Infragravity Waves Research Articles

Modelling and measurement of low-frequency surge motion associated with extreme storm conditions in the Port of Cape Town

Infra-gravity wave propagation into the Port of Cape Town (South Africa) was modelled with Delft3D-Surfbeat. The model was calibrated and verified against wave measurements outside the port and inside one of the main port basins. The numerical model results corresponded well to the measured data and theory on port basin oscillations.The calibrated model was used to develop a long wave forecasting tool, by parametrising a framework of simulations that covers the offshore wave climate. The forecasting tool relies only on the NCEP offshore wave forecasts as input and was shown to give reliable results 3 days in advance.Two storm events were analysed in detail and presented as case studies. The first event was an extreme storm with a measured significant swell wave height of 11.5 m offshore and a measured significant long wave height of 0.7 m inside the port. The second event was a significant storm with a measured significant swell wave height of 6.2 m offshore and a measured significant long wave height of 0.3 m inside the port basin.The surge motion of moored container vessels was measured during each of the two events. A tool was developed to detect and track user-defined objects on a vessel in a sequence of recorded images and then transform their positions to surge motion. The maximum surge motion measured during the first (extreme) event was 12.51 m and 3.14 m during the second (significant) event.The low-frequency motions of the vessels during the two storm events were modelled in the time domain with the moored ship motion model, Quaysim. The Delft3D-Surfbeat model output was used to determine the wave forces acting on the vessels. During the first (extreme) event, the moored vessel was under tug assistance and a line parting was observed. Several likely mooring scenarios were modelled to account for any uncertainty in the actual mooring arrangements.The modelled and measured surge motion was in good agreement, especially during the second event where there was less uncertainty about the mooring arrangement. Alternative mooring arrangements, including a dynamic tensioning system, were also modelled to determine whether vessels can be moored more effectively in such conditions.

Storm Xaver over Europe in December 2013: Overview of energy impacts and North Sea events

Abstract. Storm Xaver on 5–6 December 2013 was a serious winter storm in northern Europe with important impacts on societal and energy infrastructure. The storm's low pressure centre passed eastward north of Scotland, across the North Sea and southern Scandinavia, and into the Baltic region. The trajectory resulted in strong northwest winds and a cold air outbreak southward across the North Sea. The resultant convection system was associated with powerful wind gusts and freezing precipitation that impacted the UK, Belgium, the Netherlands, Germany, Poland, Denmark, Sweden, and Norway. The storm caused coastal flooding that was comparable with the most serious North Sea surge events of the 20th century. The primary impact for energy meteorology was a large scale electrical power loss in the northern part of the British Isles, Sweden, Poland, and parts of Germany. Petroleum production was reduced as offshore platforms were evacuated ahead of the storm. For wind energy, a number of onshore turbines were damaged by the gust field. Other societal impacts included travel and transport interruptions, building damage, forest damage, and coastal erosion. Because of the high water levels and sea state in the North Sea, the storm was important for offshore wind energy. The wind energy research tower FINO1 sustained unexpected damage during the storm, similar to previous wave strikes during Storm Britta (2006) and Storm Tilo (2007). A closer analysis is made of the tide gauge records across the North Sea to understand the progression of the storm surge and identify high amplitude, short-period features that may be linked to unusual seiches, meteotsunamis, or infragravity waves. Similar to previous storms, there is an indication that large infragravity waves during Storm Xaver may have had an impact on North Sea transport and energy infrastructure as well as coastal erosion. The review of information from different sources permits the met-ocean conditions and resultant societal/energy impacts to be related in time and space.

Hydrodynamic Modeling of a Reef-Fringed Pocket Beach Using a Phase-Resolved Non-Hydrostatic Model

The non-hydrostatic wave-flow model SWASH was used to investigate the hydrodynamic processes at a reef fringed pocket beach in southwestern Australia (Gnarabup Beach). Gnarabup Beach is a ~1.5 km long beach with highly variable bathymetry that is bounded by rocky headlands. The site is also exposed to large waves from the Southern Ocean. The model performance was evaluated using observations collected during a field program measuring waves, currents and water levels between June and July 2017. Modeled sea-swell wave heights (periods 5–25 s), infragravity wave heights (periods 25–600 s), and wave-induced setup exhibited moderate to good agreement with the observations throughout the model domain. The mean currents, which were highly-spatially variable across the study site, were less accurately predicted at most sites. Model agreement with the observations tended to be the worst in the areas with the most uncertain bathymetry (i.e., areas where high resolution survey data was not available). The nearshore sea-swell wave heights, infragravity wave heights and setup were strongly modulated by the offshore waves. The headlands and offshore reefs also had a strong impact on the hydrodynamics within the lagoon (bordered by the reefs) by dissipating much of the offshore sea-swell wave energy and modifying the pattern of the nearshore flows (magnitude and direction). Wave breaking on the reef platforms drove strong onshore directed mean currents over the reefs, resulting in off-shore flow through channels between the reefs and headlands where water exchanges from the lagoon to ocean. Our results demonstrate that the SWASH model is able to produce realistic predictions of the hydrodynamic processes within bathymetrically-complex nearshore systems.

Dangerous Sea Phenomena in the Southern Part of Matua Island, Kuril Ridge

The results of an analysis of wave observations in Dvoinaya Bay at the southern part of Matua Island (Kuril Ridge) are reported. Experimental studies were carried out in order to determine the probability of amplifying the incoming waves in the coastal zone of the island. The observations provided the records of waves at two points of the bay located at depths of ~10 and 20 m, with a discreteness of 1 s. The spectral analysis of the time series allowed wave processes with a period of 40 s and in the range of periods of 1–12 min to be identified. It is shown that the former are related to infragravity waves; in the range from 1 to 7.3 min, they are seiches of Dvoinaya Bay. In this case, oscillations with a period of about 1.8 min can be manifested in the form of harbor oscillations with a significant increase in their energy during a storm. Waves with a period of ~12 min are of seiche nature as well and are associated with the zero Helmholtz mode, while the effect of “swinging” of these oscillations may occur in the case of a tsunami wave arrival.

Operational tool for characterizing high-frequency sea level oscillations

The progressive upgrade of tide gauges to match tsunami warning requirements, tied with an upgrade of tide gauges with 1-min or less sampling and latency, has led to a huge amount of data available worldwide for studies of coastal hazards related to high-frequency sea level oscillations. This upgrade in the observation network poses a challenge in matching the operational data flow, quality control and processing, as well as an opportunity for a more immediate evaluation and understanding of the physical phenomena contained in the raw data, such as meteotsunamis and infragravity waves. The main purpose of this study is to present a new operational tool that enables, for the first time, user-friendly and fast exploitation of an up to now hidden information on high-frequency sea level oscillations. Developed and implemented for 40 tide gauges at the main ports of the Spanish coast, the new tool is based on the automatic analysis of 2-Hz raw data and the online publication of relevant products in near real time. It includes an event detection algorithm and a display calendar to select and review historical events, resulting in a revolutionary advanced toolbox, a new window to phenomena that affects ports operations and infrastructures. This toolbox, combined with the open dataset, provides the first steps for considering HFSLO in the definition of operational risk management. Dealing with these raw data in near real time requires careful selection of appropriate algorithms and quality control procedures, with therefore additional difficulties, that are discussed in this paper.

Spectral Wave-Driven Bedload Transport Across a Coral Reef Flat/Lagoon Complex

Coral reefs are an important source of sediment for reef-lined coasts by helping to maintain beaches while also providing protection in the form of wave energy dissipation. Understanding the mechanisms by which sediment is delivered to the coast as well as better constraining the total volumes generated are critical for projecting future coastal change. A month-long hydrodynamics and sediment transport study on a fringing reef/lagoon complex in Western Australia indicates that lower frequency constituents of wave energy are important to the total bedload transport of sediment across the reef flat and lagoon to the shoreline. The reef flat and the lagoon are characterized by distinctly different transport regimes, resulting in an offset in the timing of bedform migration between the two. Short-term storage of sediment is noted on the reef flat, which is subsequently washed out into the lagoon when offshore wave heights increase and strong currents due to wave breaking at the reef crest develop. This sudden influx of sediment is a significant control on bedform migration rates in the lagoon. Infragravity wave energy on the reef flat and lagoon make an important contribution to the migration of bedforms and resultant bedload transport. Given the complexity of the hydrodynamics of fringing reefs, the transfer of energy to lower frequency bands, as well as accurate estimates of sources and sinks of sediment, must but considered in order to correctly model the transport of sediment from the reef to the coast.

Development of the Physics–Based Morphology Model as the Platform for the Optimal Design of Beach Nourishment Project: A Numerical Study

In this study, a physics-based morphology model is developed and to test the feasibility of the morphology model proposed in this study as the platform for the optimal design of the beach nourishment project, the beach restoration process by the infra-gravity waves underlying the swells in a mild sea is numerically simulated. As a hydrodynamic module, the IHFOAM wave toolbox having its roots in the OpenFoam is used. Speaking of the morphology model, a transport equation for suspended load and the Exner type equation constitute the morphology model. In doing so, the probability theory first introduced by Einstein and the physical model test by Bagnold are used as the constituent sub-model of the morphology model. Numerical results show that among many flow features that are indispensable in forming sand bars over the flat bottom and swash zone, the partially skewed and asymmetric bottom shearing stresses, a shoreward Stokes drift near the free surface, boundary layer streaming near the seabed, and undertow toward the offshore were successfully simulated using the morphology model proposed in this study. It was also shown that plunging type breaker occurring at the final stage of the shoaling process, and its accompanying second breaker, sediment entrainment at the seabed, and the redistribution of suspended load by the down rush of preceding waves were successfully reproduced in the numerical simulation, and agreements with our experience in the field were very encouraging. In particular, the sand bar formation process over the flat bottom and backshore were successfully reproduced in the numerical simulation, which has been regarded as a challenging task.

Atmospheric resonances and their coupling to vibrations of the ground and waves in the ocean

Observations of the ionosphere with the airglow, GPS-TEC, and HF radar techniques reveal a resonant response of the middle and upper atmosphere to broad-band excitation by earthquakes, volcano eruptions, and convective storms. The resonances occur at such frequencies that an atmospheric wave, which is radiated at the ground level and is reflected from a turning point in the middle or upper atmosphere, upon return to the ground level satisfies boundary conditions on the ground. Using asymptotic and numerical models of atmospheric waves, this paper investigates atmospheric resonances and their excitation by seismic waves and infragravity waves in the ocean. It is found that “buoyancy” resonances with periods up to several hours arise in addition to “acoustic” resonances with periods of about 3–5 min. The acoustic and buoyancy resonances occur, respectively, on the acoustic and gravity branches of the dispersion curve of acoustic-gravity waves. Buoyancy of the atmosphere is important for the resonances of both kinds. Acoustic resonances are found to be sensitive to the temperature profile, especially around mesopause and tropopause, and are predicted to be a seasonal phenomenon in polar atmosphere. Unlike acoustic resonances, buoyancy resonances exhibit high sensitivity to the wind velocity profile and its variations. The resonances correspond to most efficient coupling between the atmosphere and its lower boundary and are promising for detection of such coupling.

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.

Laboratory study on wave transformation and run-up in a 2DH reef-lagoon-channel system

Many low-lying atolls fronted by coral reefs are vulnerable to wave-driven coastal flooding during extreme wave events. Previous laboratory studies on this issue focus mainly on the wave dynamics over one-dimensional horizontal (1DH) fringing reef profiles in wave flumes. To better understand wave transformation and run-up in a two-dimensional horizontal (2DH) reef-lagoon-channel system, a set of laboratory experiments are performed in a wave basin using an idealized physical model subjected to both regular and irregular waves. The wave height variation and wave spectrum evolution on the reef flat as well as wave run-up on the back-reef slope are first analyzed, and then the impacts of incident wave height, wave period and reef-flat wave level on these wave processes are investigated. Finally, the excitation of resonant modes is identified to account for the infragravity (IG) wave characteristics in the system.

Evolution of Infragravity Waves Over a Shoal Under Nonbreaking Conditions

AbstractThe present paper aims to clarify the mechanism of infragravity wave (IGW) energy amplification over nearshore shoals reported in recent studies. Wave transformation and energy transfer between short waves (SWs) and IGWs were investigated using SWASH model for nonbreaking random waves propagating over trapezoid shoals with different bottom slopes. It was found that the time lag of IGWs relative to SW groups is the major mechanism for energy transfer from SWs to IGWs and the amplification of IGW energy over all segments of the shoal. The time lag is generated on the front slope and enlarged on the plateau of the shoal, decreases on the rear slope with higher decreasing rates on milder slopes. Over the rear slope, the evolution of IGWs depends on the relative importance of deshoaling and nonlinear energy transfer. It was found that nonlinear energy transfer dominates over the rear slope gentler than 1/60, causing the IGW energy to increase over the first half of the deshoaling process and decrease over the second half whereas deshoaling dominates for steeper rear slope and the IGW energy decay over the whole slope. It is demonstrated numerically and theoretically that the shoals with gentler bottom slopes amplify the IGW energy more effectively by providing longer distance for nonlinear energy transfer to build up. The persistent nonlinear energy transfer on the plateau indicates the important role of wave spatial evolution history in the subsequent IGWs evolution. Strong free IGWs were detected on leeward of the shoal, possibly due to release of topography‐induced additional bound IGWs.

Transformation of Infragravity Waves during Hurricane Overwash

Infragravity (IG) waves are expected to contribute significantly to coastal flooding and sediment transport during hurricane overwash, yet the dynamics of these low-frequency waves during hurricane impact remain poorly documented and understood. This paper utilizes hydrodynamic measurements collected during Hurricane Harvey (2017) across a low-lying barrier-island cut (Texas, U.S.A.) during sea-to-bay directed flow (i.e., overwash). IG waves were observed to propagate across the island for a period of five hours, superimposed on and depth modulated by very-low frequency storm-driven variability in water level (5.6 min to 2.8 h periods). These sea-level anomalies are hypothesized to be meteotsunami initiated by tropical cyclone rainbands. Estimates of IG energy flux show that IG energy was largely reduced across the island (79–86%) and the magnitude of energy loss was greatest for the lowest-frequency IG waves (<0.01 Hz). Using multitaper bispectral analysis, it is shown that, during overwash, nonlinear triad interactions on the sea-side of the barrier island result in energy transfer from the low-frequency IG peak to bound harmonics at high IG frequencies (>0.01 Hz). Assuming this pattern of nonlinear energy exchange persists across the wide and downward sloping barrier-island cut, it likely contributes to the observed frequency-dependence of cross-barrier IG energy losses during this relatively low surge event (<1 m).

Morphodynamic response to extreme wave events of microtidal dissipative and reflective beaches

In this work, the morphodynamic response of microtidal dissipative and reflective beaches to extreme wave events generated by hurricanes and cold fronts in the Colombian Caribbean was established. Two sandy beaches were used as case studies, Bocagrande (Cartagena, Bolívar Department) (dissipative) and Costa Verde (Ciénaga, Magdalena Department) (intermediate–reflective). The numerical models SWAN and XBeach were used to investigate wave propagation during storm conditions for selected cases and to determine morphological changes on the two beaches. Both models were calibrated using hydrodynamic and bathymetric data obtained in field campaigns pre and post-cold fronts. The results of the numerical models satisfactorily represented the real conditions on both beaches. It was shown that regardless of beach configuration, whether dissipative or intermediate–reflective, erosive processes had a greater magnitude during cold fronts (March 2009 and December 2017) than under hurricanes conditions. Bocagrande beach is more exposed to extreme wave events, mainly due to its characteristic dissipative profile. Infragravity wave energy, fundamental in sediment transport dynamics, is not reflected on the beach front, but is dissipated along the swash zone and drags sediment to the sea. By contrast, Costa Verde shows conservative coastline retreats: its intermediate–reflective morphologic configuration played a transcendental role. Infragravity wave reflection on the beach front does not allow a longer water extension over dry areas in the swash zone under storms.

Distant-Source Swells Cause Coastal Inundation on Fiji’s Coral Coast

Distant-source swells are known to regularly inundate low-lying Pacific Island communities. Here we examine extreme total water level (TWL) and inundation, driven by a distant-source swell on Fiji’s Coral Coast using observations and a phase-resolving wave model (XBeach). The objective of this study is to increase understanding of swell-driven hazards in fringing reef environments, to identify the contribution of wave setup and infragravity waves to extreme TWL, and to investigate coastal flooding during present and future sea levels. The maximum TWL near the shore was caused by compounding mechanisms, where tides, wave setup, infragravity waves, and waves in the sea swell frequencies contributed to the TWL. Waves and wave setup on the reef were modulated by offshore wave heights and tides, with increased setup during low tide and increased wave heights during high tide. Numerical simulations were able to reproduce the mechanisms contributing to the extreme TWL and allowed an estimation of the inundation extent. Simulations of the same swell under the RCP8.5 sea-level rise scenario suggested the area of inundation would increase by 97% by 2100. A comparison between the numerical model, a multiple linear regression model, and two commonly used parametric models revealed that both XBeach and the linear regression model were better suited to reproduce the nearshore wave setup and TWL than the empirical equations. The results highlight the need for customized, site-specific coastal hazard assessments and inundation forecast systems in the South Pacific.

Relative Magnitude of Infragravity Waves at Coastal Dikes with Shallow Foreshores: A Prediction Tool

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-to-SS-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.

In situ Observations of Wave Transformation and Infragravity Bore Development Across Reef Flats of Varying Geomorphology

The character and energetics of infragravity (‘IG’, 25 s 250 s) waves over coral reef flats can enhance shoreline erosion or accretion, and also govern extreme shoreline events such as runup, overwash, and flooding on coral reef-lined coasts. Here we use in situ wave measurements collected along cross-reef transects at 7 sites on Pacific islands with varying reef geomorphologies to examine under what conditions IG waves occur and what factors enhance their irregularity. In general, a greater fraction of total wave energy was transferred to the IG band on reefs with steeper fore reef slopes and shallower reef flats. The IG wave amplitudes scaled with increasing water levels, but it was primarily at lower water levels when these waves became pitched onshore (negatively asymmetrical) and peaked (positively skewed). However, our results also highlight the importance of reef-flat width and slope as important morphological controls on IG waves, as the most asymmetric, bore-like, IG waves occurred on the wider reef flats, and the most skewed IG waves at the site with the steepest reef-flat. On the wider reef flats, IG wave-wave capture was observed during periods of large offshore wave forcing and enhanced VLF wave energy. Because similar IG wave motions over plane beaches enhance sea-swell (‘SS’, period < 25 s) bore-merging in the surf zone, we posit that VLF waves over reef flats may facilitate IG bore merging, and this may lead to larger, more pitched-onshore bores at the shoreline. In addition, greater IG wave heights appear to support the transmission of larger secondary short-period waves over the reef flat, independent of overall water levels. As irregular IG waves may be strong drivers of cross-reef sediment transport as well as runup, understanding the conditions and reef geomorphologies that lead to low-frequency, energetic bores on reef flats is critical to forecasting how coral reef-lined coasts will respond to sea-level rise and climate change.

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.

Occurrence of Gravity and Infra Gravity Waves in the Nearshore Region at Ratnagiri, West Coast of India

Glejin, J.; Sanil Kumar, V., and Sheela Nair, L., 2020. Occurrence of gravity and infra gravity waves in the nearshore region at Ratnagiri, west coast of India. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 92-96. Coconut Creek (Florida), ISSN 0749-0208.This study analyses the sea level data measured simultaneously using S4DW and directional Wave Rider Buoy (WRB) off Ratnagiri, eastern Arabian Sea, located along the west coast of the Indian sub-continent. The objective of the study is to investigate the presence of infra-gravity waves as well as the source of infra-gravity waves that reach the near-shore regions of eastern Arabian Sea. Based on FFT analysis of the wave data, the estimated mean infragravity wave height is 0.04 m with a peak value of 0.08 m whereas the mean wave period of the infra-gravity waves is 60 s with a standard deviation of 8.9 s. Detailed analysis of the WRB data (wave period in the range of 0-30 s) provide reliable evidence indicating the presence of infra-gravity waves in the eastern Arabian Sea. The source of infra-gravity waves that propagate into the nearshore region could be either remotely or locally generated waves. However, it is observed that the presence of long-period gravity waves in the region has a direct influence on the infra-gravity waves. The amplitude of the infra-gravity is found to increase with the arrival of long period swell waves and vice-versa.

Source Regions of Infragravity Waves Recorded at the Bottom of the Equatorial Atlantic Ocean, Using OBS of the PI‐LAB Experiment

AbstractInfragravity waves are generated along coasts, and some small fraction of their energy escapes to the open oceans and propagates with little attenuation. Due to the scarcity of deep‐ocean observations of these waves, the mechanism and the extent of the infragravity waves energy leakage from the coasts remains poorly understood. Understanding the generation and pathways of infragravity wave energy is important among others for understanding the breakup of ice‐shelves and the contamination of high‐resolution satellite radar altimetry measurements of sea level. We examine data from 37 differential pressure gauges of Ocean Bottom Seismometers (OBS) near the equatorial mid‐Atlantic ridge, deployed during the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We use the beamforming technique to investigate the incoming directions of infragravity waves. Next, we develop a graph‐theory‐based global back‐projection method of noise cross‐correlation function envelopes, which minimizes the effects of array geometry using an adaptive weighting scheme. This approach allows us to locate the sources of the infragravity energy. We assess our observations by comparing to a global model of infragravity wave heights. Our results reveal strong coherent energy from sources and/or reflected phases at the west coast of Africa and some sources from South America. These energy sources are in good agreement with the global infragravity wave model. In addition, we also observe infragravity waves arriving from North America during specific events that mostly occur during October–February 2016. Finally, we find indications of waves that propagate with little attenuation, long distances through sea ice, reflecting off Antarctica.

Quantifying Bore‐Bore Capture on Natural Beaches

AbstractBore‐bore capture occurs when a faster moving bore captures a slower moving bore or the shoreline while both are propagating shoreward in the surf or swash zones and is quantified here for the first time using wave tracking methods applied to data collected at seven sandy Australian beaches. The results showed that, for the locations where bore‐bore capture occurred, there was a high probability (≈40%) of bore‐bore capture occurring in the surf or swash zones. Of these ≈45% consisted of a bore capturing the instantaneous shoreline (i.e., they occurred in the swash zone). The landward‐most 10% of the nearshore region (i.e., the time‐varying surf‐swash extent) was found to be the most likely location for a capture event. Infragravity wave energy (Eig) at the shoreline, number of bore‐bore capture events, and beach morphodynamic state are shown to be correlated: more dissipative beaches had higher levels of Eig at the shoreline which directly correlated with higher rates of bore‐bore capture. Approximately 20% of the capture events led to extreme horizontal shoreline maxima; however, &gt;97% of extreme shoreline maxima were directly driven by bore‐bore capture events. There was a clear relation between the probability of bore‐bore capture driving an extreme shoreline maxima and beach morphodynamic state: The more dissipative the beach, the lower the probability of a bore‐bore capture event causing an extreme shoreline maxima event. Such correlation has direct importance for the future development of predictive runup models, which do not currently account for this phenomenon.