Large Breaking Waves Research Articles

Mid-frequency acoustic localization of breaking waves.

During an experiment in deep water off the coast of Southern California, wind speeds ranged from 10 to 15 m/s and wind forcing produced large breaking waves. A mid-frequency vertical planar hydrophone array recorded underwater ambient noise while an airplane equipped with a high-resolution video camera captured images of the sea surface above the array. Beams of ambient noise between 5 and 6 kHz were projected onto the sea surface and synchronized in space and time with the aerial images. Despite the array's limited azimuthal resolution of the surface, due to its modest 1 m horizontal aperture and relatively deep 130 m deployment depth, concentrated areas of high intensity in the acoustic surface projection were observed to match visible breaking events in the aerial images.

Modelling the air-sea-land interactions responsible for the direct trigger of turbidity currents by tropical cyclones

Tropical cyclones directly trigger turbidity currents in submarine canyons as a consequence of storm surges, high waves, onshore blowing winds and extreme currents. The resultant supply of sediment at the heads of the canyons plays a crucial role in the genesis of turbidity currents and thus is key in understanding frequency and duration of their flows. Here we present a single numerical framework capable of modelling turbidity currents driven by cyclone-induced winds and waves through resolved, quasi-3D hydrostatic equations. Our simulations predict the occurrence of a canyon-confined turbid underflow induced by a tropical cyclone that caused a seafloor pipeline to shift. Turbidity current occurrence is shown to be related not just to tropical cyclone intensities but also to their tracks with respect to the canyon head. The modelled underflow is well approximated by similarity profiles from laboratory and field observations which demonstrates the reliability of the model in capturing the structure of turbidity currents. The proposed triggering of turbidity currents off the centre of coastal embayments is likely to occur when the abrupt rotation of incoming winds induced by a passing cyclone remains always coastal-bound all across the cyclone’s waxing and waning stages. Our results show that these conditions can give rise to simultaneous, opposite alongshore currents and eventually result in offshore-bound rip currents. Conversely, it is unlikely that turbidity currents will be triggered by cyclone-induced rip currents when the cyclonic rotation results in peak offshore winds (coming from the land), as no fetch is available for generating large breaking waves to induce simultaneous, opposite alongshore currents. Nevertheless, the sole presence of strong alongshore currents deflected at the headlands of a coastal embayment (or delta) is likely to trigger sediment-gravity flows and eventually result in turbidity currents offshore the edge of the embayment without the aid of rip currents.

Mid-frequency acoustic localization of breaking waves

During an experiment in deep water off the coast of Southern California, winds between 20 and 25 knots resulted in large breaking waves. A mid-frequency planar hydrophone array recorded underwater ambient noise while an airplane equipped with a high-resolution video camera recorded images of the sea surface above the array. Beams of ambient noise between 5 and 6kHz were projected onto the sea surface and aligned in space and time with the aerial images. The array’s resolution of the surface is coarse (20 by 50 m) due to its modest 1 meter horizontal aperture and relatively deep 130meter deployment depth. Nonetheless, concentrated regions of high intensity in the acoustic surface projection match visible breaking events in the aerial images. [Work supported by the Office of Naval Research.]

Observations of River Plume Mixing in the Surf Zone

Abstract We use salinity observations from drifters and moorings at the Quinault River mouth to investigate mixing and stratification in a surf-zone-trapped river plume. We quantify mixing based on the rate of change of salinity DS/Dt in the drifters’ quasi-Lagrangian reference frame. We estimate a constant value of the vertical eddy diffusivity of salt of Kz = (2.2 ± 0.6) × 10−3 m2 s−1, based on the relationship between vertically integrated DS/Dt and stratification, with values as high as 1 × 10−2 m2 s−1 when stratification is low. Mixing, quantified as DS/Dt, is directly correlated to surf-zone stratification, and is therefore modulated by changes in stratification caused by tidal variability in freshwater volume flux. High DS/Dt is observed when the near-surface stratification is high and salinity gradients are collocated with wave-breaking turbulence. We observe a transition from low stratification and low DS/Dt at low tidal stage to high stratification and high DS/Dt at high tidal stage. Observed wave-breaking turbulence does not change significantly with stratification, tidal stage, or offshore wave height; as a result, we observe no relationship between plume mixing and offshore wave height for the range of conditions sampled. Thus, plume mixing in the surf zone is altered by changes in stratification; these are due to tidal variability in freshwater flux from the river and not wave conditions, presumably because depth-limited wave breaking causes sufficient turbulence for mixing to occur during all observed conditions. Significance Statement River outflows are important sources of pollutants, sediment, and nutrients to the coastal ocean. Small rivers often meet large breaking waves in the surf zone close to shore, trapping river water and river-borne material near the beach. Such trapped material can influence coastal public health, beach morphology, and nearshore ecology. This study investigates how trapped fresh river water mixes with salty ocean water in the presence of large breaking waves by using high-resolution measurements of waves, salinity, and turbulence. We find that the surf zone is often fresh and stratified, which could have significant implications for the fate of riverine material. Wave breaking provides a constant source of turbulence, and the amount of mixing is limited by the degree of vertical salt stratification; more mixing occurs when stratification is higher.

On the role of sea‐state in bubble‐mediated air‐sea gas flux during a winter storm

AbstractOceanic bubbles play an important role in the air‐sea exchange of weakly soluble gases at moderate to high wind speeds. A Lagrangian bubble model embedded in a large eddy simulation model is developed to study bubbles and their influence on dissolved gases in the upper ocean. The transient evolution of mixed‐layer dissolved oxygen and nitrogen gases at Ocean Station Papa (50°N, 145°W) during a winter storm is reproduced with the model. Among different physical processes, gas bubbles are the most important in elevating dissolved gas concentrations during the storm, while atmospheric pressure governs the variability of gas saturation anomaly (the relative departure of dissolved gas concentration from the saturation concentration). For the same wind speed, bubble‐mediated gas fluxes are larger during rising wind with smaller wave age than during falling wind with larger wave age. Wave conditions are the primary cause for the bubble gas flux difference: when wind strengthens, waves are less‐developed with respect to wind, resulting in more frequent large breaking waves. Bubble generation in large breaking waves is favorable for a large bubble‐mediated gas flux. The wave‐age dependence is not included in any existing bubble‐mediated gas flux parameterizations.

Relationship between latent heat of sea spray and uncertainty of a meteorological field

A surf zone with large breaking waves produces more spray than do offshore regions. Latent heat of spray evaporation causes change in the surrounding temperature and wind velocity, resulting in further alterations in temperature, wind velocity and heat flux. Spray in a surf zone with large breaking waves may have unignorable effect on determination of a local meteorological field because of this interconnected relationship as well as its higher population than in the open ocean. In this study, the effects of the spray latent heat on a meteorological field were investigated. The authors propose a method for estimating latent heat of spray vaporization over the ocean. The method was applied to a meso-scale meteorological model to perform numerical experiments with consideration of heat flux by spray. Although the contribution of heat flux on the ocean was as small as 2.5%, fluctuations of air temperature and wind velocity increased over time due to the effects of spray. The fluctuations are thought to cause uncertainty in weather prediction. Numerical experiments with spray provided predictions of air temperature and wind velocity near a coast line that were consistent with observational data, especially when the population of spray droplets increased by two orders of magnitude as is often observed in a coastal area.

Sakhalin-1: Technology Development for Frontier Arctic Projects

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 20208, "Sakhalin-1: Technology Development for Frontier Arctic Projects," by J.M. Hamilton, ExxonMobil Upstream Re - search, and J.E. Jones, ExxonMobil Development Company, originally prepared for the 2009 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. As the oil and gas industry pushes into frontier Arctic areas, new technologies and design criteria will be required to ensure safety and cost effectiveness of increasingly challenging developments. Sakhalin-1 provides a good example of a development in a frontier subarctic area where location-specific design criteria had to be developed and innovative-technology solutions were required to deal with demanding environmental conditions including sea ice, large breaking waves, and earthquakes. Introduction First production from the Exxon Neftegas Limited (ENL) Sakhalin-1 Phase-1 development of Chayvo field began in October 2005, marking the culmination of nearly 10 years of design-criteria and technology development. The development concept consists of an offshore drilling platform, an onshore drilling and treating facility at Chayvo, an oil-export pipeline that runs east/west across the island and Tatar Strait to mainland Russia, and a storage and tanker export terminal at DeKastri. The oil is exported by ice-strengthened tankers south through the Tatar Strait, which is partially covered by up to medium first-year ice (70 to 120 cm) in the winter. The offshore drilling platform, Orlan, is approximately 10 km offshore in approximately 15-m water depth. The platform annually is exposed to approximately 4000 km of drifting first-year pack ice up to 1.5-m (level ice) thickness, which originates in the Sea of Okhotsk north of the island and drifts south along the eastern coast. The controlling ice load features are pressure ridges, with keels extending to depths greater than 30 m (2000-year design value). The design ice load on the 80-m-wide, vertical-sided structure is approximately 385 MN. Design wave loads are generated by late-summer and fall storms that can produce breaking waves at the platform. Global wave loads are on the order of the ice loads, and wave run-up and locally high slamming pressures represent design challenges. Sakhalin Island seismicity is moderate to high, with the active Piltun fault system running north/south on the island as close as 10 km from onshore facilities and 20 km from the offshore platform. The 1995 Neftegorsk earthquake, which registered moment magnitude 7.1, had its epicenter approximately 50 km from the Sakhalin-1 facilities. All facilities must be designed for earthquakes, and the export pipeline, which crosses multiple faults, must be able to withstand several meters of fault displacement.

Dynamic Knockdown of Canting Keel Yachts in Breaking Waves

Yacht canting keel configurations have been developed to maximize the available righting moment by rotating their keel bulb to windward. Regulatory authorities have been required to establish rules covering the design and operation of such systems; however, significant dynamic investigations into their behavior have not been performed. This paper presents results from a theoretical and experimental investigation into the dynamic stability of canting keel sailing yachts when experiencing a knockdown by large breaking waves. Towing tank experiments were conducted on a yacht model with a fixed keel and two canted keel configurations, beam-on to large solitary breaking waves. The motion of the model during the tests was recorded by four video cameras and analyzed using photogrammetry. A comparison of the hydrostatic predictions and experimental results indicate that the traditional GZ approach of assessing the energy required to heel may not be appropriate for yachts with canting keels. In addition, a correlation of the experimental results with the ISO stability index highlights that this index may be unsuitable for categorizing yachts with canting keels with respect to their ability to recover after a knockdown event.

Weak or strong nonlinearity: the vital issue

Marine hydrodynamics is characterised by both weak nonlinearities, as seen for example in drift forces, and strong nonlinearities, as seen for example in wave breaking. In many cases their relative importance is still a controversial matter. The phenomenon of particle escape, seen in linear theory, appears to offer a guide to when strongly nonlinear effects will start to become important, and what will happen when they do. In the case of the “ringing” of vertical cylinders in steep waves, particle escape is shown to correspond approximately to local wave breaking, which leads to the cavitation responsible for “ringing”. Another example is rogue waves, where recent results from weakly nonlinear theory are disappointing, and also fail to explain the rogue waves seen in relatively shallow water, as in the data from the Draupner and Gorm platforms. Recent laboratory experiments, too, show wave crests continuing to grow in height after all frequency components have come into phase, which is inconsistent with weakly nonlinear theory. Particle escape, which is more frequent in shallow water, offers a simple alternative explanation for these observations, as well as for the violent motion at the wave crests, which often confuses rogue-wave data. Extreme wave crests have long been known to be strongly nonlinear, so it appears possible that rogue waves are primarily a strongly nonlinear phenomenon. Fully nonlinear computations of two interacting regular waves are presented, to explore further the connection between particle escape and wave breaking. They are combined with Monte-Carlo simulations of particle escape in hurricane conditions, and the very few measurements of large breaking waves during hurricanes. It is concluded that large breaking waves will have occurred about once per hour, and once per 100 h, respectively, in the recent hurricanes LILI and IVAN. These findings call into question the use of non-breaking wave models in the design codes for fixed steel offshore structures.

Breaking waves and the equilibrium range of wind-wave spectra

When scaled properly, the high-wavenumber and high-frequency parts of wind-wave spectra collapse onto universal curves. This collapse has been attributed to a dynamical balance and so these parts of the spectra have been called the equilibrium range. We develop a model for this equilibrium range based on kinematical and dynamical properties of breaking waves. Data suggest that breaking waves have high curvature at their crests, and they are modelled here as waves with discontinuous slope at their crests. Spectra are then dominated by these singularities in slope. The equilibrium range is assumed to be scale invariant, meaning that there is no privileged lengthscale. This assumption implies that: (i) the sharp-crested breaking waves have self-similar shapes, so that large breaking waves are magnified copies of the smaller breaking waves; and (ii) statistical properties of breaking waves, such as the average total length of breaking-wave fronts of a given scale, vary with the scale of the breaking waves as a power law, parameterized here with exponent D.

On the analysis of sloshing of water in rectangular containers: Numerical study and experimental validation

In this work the analysis of sloshing of water in rectangular open tanks has been extensively carried out. Two mathematical models are employed, respectively the Reynolds Averaged Navier Stokes Equations (RANSE) and the Shallow Water Equations (SWE). The RANSE are solved using a modified form of the well established MAC method (SIMAC) able to treat both the free surface motion and the viscous stresses over the rigid walls accurately. The Shallow Water Equations are solved by means of a simple and powerful algorithm (CE-SE) able to deal with large impacting waves over the tank walls. Successively, in order to validate the mentioned algorithms and for a better understanding of the sloshing phenomenon, experimental tests have been carried out using a 0.5 m breadth rectangular tank in periodic roll motion. It has been shown that RANSE provide more accurate solutions than SWE for small or moderate amplitudes of excitation. In particular in this paper it is proved that the shallow water approximation can be efficiently adopted within liquid depth to tank breadth ratio = 0.15, when examining the sloshing problem. By increasing the water level inside the tank, results by SWE show large qualitative and quantitative disagreement with experiments. Nevertheless, in the case of large amplitude excitation, when sprays and large breaking waves are expected, SWE provide a fairly good estimate of the sloshing induced waves. Finally a simple baffle configuration inside the tank has been considered. By the analysis of numerical results, it has been observed that the presence of a vertical baffle at the middle of the tank dramatically changes the sloshing response compared to the unbaffled configuration. It produces a jump-like effect, resulting in a weak magnification of the dynamic loads on the vertical walls out of resonance, and a strong reduction of the dynamic loads in the resonance condition.

Wave Breaking in Deep Water

Every mariner is aware that dangerous large breaking waves occur on the world's oceans. The scope of this review is somewhat greater. Wave breaking occurs at a large range of scales and we do not restrict ourselves to the deep ocean. Deep water in the context of wave studies implies sufficiently deep that the surface waves are unaffected by the direct effects of variations in bed topography. Thus even a small pond can support breaking deep-water waves. Shallow breaking is reviewed in Peregrine (1983). Some comments on the visual aspect of breakers are in order, since direct observation still has a role to play in the study of this complex phenomenon. The most dramatic breakers are plunging breakers where the breaking commences by the wave overturning and forming a forward moving sheet of which plunges down into the in front causing splashes, air entrainment, and eddies. Although plunging breakers are common on beaches they are less common on deep water, so much so that some people have argued that they do not occur naturally. However, read Coles (1991) for a distillation of an experienced yachtsman's account of waves at sea. Most other breakers are described as spilling breakers. From their

New instrumentation for the investigation of sediment suspension processes in the shallow marine environment

An instrumentation system has been developed that continuously measures: flow velocity, water depth, and suspended sediment concentration profiles in the nearshore zone. The system consists of four components: (1) an array of electronic sensors, including: an electromagnetic current meter, a surface-piercing resistance wave gauge, and a five-element array of optical back-scatterance sensors; (2) a signal processing electronics unit; (3) a guyed mast support structure; and (4) a data acquisition system. The small size of the sensors and their response characteristics make the array suitable for monitoring sediment transport to within a few centimeters of the bed under moderately large breaking waves (∼2 m). Results from an experiment on an exposed ocean beach on the coast of Washington, U.S.A., indicate that sand suspension events of long duration (15–40 sec) relative to the period of incident waves contribute a significant quantity of sand to the total littoral drift. Work is now in progress to modify the system for self-contained operation on the continental shelf. Field investigations with the modified system will focus on the acquisition of a data set to test the validity of the suspended sediment distribution equation.

Discus Buoy Stability and the Spectrum of Steep Waves

Thick discus buoys are used in the ocean as platforms from which oceanographic and meteorological measurements are made and reported to shore. The stability of these buoys is sensitive to steep breaking waves, and a few have capsized in the ocean from large breaking waves. This occurrence can be simulated in laboratory scale-model studies. Generally, in the laboratory, the buoys will not capsize for a breaking wave that is small relative to buoy diameter. The problem is predicting the probability of capsizing for engineering planning purposes. This paper describes how first approximations can be made from a combination of laboratory scale-model testing of buoy capsizings in random waves, a few reported occurrences of buoy capsizings in the open ocean, and the statistics of ocean waves. An estimate of the joint probability density distribution function of a wave steepness parameter and a wave height parameter is made from buoy motion measurements. It is found that in the northeastern Pacific Ocean the wave climate is quite severe and includes a large percentage of relatively large breaking waves. Under such circumstances, buoy diameters of 10 m have a relatively low reliability. The reliability increases rapidly as buoy diameter increases.

Observations on the formation and location of transient rip currents

A model for the formation and location of transient rip currents is proposed. It is suggested that sucking and erosive effects at the peak site (highest part) of a set of large breaking waves initiate a seaward current in a scoured breaker zone, at the same time that water is accumulating inside the breakers. Accumulated water inside the breakers immediately begins to drain seaward through the peak site, feeding this embryonic rip current, which develops into a mature rip current. Rip-current formation in this manner best occurs following storms when a moderate swell, negligible or offshore winds and a flat shoreface profile are present.