Showing papers on "Breaking wave published in 2002"

Ocean waves and oscillating systems : linear interactions including wave-energy extraction

Abstract: 1. Introduction 2. Mathematical description of oscillations 3. Interaction between oscillations and waves 4. Gravity waves on water 5. Wave-body interactions 6. Wave-energy absorption by oscillating bodies 7. Wave interactions with oscillating water columns Bibliography Index.

Scale dependence of bubble creation mechanisms in breaking waves

Abstract: Breaking ocean waves entrain air bubbles that enhance air-sea gas flux, produce aerosols, generate ambient noise and scavenge biological surfactants. The size distribution of the entrained bubbles is the most important factor in controlling these processes, but little is known about bubble properties and formation mechanisms inside whitecaps. We have measured bubble size distributions inside breaking waves in the laboratory and in the open ocean, and provide a quantitative description of bubble formation mechanisms in the laboratory. We find two distinct mechanisms controlling the size distribution, depending on bubble size. For bubbles larger than about 1 mm, turbulent fragmentation determines bubble size distribution, resulting in a bubble density proportional to the bubble radius to the power of -10/3. Smaller bubbles are created by jet and drop impact on the wave face, with a -3/2 power-law scaling. The length scale separating these processes is the scale where turbulent fragmentation ceases, also known as the Hinze scale. Our results will have important implications for the study of air-sea gas transfer.

INTERNAL GRAVITY WAVES: From Instabilities to Turbulence

Abstract: ▪ Abstract We review the mechanisms of steepening and breaking for internal gravity waves in a continuous density stratification. After discussing the instability of a plane wave of arbitrary amplitude in an infinite medium at rest, we consider the steepening effects of wave reflection on a sloping boundary and propagation in a shear flow. The final process of breaking into small-scale turbulence is then presented. The influence of those processes upon the fluid medium by mean flow changes is discussed. The specific properties of wave turbulence, induced by wave-wave interactions and breaking, are illustrated by comparative studies of oceanic and atmospheric observations, as well as laboratory and numerical experiments. We then review the different attempts at a statistical description of internal gravity wave fields, whether weakly or strongly interacting.

Complex Wave Dynamics on Thin Films

Abstract: Formulation and Linear Orr-Sommerfeld Theory: Navier-Stokes Equation with interfacial conditions Linear stability of the trivial solution to two- and three-dimensional perturbations Longwave expansion for surface waves Unusual case of zero surface tension Surface waves - Numerical solution of the Orr-Sommerfeld equations. Hierarchy of Model Equations: Kuramoto-Sivashinsky (KS), KdV and related weakly nonlinear equations Lubrication theory to derive Benney's longwave equation Depth-averaged integral equations Combination of Galerkin-Petrov method with weighted residuals Validity of the equations Spatial and temporal primary instability of the Shkadov model. Experiments and Numerical Simulation: Experiments on falling-film wave dynamics Numerical formulation Numerical simulation of noise-driven wave transitions Pulse formation and coarsening. Periodic and Solitary Wave Families: Main properties of weakly nonlinear waves in an active/dissipative medium Phase space of stationary KS equation Solitary waves and Shilnikov theorem Bifurcations of spatially periodic travelling waves and their stability Normal Form analysis for the Kawahara equation Nonlinear waves far from criticality - the Shkadov model Stationary waves of the boundary layer equation and Shkadov model Navier-Stokes equation of motion - the effects of surface tension. Floquet Theory and Selection of periodic Waves: Stability and selection of stationary waves Stable intervals from a Coherent Structure Theory Evolution towards solitary waves. Spectral Theory for gKS Solitary Pulses: Pulse spectra Some numerical recipes to construct eigenfunctions and obtain spectra Stability of gKS pulses Attenuation of radiation wave packet by stable pulses Resonance pole-a discrete culmination of the continuous spectrum Resonance pole description of mass drainage Suppression of wave packets by a periodic train of pulses. (Part contents).

The velocity field under breaking waves: coherent structures and turbulence

Abstract: Digital particle image velocimetry (DPIV) measurements of the velocity eld under breaking waves in the laboratory are presented. The region of turbulent fluid directly generated by breaking is too large to be imaged in one video frame and so an ensemble-averaged representation of the flow is built up from a mosaic of image frames. It is found that breaking generates at least one coherent vortex that slowly propagates downstream at a speed consistent with the velocity induced by its image in the free surface. Both the kinetic energy of the flow and the vorticity decay approximately as t 1 . The Reynolds stress of the turbulence also decays as t 1 and is, within the accuracy of the measurements, everywhere negative, consistent with downward transport of streamwise momentum. Estimates of the mometum flux from waves to currents based on the measurements of the Reynolds stress are consistent with earlier estimates. The implications of the measurements for breaking in the eld are discussed. Based on geometrical optics and wave action conservation, we suggest that the presence of the breaking-induced vortex provides an explanation for the suppression of short waves by breaking. Finally, in Appendices, estimates of the majority of the terms in the turbulent kinetic energy budget are presented at an early stage in the evolution of the turbulence, and comparisons with independent acoustical measurements of breaking are presented.

Distribution of breaking waves at the ocean surface

Abstract: Surface waves play an important role in the exchange of mass, momentum and energy between the atmosphere and the ocean. The development of the wave field depends on wind, wave-wave and wave-current interactions and wave dissipation owing to breaking, which is accompanied by momentum fluxes from waves to currents. Wave breaking supports air-sea fluxes of heat and gas, which have a profound effect on weather and climate. But wave breaking is poorly quantified and understood. Here we present measurements of wave breaking, using aerial imaging and analysis, and provide a statistical description of related sea-surface processes. We find that the distribution of the length of breaking fronts per unit area of sea surface is proportional to the cube of the wind speed and that, within the measured range of the speed of the wave fronts, the length of breaking fronts per unit area is an exponential function of the speed of the front. We also find that the fraction of the ocean surface mixed by breaking waves, which is important for air-sea exchange, is dominated by wave breaking at low velocities and short wavelengths.

Numerical Experiments on the Breaking of Solitary Internal Wavesover a Slope–Shelf Topography

Abstract: A theoretical study of the transformation of large amplitude internal solitary waves (ISW) of permanent form over a slope–shelf topography is considered using as basis the Reynolds equations. The vertical fluid stratification, amplitudes of the propagating ISWs, and the bottom parameters were taken close to those observed in the Andaman and Sulu Seas. The problem was solved numerically. It was found that, when an intense ISW of depression propagates from a deep part of a basin onto the shelf with water depth Hs, a breaking event will arise whenever the wave amplitude am is larger than 0.4(Hs − Hm), where Hm is the undisturbed depth of the isopycnal of maximum depression. The cumulative effect of nonlinearity in a propagating ISW leads to a steepening and overturning of a rear wave face over the inclined bottom. Immediately before breaking the horizontal orbital velocity at the site of instability exceeds the phase speed of the ISW. So, the strong breaking is caused by a kinematic instability of t...

On the distortion of turbulence by a progressive surface wave

Abstract: A rapid-distortion model is developed to investigate the interaction of weak turbulence with a monochromatic irrotational surface water wave. The model is applicable when the orbital velocity of the wave is larger than the turbulence intensity, and when the slope of the wave is sufficiently high that the straining of the turbulence by the wave dominates over the straining of the turbulence by itself. The turbulence suffers two distortions. Firstly, vorticity in the turbulence is modulated by the wave orbital motions, which leads to the streamwise Reynolds stress attaining maxima at the wave crests and minima at the wave troughs; the Reynolds stress normal to the free surface develops minima at the wave crests and maxima at the troughs. Secondly, over several wave cycles the Stokes drift associated with the wave tilts vertical vorticity into the horizontal direction, subsequently stretching it into elongated streamwise vortices, which come to dominate the flow. These results are shown to be strikingly different from turbulence distorted by a mean shear flow, when `streaky structures' of high and low streamwise velocity fluctuations develop. It is shown that, in the case of distortion by a mean shear flow, the tendency for the mean shear to produce streamwise vortices by distortion of the turbulent vorticity is largely cancelled by a distortion of the mean vorticity by the turbulent fluctuations. This latter process is absent in distortion by Stokes drift, since there is then no mean vorticity. The components of the Reynolds stress and the integral length scales computed from turbulence distorted by Stokes drift show the same behaviour as in the simulations of Langmuir turbulence reported by McWilliams, Sullivan & Moeng (1997). Hence we suggest that turbulent vorticity in the upper ocean, such as produced by breaking waves, may help to provide the initial seeds for Langmuir circulations, thereby complementing the shear-flow instability mechanism developed by Craik & Leibovich (1976). The tilting of the vertical vorticity into the horizontal by the Stokes drift tends also to produce a shear stress that does work against the mean straining associated with the wave orbital motions. The turbulent kinetic energy then increases at the expense of energy in the wave. Hence the wave decays. An expression for the wave attenuation rate is obtained by scaling the equation for the wave energy, and is found to be broadly consistent with available laboratory data.

Experimental study of nearshore dynamics on a barred beach with rip channels

Abstract: [1] Wave and current measurements are presented from a set of laboratory experiments performed on a fixed barred beach with periodically spaced rip channels using a range of incident wave conditions. The data demonstrate that the presence of gaps in otherwise longshore uniform bars dominates the nearshore circulation system for the incident wave conditions considered. For example, nonzero cross-shore flow and the presence of longshore pressure gradients, both resulting from the presence of rip channels, are not restricted to the immediate vicinity of the channels but instead are found to span almost the entire length of the longshore bars. In addition, the combination of breaker type and location is the dominant driving mechanism of the nearshore flow, and both are found to be strongly influenced by the variable bathymetry and the presence of a strong rip current. The depth-averaged currents are calculated from the measured velocities assuming conservation of mass across the measurement grid. The terms in both the cross-shore and longshore momentum balances are calculated, and their relative magnitudes are quantified. The cross-shore balance is shown to be dominated by the cross-shore pressure and radiation stress gradients in general agreement with previous results, however, the rip current is shown to influence the wave breaking and the wave-induced setup in the rip channel. Analysis of the longshore balance shows that the overall strength of the longshore feeder currents is governed by a balance between longshore pressure gradient forcing and an opposing radiation stress gradient. In addition, the longshore feeder currents show maxima in the bar trough region, providing experimental evidence that longshore pressure gradients can shift longshore current maxima shoreward from the bar crest. Finally, since the longshore radiation stress gradient in the surf zone is governed by the amount of wave dissipation on the rip current, there exists a positive feedback mechanism whereby a strong rip current can weaken the radiation stress gradient opposing the feeder currents and lead to even stronger feeder currents and rips.

New method for numerical simulation of a nonstationary potential flow of incompressible fluid with a free surface

Abstract: New method for numerical simulation of potential flows with a free surface of two-dimensional fluid, based on combination of the conformal mapping and Fourier Transform is proposed. The method is efficient for study of strongly nonlinear effects in gravity waves including wave breaking and formation of rogue waves.

Full waveform numerical simulations of seismoelectromagnetic wave conversions in fluid‐saturated stratified porous media

Abstract: [1] We present a full-waveform modeling technique of the coupled seismoelectromagnetic wave propagation in fluid-saturated stratified porous media. Our simulation code uses the macroscopic governing equations derived by Pride [1994], which couple Biot's theory and Maxwell equations via flux/force transport equations. In this theory the coupling mechanism is explained by electrokinetic effects taking place at the pore level. The synthetic seismoelectrograms and seismomagnetrograms are computed by extending the generalized reflection and transmission matrix method and by using a discrete wave number integration of the global reflectivity obtained in the frequency wave number domain. Synthetic time sections and snapshots of the wave propagation are used to study the seismic, electromagnetic, and seismoelectromagnetic waves properties in fluid-saturated layered porous media. Two wave phenomena are investigated: (1) the electric and magnetic fields induced by the propagation of a seismic perturbation in a homogeneous porous medium and (2) the electromagnetic waves generated at depth when seismic waves propagate through a vertically heterogeneous porous medium. Concentrating on the second effect, we show that the zone which effectively contributes to the generation of EM disturbances along a plane interface coincides with the first Fresnel zone associated with a seismic-to-electromagnetic wave conversion. A numerical sensitivity study shows that the EM waves generated at depth by the passage of seismic waves through an interface are particularly sensitive to contrasts in porosity, permeability, fluid salinity, and fluid viscosity. Our numerical simulations highlight the potential of artificially generated seismoelectromagnetic converted waves for the characterization of the subsurface and its fluid content.

A numerical investigation of solitary internal waves with trapped cores formed via shoaling

Abstract: The formation of solitary internal waves with trapped cores via shoaling is investigated numerically. For density fields for which the buoyancy frequency increases monotonically towards the surface, sufficiently large solitary waves break as they shoal and form solitary-like waves with trapped fluid cores. Properties of large-amplitude waves are shown to be sensitive to the near-surface stratification. For the monotonic stratifications considered, waves with open streamlines are limited in amplitude by the breaking limit (maximum horizontal velocity equals wave propagation speed). When an exponential density stratification is modified to include a thin surface mixed layer, wave amplitudes are limited by the conjugate flow limit, in which case waves become long and horizontally uniform in the centre. The maximum horizontal velocity in the limiting wave is much less than the wave's propagation speed and as a consequence, waves with trapped cores are not formed in the presence of the surface mixed layer.

Asymptotic modal approximation of nonlinear resonant sloshing in a rectangular tank with small fluid depth

Abstract: The modal system describing nonlinear sloshing with inviscid flows in a rectangular rigid tank is revised to match both shallow fluid and secondary (internal) resonance asymptotics. The main goal is to examine nonlinear resonant waves for intermediate depth/breadth ratio 0.1 [lsim ] h/l [lsim ] 0.24 forced by surge/pitch excitation with frequency in the vicinity of the lowest natural frequency. The revised modal equations take full account of nonlinearities up to fourth-order polynomial terms in generalized coordinates and h/l and may be treated as a modal Boussinesq-type theory. The system is truncated with a high number of modes and shows good agreement with experimental data by Rognebakke (1998) for transient motions, where previous finite depth modal theories failed. However, difficulties may occur when experiments show significant energy dissipation associated with run-up at the walls and wave breaking. After reviewing published results on damping rates for lower and higher modes, the linear damping terms due to the linear laminar boundary layer near the tank's surface and viscosity in the fluid bulk are incorporated. This improves the simulation of transient motions. The steady-state response agrees well with experiments by Chester & Bones (1968) for shallow water, and Abramson et al. (1974), Olsen & Johnsen (1975) for intermediate fluid depths. When h/l [lsim ] 0.05, convergence problems associated with increasing the dimension of the modal system are reported.

Large fully nonlinear internal solitary waves: The effect of background current

Abstract: In this paper we consider what effect the presence of a nonconstant background current has on the properties of large, fully nonlinear solitary internal waves in a shallow, stratified ocean. In particular, we discuss how the amplitude of the largest nonbreaking wave that it is possible to calculate depends on the background current as well as the nature of the upper bound. We find that the maximum wave amplitude is given by one of three possibilities: The onset of wave breaking, the conjugate flow amplitude or a failure of the wave calculating algorithm to converge (associated with shear instability). We also discuss how wave properties such as propagation speed, half-width, etc. vary with background current amplitude.

Non-breaking and breaking solitary wave run-up

Abstract: The run-up of non-breaking and breaking solitary waves on a uniform plane beach connected to a constant-depth wave tank was investigated experimentally and numerically. If only the general characteristics of the run-up process and the maximum run-up are of interest, for the case of a breaking wave the post-breaking condition can be simplified and represented as a propagating bore. A numerical model using this bore structure to treat the process of wave breaking and subsequent shoreward propagation was developed. The nonlinear shallow water equations (NLSW) were solved using the weighted essentially non-oscillatory (WENO) shock capturing scheme employed in gas dynamics. Wave breaking and post-breaking propagation are handled automatically by this scheme and ad hoc terms are not required. A computational domain mapping technique was used to model the shoreline movement. This numerical scheme was found to provide a relatively simple and reasonably good prediction of various aspects of the run-up process. The energy dissipation associated with wave breaking of solitary wave run-up (excluding the effects of bottom friction) was also estimated using the results from the numerical model.

On Determining the Onset and Strength of Breaking for Deep Water Waves. Part I: Unforced Irrotational Wave Groups

Abstract: Finding a robust threshold variable that determines the onset of breaking for deep water waves has been an elusive problem for many decades. Recent numerical studies of the unforced evolution of two-dimensional nonlinear wave trains have highlighted the complex evolution to recurrence or breaking, together with the fundamental role played by nonlinear intrawave group dynamics. In Part I of this paper the scope of two-dimensional nonlinear wave group calculations is extended by using a wave-group-following approach applied to a wider class of initial wave group geometries, with the primary goal of identifying the differences between evolution to recurrence and to breaking onset. Part II examines the additional influences of wind forcing and background shear on these evolution processes. The present investigation focuses on the long-term evolution of the maximum of the local energy density along wave groups. It contributes a more complete picture, both long-term and short-term, of the approach to b...

On temperature inversions and the mesospheric surf zone

Abstract: [1] Mesospheric thermal inversions are investigated in a numerical simulation with the Whole Atmosphere Community Climate Model, an upward extension of the National Center for Atmospheric Research's Community Climate Model. The seasonal character, spatial extent, and magnitude of the inversion layers are realistic during winter. In the model, the occurrence of wintertime inversions is a direct consequence of the rapid decay with height of vertically propagating planetary waves, which induces large temperature perturbations in the upper mesosphere to maintain hydrostatic equilibrium. The magnitude of the inversions is highly correlated with planetary wave amplitude, so that large inversions develop during episodes of planetary wave amplification. Gravity waves do not play a major direct role in the formation of the inversions because the largest thermal tendencies associated with gravity wave breaking occur well above the range of altitudes where inversions are found. However, gravity waves play an essential indirect role because they set up a critical line in the upper mesosphere where Rossby waves break in the mesospheric surf zone.

Monochromatic and random wave breaking at blocking points

Abstract: [1] In this paper we study the energy dissipation due to current-limited wave breaking in monochromatic and random waves with the help of experimental tests. The opposing currents are strong enough for wave blocking to occur. A modified bore model is used to simulate the dissipation rate in the monochromatic waves, and an empirical bulk dissipation formula for wave breaking in random waves is proposed. The effects of wave blocking on the dynamics of the wave field are also discussed. INDEX TERMS: 4546 Oceanography: Physical: Nearshore processes; 4560 Oceanography: Physical: Surface waves and tides (1255); 4512 Oceanography: Physical: Currents; 4528 Oceanography: Physical: Fronts and jets; KEYWORDS: wave-current interaction, wave breaking, wave action conservation, nonlinear dispersion, inlets

Multiscale Measurements of Ocean Wave Breaking Probability

Abstract: Recent numerical model studies of nonlinear deep water wave group evolution suggest that wave breaking onset is associated primarily with a threshold behavior linked to the nonlinear wave group hydrodynamics Motivated by these findings, a recently published probability analysis of observed dominant ocean wind wave breaking events reported a threshold behavior using the significant wave steepness as a measure of the mean nonlinearity of these waves The present study investigates whether a similar threshold dependence in terms of an appropriate spectral measure of wave steepness, the spectral saturation, may be found for the breaking probability of shorter wind waves above the spectral peak Extensive data records of open ocean whitecap breaking wave occurrences for wind speeds up to 18 m s−1 were analyzed for breaking probability dependence on spectral saturation in spectral bands with center frequencies ranging from 1 to 248 times the spectral peak frequency Results are based on the measured

Breaking criteria and energy losses for three‐dimensional wave breaking

Abstract: [1] Laboratory experiments were used to explore the influence of spatial focusing and diffraction on the evolution of unsteady, three-dimensional, deep water wave packets with a constant-steepness spectrum. The wave packets were generated by 13 independently programmed paddles and evolved to breaking near the midpoint of a 4 m × 11 m test section. Detailed measurements of surface displacements were made across the entire test section and were used to examine energy losses and breaking criteria. Three forms of breaking criteria were considered: (1) geometric criteria based on local and global wave steepness, (2) a kinematic criterion based on particle and phase velocities, and (3) a dynamic criterion based on higher harmonic energy evolution. The results indicate that directionality of the waves can either increase (focusing waves) or decrease (diffracting waves) the geometric breaking criterion as well as breaking severity. In contrast, the directionality of waves had little effect on the kinematic criterion. At breaking, the ratio of local particle velocity and phase velocity was shown to be larger than unity for both focusing and diffracting waves. Indeed, the robustness and simplicity of the kinematic criterion make it an excellent choice for field application. Finally, the directionality of waves did not alter the up-frequency energy transfer associated with wave steepening. The three-dimensional, spatially focusing and diffracting wave packets lost 34% and 18% of their energy, respectively, as a result of plunging breakers and lost 12% and 9%, respectively, as a result of spilling breakers. Comparable two-dimensional breakers with the same spectral shape lost 16% for plunging and 12% for spilling.

Convectively generated mesoscale gravity waves simulated throughout the middle atmosphere

Abstract: [1] A three-dimensional simulation was conducted with a cloud-resolving model to investigate mesoscale gravity waves generated by cumulus convection that propagate to the mesospheric and lower thermospheric (MLT) region It is found that both individual convective turrets and mesoscale convective systems excite gravity waves, resulting in a broad scale distribution Waves excited by the former have conically shaped phase surfaces as previously reported and are conspicuous up to around the stratopause Waves excited by the latter dominate in the MLT region Frequent wave breaking is found above 85 km, where convective instability is found typically in the initial stage, leading to unstable shear with billows resembling the ripple-type structures observed frequently in airglow imaging

Experimental Investigation of Wave Breaking Criteria Based on Wave Phase Speeds

Abstract: Experiments were performed that test the kinematic breaking criterion, which states that the horizontal fluid particle velocity at the surface of a crest exceeds the local phase speed of the crest prior to breaking. Three different definitions of phase speed are used to calculate phase velocities of the wave crests from detailed surface elevation measurements. The first definition, based on the equivalent linear wave, is constant over the wavelength of the wave. The second, based on partial Hilbert transforms of the surface elevation data, is local in space and time giving instantaneous values at all space and time measurements. The third, based on the speed of the position of the crest maximum, is local in time but not in space. Particle image velocimetry is used to obtain horizontal components of fluid particle velocity at the surface of crests of breaking and nonbreaking waves produced in a wave flume using the chirp-pulse focusing technique. The ratios of these fluid particle speeds to crest phase speeds are calculated and are consistently less than unity. The speed ratio for the rounded crest of a plunging breaker is at most 0.81, implying that the kinematic breaking criterion is far from satisfied. The ratio for the more sharply pointed crest of a spilling breaker is at most 0.95, implying that the kinematic breaking criterion is closer to being satisfied. Thus, for the breaking waves in this study the kinematic breaking criterion is not satisfied for any of the three definitions of phase speed and so it cannot be regarded as a universal predictor of wave breaking.

Long-wave forcing by the breaking of random gravity waves on a beach

Abstract: This paper presents new laboratory data on long-wave (surf-beat) forcing by the random breaking of shorter gravity water waves on a plane beach. The data include incident and outgoing wave amplitudes, together with shoreline oscillation amplitudes at long-wave frequencies, from which the correlation between forced long waves and short-wave groups is examined. A detailed analysis of the cross-shore structure of the long-wave motion is presented, and the observations are critically compared with existing theories for two-dimensional surf-beat generation. The surf beat shows a strong dependency on normalized surf-zone width, consistent with long-wave forcing by a time-varying breakpoint, with little evidence of the release and reflection of incident bound long waves for the random-wave simulations considered. The seaward-propagating long waves show a positive correlation with incident short-wave groups and are linearly dependent on short-wave amplitude. The phase relationship between the incident bound long waves and radiated free long waves is also consistent with breakpoint forcing. In combination with previous work, the present data suggest that the breakpoint variability may be the dominant forcing mechanism during conditions with steep incident short waves.

Self-organization mechanisms for the formation of nearshore crescentic and transverse sand bars

Abstract: The formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow-water equations and on a simplified sediment transport parameterization. By assuming normally approaching waves and a saturated surf zone, rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed. Two types of bedforms appear: one is a crescentic bar pattern centred around the breakpoint and the other, herein modelled for the first time, is a transverse bar pattern. The feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations. Basically, the waves stir up the sediment and keep it in suspension with a certain cross-shore distribution of depth-averaged concentration. Then, a current flowing with (against) the gradient of sediment concentration produces erosion (deposition). It is shown that inside the surf zone, these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms. Numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro-morphodynamic input conditions to which of the patterns develops. It is suggested that crescentic bar growth would be favoured by high-energy conditions and fine sediment while transverse bars would grow for milder waves and coarser sediment. In intermediate conditions mixed patterns may occur.

Video‐imaged surf zone wave and roller structures and flow fields

Abstract: [1] The measurement of mean water levels, roller geometries, and phase ensemble-averaged velocity and turbulence intensity fields under spilling and plunging waves breaking in a two-dimensional laboratory surf zone is presented. The velocities were measured using digital correlation image velocimetry, while water levels and roller geometries were determined through gray scale filtering of video images. The phase ensemble-averaged horizontal and vertical components of velocity and turbulence intensities are measured throughout the entire flow domain, including the wave roller area, by utilizing the aerated areas as part of the flow structure. The time-averaged horizontal velocities (undertow), turbulence intensities, and turbulent kinetic energies are determined by averaging across the wave phase. Turbulence magnitudes are found to compare favorably with existing laser Doppler anemometry measurements below the wave trough level, where such measurements have generally been confined because of aeration contamination effects. The significantly higher velocity and turbulence intensity magnitudes measured above the trough level in the present experiments highlight the novel nature of the present investigation for describing flow regimes in the surf zone.