which document for the first time the plume response to infragravity (IG) wave forcing. We find that inlet dynamics are strongly modulated by IG waves; inlet discharge velocity varies by 20% in the IG band relative to peak ebb velocity. Drone imaging and measurements of surfzone salinity show that the plume edge is forced offshore and onshore, modifying plume width by 20%–50% at IG frequencies. We conclude that the observed modulation results from surfzone forcing at IG frequencies consistent with a moving break point; increased (decreased) wave amplitude generates stronger (weaker) wave‐driven circulation and forces the plume onshore (offshore). Although river water is often trapped in the surfzone, these dynamics may enhance mixing and cross‐shore dispersion of river‐borne materials.

Characteristics of maximal energy component of waves in Terpeniya Bay (Sakhalin Island)

Analysis of natural sea surface oscillations was performed in order to determine maximal spectral energy components in sea waves in the area of Terpeniya Bay. It has been found out that maximal components appear at periods of 5 and 8 seconds and reach energy of 3 · 106 cm2·s during storms. During calm weather maximums can be observed in the range of swell waves, herewith maximums exist at periods of 12 and 15 seconds. Energy of these maximums doesn’t go upper then 8 · 105 cm2·s and 3 · 105 cm2·s accordingly. Maximum components in the infragravity band of energy spectrum were estimated as well. Two steady energy peaks were determined at periods of 75 and 135 seconds, which are most probably relate to edge waves in considered water area.

Resonance and interactions of infragravity waves with sea ice

Flexural-gravity waves propagating under continuous sea ice can potentially travel considerable distances with negligible reduction of their amplitude, particularly waves that have longer periods such as those in the IG (infragravity) band. This paper investigates resonance from that perspective, explicitly identifying where it might be influencing the formative composition and subsequent evolution of the flexural-gravity wave spectrum by moving energy to longer periods. Several possible manifestations of resonance are appraised, where the term is being applied loosely; some obvious and some less so. For example, the heaving, rolling and flexing of ice floes using NAVMI (nondimensionalized added virtual mass incremental) factors; the natural frequencies of finite sheets of fast ice constrained along some of their boundaries; the initiation and maintenance of trapped edge modes due to reflection at the shore followed by partial total internal reflection as the reflected waves venture out into deeper water again, with the attendant exiting leaky waves that are liberated as part of that process—each shaped by topographical subtleties of local terrain; and a proposition that ice sheets select a preferred frequency at which the wave moves at the same speed in ice as it does in open sea. A data set of observations from the Sea of Okhotsk that includes a broad span of IG waves is employed, as it is these flexural-gravity waves that have the greatest tenacity to propagate over vast distances. Because IG wave energy within the period range 15–70 s is a recurring feature of the measurements collected whilst the sea is ice-covered, this band is a primary focus of the paper. A conceptual model based upon the van der Pol oscillator is also briefly mentioned, to illustrate qualitatively how synchronization could act as a catalyst to some of the resonances that occur.

Variability in infragravity wave processes during estuary artificial entrance openings

ABSTRACTIntermittently open/closed estuaries (IOCE) are wave‐dominated estuaries with entrances that temporarily close to the ocean. Wave–current interactions play a major role in estuary entrance morphodynamics and influence the degree of energy transfer from the ocean into the lagoon. This study utilizes artificial entrance openings of multiple ICOE in Victoria, Australia, to capture continuous hydrodynamic and geomorphic data throughout the opening cycle. We illustrate that water level oscillations in the infragravity (IG) band) are present in the basin during open entrance conditions. IG waves were observed to propagate up to 1.8 km upstream of the mouth while the entrance was open. Our work identifies that changes in cross‐sectional area, bed depth at the berm position, and offshore wave height control the magnitude of IG waves within the estuary basin. IG wave magnitude is also tidally modulated and increases with high tides when the nearshore water level is higher. Late during the drainage phase, waves were observed to track the margins of the channel, away from the thalweg, and reach the basin. IG wave energy was highest immediately after the basin had ceased draining and while channel dimensions at the mouth were within 10% of their maximum value. As the entrance aggrades, IG wave magnitude decreases in the absence of energetic offshore wave conditions. We relate the changes in IG wave magnitude and frequency to a six‐stage conceptual model of the opening–closure sequence. Within the basin, IG wave energy, height and frequency were also consistently highest closer to the mouth and decreased with distance upstream. Our findings indicate that water level oscillations in the IG band are a persistent feature in IOCE and may be the norm rather than the exception in these systems. As IG waves were captured serendipitously as part of a larger field campaign, future work will focus on instrumenting IOCE to gain high‐resolution data to quantify IG wave processes during entrance openings. © 2020 John Wiley & Sons, Ltd.

Sea Surface Gravity Waves Excited by Dynamic Ground Motions from Large Regional Earthquakes

Abstract Infragravity waves on the sea surface near coastlines are occasionally excited by static displacement caused by large local earthquakes and recorded as tsunamis. However, tsunamis induced by ground motions from seismic waves are rarely observed, especially far from earthquake focal areas. We investigated seafloor pressure variations in the infragravity band at the Hikurangi subduction zone following the M 7.8 Kaikōura and M 7.1 Te Araroa earthquakes. Anomalous infragravity waves were observed at 0.2–20 mHz at sites overlying a low-velocity accretionary wedge offshore of the east coast of New Zealand’s North Island accompanying the Rayleigh-wave arrivals. The maximum amplitude of these ultra-low-frequency waves was similar to the tsunami that propagated from the earthquake focal area hours later. The amplitude of the pressure signal from these waves observed offshore varied inversely with water depth, suggesting that sea surface gravity waves were excited by Rayleigh or Love waves amplified within the accretionary wedge.

In-situ Observations of Infragravity Response during Extreme Storms on Sand and Gravel Beaches

Billson, O.J.; Russell, P.; Davidson, M.; Poate, T.; Amoudry, L.O., and Williams, M.E., 2020. In-situ observations of infragravity response during extreme storms on sand and gravel beaches. Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 382–386. Coconut Creek (Florida), ISSN 0749-0208.Infragravity waves (frequency = 0.005 – 0.05 Hz) play a key role in coastal storm impacts such as flooding and beach/dune erosion. They are known to dominate the inner surf zone on low-sloping sandy beaches during storms. However, in large wave conditions, their importance on different beach types, of variable swell and wind-waves dominance, is largely unknown. Here, a new dataset is presented comprising in-situ observations during storm wave conditions (significant wave height of 3.3 m, peak periods of 18 s and return periods up to 1 in 60 years) from two contrasting sites: a low-sloping sandy beach and a steep gravel beach. Wave measurements were collected seaward of the breakpoint by wave buoys and bed-mounted acoustic Doppler current profilers, and through the surf zone using arrays of pressure transducers. Wave spectra showed contrasting evolution from the shoaling zone to the inner surf zone at the two sites. At the sandy beach, gravity band energy dissipated gradually as depth reduced, while infragravity band energy simultaneously increased, resulting in strongly infragravity-dominated wave spectra in the inner surf zone. At the steep gravel site, a rapid drop in short wave energy was observed, with limited growth of infragravity energy so that inner surf zone spectra showed a low energy peak in the infragravity band. The normalized bed slope parameter indicated whether infragravity waves were generated by bound long wave release or breakpoint forcing, showing that the former (latter) was dominant on the sandy (gravel) beach. In spite of these differences, the shoreline wave spectra under storm wave conditions were infragravity-dominated on both the sandy and gravel beaches.

Numerical modeling of wave runup on steep and mildly sloping natural beaches

Runup on ocean beaches includes steady wave setup and oscillating swash, often decomposed into wind generated sea-swell (SS), and lower frequency infragravity (IG) waves. We show that the numerically fast, open-source numerical model 1D SWASH predicts accurately the bulk properties of runup observed on two natural beaches (one steep and one shallow sloped) for a range of incident wave conditions. The runup tongue shape was measured with a scanning lidar, and the waterline location was defined in both the observations and model with a 10 cm depth threshold. Runup is reasonably accurately predicted with energetic (e.g. 5 m significant height) incident waves, even though the assumption of 1D bound waves significantly overpredicts infragravity energy at the offshore boundary in 10 m depth. The model-data comparisons are limited by statistical chatter, often larger in runup than offshore because runup energy is concentrated in the relatively narrow infragravity IG band with low effective degrees of freedom.

Tsunami and infragravity waves impacting Antarctic ice shelves

AbstractThe responses of the Ross Ice Shelf (RIS) to the 16 September 2015 8.3 (Mw) Chilean earthquake tsunami (>75 s period) and to oceanic infragravity (IG) waves (50–300 s period) were recorded by a broadband seismic array deployed on the RIS from November 2014 to November 2016. Here we show that tsunami and IG‐generated signals within the RIS propagate at gravity wave speeds (∼70 m/s) as water‐ice coupled flexural‐gravity waves. IG band signals show measureable attenuation away from the shelf front. The response of the RIS to Chilean tsunami arrivals is compared with modeled tsunami forcing to assess ice shelf flexural‐gravity wave excitation by very long period (VLP; >300 s) gravity waves. Displacements across the RIS are affected by gravity wave incident direction, bathymetry under and north of the shelf, and water layer and ice shelf thicknesses. Horizontal displacements are typically about 10 times larger than vertical displacements, producing dynamical extensional motions that may facilitate expansion of existing fractures. VLP excitation is continuously observed throughout the year, with horizontal displacements highest during the austral winter with amplitudes exceeding 20 cm. Because VLP flexural‐gravity waves exhibit no discernable attenuation, this energy must propagate to the grounding zone. Both IG and VLP band flexural‐gravity waves excite mechanical perturbations of the RIS that likely promote tabular iceberg calving, consequently affecting ice shelf evolution. Understanding these ocean‐excited mechanical interactions is important to determine their effect on ice shelf stability to reduce uncertainty in the magnitude and rate of global sea level rise.

The differential response of kelp to swell and infragravity wave motion

AbstractWe present field measurements of the movement of the giant kelp Macrocystis pyrifera under wave forcing. We resolve the depth and frequency‐dependent responses along the stipe and find different and counterintuitive patterns of response at the infragravity and swell wave forcing frequencies. At swell frequencies, tilting of the stipe is largest toward the holdfast, whereas at infragravity frequencies, the stipe tilting is largest closer to the water surface. It is postulated that the stretching of blades and subsequent pull on the stipe is, in part, responsible for these patterns. This conclusion is supported by results of manipulative experiments, which show a more along‐stipe uniform response after removal of blades from the kelp. The length of the kelp also exerts a strong control on the relative magnitudes of movements in the different frequency bands, with the swell band becoming more important relative to the infragravity band for shorter length kelp. These results indicate that kelp will differentially dissipate energy over both frequencies and varying depths within the water column. The variety of movement responses over differing wave forcing frequencies may also imply that there exist differing rates of breakage for kelp exposed to hydrodynamics stressors of multiple frequencies.

Experimental analysis of infragravity waves in two eroded microtidal beaches

This work aims to contribute to the characterization and understanding of infragravity waves on two beaches with erosion problems. For this reason, we have used an array of ADCP and a pressure sensor to measure wave parameters and pressure inside and outside of the surf zone during the dry and rainy period in the beaches of Galerazamba and Manzanillo del Mar (both dissipative and eroded beaches) located in the Colombian Caribbean coast. Based on these measurements, we have carried out a spectral analysis in order to identify the frequency components that characterize the wave and its energy; thus, we identified the characteristic frequencies of infragravity waves to finally filter the infragravity signal on each beach in different seasonal periods. Among the results of the Welch spectrum applied to surface elevation time series, we found that, the frequencies’ energy of the sea-swell band decreases due to bottom friction and wave breaking as the wave approaches the shore, while the frequencies’ energy of the infragravity band increases significantly. In addition, for the wavelet analysis, we could observe how the energy of the infragravity band, especially the lowest frequencies gain energy as the waves approaches the coast. Furthermore, based on the infragravity wave obtained from the extreme wave event registered during the field campaign we can conclude that the contribution of this signal is important in the erosion problems presented in the beaches of Galerazamba and Manzanillo del Mar. Finally, these results show the need to realize other studies that allow us to understand deeply, the role of infragravity waves on the morphological changes that occurs in these beaches.

Modeling the effect of wave‐vegetation interaction on wave setup

AbstractAquatic vegetation in the coastal zone attenuates wave energy and reduces the risk of coastal hazards, e.g., flooding. Besides the attenuation of sea‐swell waves, vegetation may also affect infragravity‐band (IG) waves and wave setup. To date, knowledge on the effect of vegetation on IG waves and wave setup is lacking, while they are potentially important parameters for coastal risk assessment. In this study, the storm impact model XBeach is extended with formulations for attenuation of sea‐swell and IG waves, and wave setup effects in two modes: the sea‐swell wave phase‐resolving (nonhydrostatic) and the phase‐averaged (surfbeat) mode. In surfbeat mode, a wave shape model is implemented to capture the effect of nonlinear wave‐vegetation interaction processes on wave setup. Both modeling modes are verified using data from two flume experiments with mimic vegetation and show good skill in computing the sea‐swell and IG wave transformation, and wave setup. In surfbeat mode, the wave setup prediction greatly improves when using the wave shape model, while in nonhydrostatic mode (nonlinear) intrawave effects are directly accounted for. Subsequently, the model is used for a range of coastal geomorphological configurations by varying bed slope and vegetation extent. The results indicate that the effect of wave‐vegetation interaction on wave setup may be relevant for a range of typical coastal geomorphological configurations (e.g., relatively steep to gentle slope coasts fronted by vegetation).

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.

Observations of infragravity period oscillations in a small marina

We measured water levels in Two Rocks Marina, Western Australia, to investigate infragravity-period (25–300s) oscillations and their forcing mechanisms. Spectral analyses identified four dominant oscillations in the infragravity band, which were generated through excitation of the marina’s natural periods. The oscillations were present at all times, independent of the offshore conditions, indicating that they were forced by a continuous external energy source. The spectral energy of the oscillations increased by ~50 times during storm events (higher incident wave heights), in comparison to calm events (lower incident wave heights). Wave heights of oscillations within the marina were strongly correlated with offshore incident swell wave heights and reached maximum of 0.5m. The groupiness factor of swell waves around the marina was 0.6–0.85. Bound infragravity waves associated with swell wave groups were identified as potential forcing mechanism of infragravity-period oscillations within the marina. The bound infragravity waves have broad frequency spectrum without dominant periods matched the marina’s natural periods however, bound infragravity waves of periods in the proximity of the marina NOPs were adequate to generate oscillations at the NOPs of the marina. Frequencies of the oscillations were independent of the forcing frequency, and determined by the marina׳s geometry.

The character of seafloor ambient noise recorded offshore New Zealand: Results from the MOANA ocean bottom seismic experiment

We analyze the characteristics of ambient noise recorded on ocean‐bottom seismographs using data from the 2009–2010 MOANA (Marine Observations of Anisotropy Near Aotearoa) seismic experiment deployed west and east of South Island, New Zealand. Microseism and infragravity noise peaks are clear on data recorded on the vertical channel of the seismometer and on the pressure sensor. The noise levels in the infragravity band (<0.03 Hz) on the horizontal seismometer channels are too high to show the infragravity peak. There is a small difference (∼0.25 Hz versus ∼0.2 Hz) in microseism peak frequencies between the two sides of the South Island on all three seismic channels. Our results show clear depth dependence between the peak frequency of infragravity waves and the water depth. We find that the product of water depth and wave number at the peak frequency is a constant,koH = 1.5. This relationship can be used to determine the variation of phase and group velocity of infragravity waves with water depth, and the location of the infragravity peak and corresponding noise notch at any water depth. These estimates of spectral characteristics, particularly low noise bands, are useful for future OBS deployments.

Observations of alongshore variability of swash motions on an intermediate beach

Alongshore variability in swash motions – shoreline oscillations about the mean water level on the beach face – were investigated using video images and a high-resolution morphology survey on an intermediate beach. Under mild, swell-dominated offshore wave conditions, alongshore variation of up to 78% in significant runup height Rs (defined as 4 times the standard deviation of the swash time series) was observed. This variation was predominantly driven by energy at the incident (>0.05Hz) frequencies (where most of the swash energy was observed), and, consistent with previous observations, was mainly controlled by changes in the slope of the beach face (measured at the mean swash location). However, alongshore patterning in wave breaking over the sandbar caused variation in the degree of wave dissipation along the beach and also resulted in alongshore changes to swash motions. Although alongshore changes in beach slope and wave breaking patterning over the bar were observed to be typically correlated, both were needed in a regression model to provide the best explaination of alongshore changes in Rs. At infragravity frequencies (<0.05Hz), alongshore variability was not well associated either with changes in beach slope or wave breaking patterning. Low-mode edge waves were observed in the swash measurements and their contribution to the total energy spectrum was greatest near the location where a shoal was observed, suggesting this shoal may play a role in forcing. The edge waves may have contributed to the swash variability observed at infragravity frequencies. However, in these reflective conditions, the infragravity band plays a secondary role in controlling alongshore variations to swash motions.

Spectral wave modeling in fringing reef environments

This study examines the implementation and validity of phase-averaged spectral models in describing wave transformation over fringing reefs. The wave breaking mechanism over the abrupt reef crest, dissipation due to bottom friction, and nonlinear energy transfer over the reef flat set the problem apart from general applications in non-tropical settings. The Simulating Wave Nearshore (SWAN) model, which is widely used in coastal engineering applications, has embedded parameterizations of these surf-zone processes for examination. In addition, we extend the model to include a recently implemented bottom-friction source term for tropical reefs. Comparisons of model predictions with laboratory and field observations over fringing reefs identify promising wave-breaking and bottom-friction parameterizations for calibration. The calibrated parameterizations enable SWAN to reproduce the observed wave height and setup and extend the applicability of spectral models to fringing reef environments. SWAN, however, lacks the parameterization to describe the nonlinear energy transfer toward the infra-gravity band that was observed in the laboratory and field studies.