Breaking Wave Characteristics Research Articles

IMAGE-BASED STUDY OF BREAKING AND BROKEN WAVE CHARACTERISTICS IN FRONT OF THE SEAWALL

This study aims to study the breaking and broken wave characteristics in front of the vertical seawall. Laboratory experiments were performed to represent such phenomena. A physical model of a vertical seawall installed on the mild slope was made in the wave flume. Image-based measuring technique was developed and applied to capture the wave characteristics as high-resolution data sets both in spatial and temporal domains. By analyzing the high-resolution data, behaviors of partial standing waves were successfully captured in front of the seawall. Comparisons of obtained data with and without seawall clearly showed the difference in cross-shore distributions of wave height, mean water level and skewness while no clear difference was observed in asymmetry. Obtained experimental data was then used to validate the applicability of Boussinesq wave model attached with existing breaking models. It was found that both viscosity-type and surface roller-type breaking models tended to overestimate the energy dissipation in front of the seawall.

REMOTE MEASUREMENT AND PREDICTION OF BREAKING WAVE PARAMETERS

The analysis of wave breaking in shallow water has been on-going for almost 150 years. Numerous research papers have been published that investigate methods to predict breaking conditions and the geometric characteristics of breaking waves. This study presents a novel, safe, and low cost method to extract breaking wave properties from irregular waves in the surf zone, using optical and in-situ measurement systems. Sensitivities studies on methods of measuring the breaking water depth are compared and the water depth at the wave trough depth, corrected for optical offsets using a still water correction of 1/3 wave height, is found to be exhibit the least variability. A new effective seafloor slope definition, based on individual breaking wavelength to depth ratios, was found to increase predictive ability over previously variable seafloor slope extraction methods. Collected field data is compared against established breaking wave height formulas with general exponential form consistently finding best correlation. An optimized breaking wave height predictor featured a root mean square relative error of only 1.672% against the measured dataset. Finally, the study of the geometric shape of the plunging wave vortex as a possible indicator for the breaking intensity of ocean waves has been ongoing for almost 50 years with limited success. The validity of using the vortex ratio and vortex angle as a method of predicting breaking intensity is examined. Through the first complete analysis of field collected irregular wave breaking vortex parameters it is illustrated that the vortex ratio and vortex angle cannot be accurately predicted using standard breaking wave characteristics and hence are not suggested as a possible indicator for breaking intensity

Study Based on FLUENT of Broken Internal Waves in Different Slope Angles

Internal waves will break in the process of communication, the broken will make water in upper and lower mixing, which has significant influence on the hydrodynamic and layered characteristics of density stratification of the water. In order to reveal the propagation of internal solitary waves, a 3d numerical wave flume was built. The research of the propagation of internal solitary waves in the regular topography and broken on slopes was based on FLUENT. Comparing the fragmentation degree of different slope angle and researching the energy dissipation of the wave propagation process , which are supposed to successfully match the results with the experiment results, can provide new methods and means for the further study of internal wave breaking characteristics and the improvement of ecological environment of water bodies.

Ultrashort pulse breaking in optical fiber with third-order dispersion and quintic nonlinearity

The optical wave breaking (OWB) characteristics in terms of the pulse shape, spectrum, and frequency chirp, in the normal dispersion regime of an optical fiber with both the third-order dispersion (TOD) and quintic nonlinearity (QN) are numerically calculated. The results show that the TOD causes the asymmetry of the temporal- and spectral-domain, and the chirp characteristics. The OWB generally appears near the pulse center and at the trailing edge of the pulse, instead of at the two edges of the pulse symmetrically in the case of no TOD. With the increase of distance, the relation of OWB to the TOD near the pulse center increases quickly, leading to the generation of ultra-short pulse trains, while the OWB resulting from the case of no TOD at the trailing edge of the pulse disappears gradually. In addition, the positive (negative) QN enhances (weakens) the chirp amount and the fine structures, thereby inducing the OWB phenomena to appear earlier (later). Thus, the TOD and the positive (negative) QN are beneficial (detrimental) to the OWB and the generation of ultra-short pulse trains.

Wave-Breaking Characteristics of Northern Hemisphere Winter Blocking: A Two-Dimensional Approach

Abstract This paper generalizes and applies recently developed blocking diagnostics in a two-dimensional (2D) latitude–longitude context, which takes into consideration both mid- and high-latitude blocking. These diagnostics identify characteristics of the associated wave breaking as seen in the potential temperature θ on the dynamical tropopause, particularly the cyclonic or anticyclonic direction of wave breaking (“DB index”) and the relative intensity (“RI index”) of the air masses that contribute to blocking formation. The methodology is extended to a 2D domain and a cluster technique is deployed to classify mid- and high-latitude blocking according to the wave-breaking characteristics. Midlatitude blocking is observed over Europe and Asia, where the meridional gradient of θ is generally weak, whereas high-latitude blocking is mainly present over the oceans, to the north of the jet stream, where the meridional gradient of θ is much stronger. They occur on the equatorward and poleward flank of the jet stream, respectively, where the horizontal shear ∂u/∂y is positive in the first case and negative in the second case. A regional analysis is also conducted. Warm-cyclonic blocking over the Pacific and cold-anticyclonic blocking over Europe are identified as the most persistent types and are associated with large amplitude anomalies in temperature and precipitation. Finally, the high-latitude cyclonic events seem to correlate well with low-frequency modes of variability over the Pacific and Atlantic Oceans.

Prediction formula for longshore sediment transport rate with M5' algorithm

ABSTRACT Mafi, S., Yeganeh-Bakhtiary, A., Kazeminezhad, M.H., 2013. Prediction formula for longshore sediment transport rate with M5' algorithm One of the most vital tasks for coastal engineers is to calculate the gross longshore sediment transport rate (LSTR) to control the shoreline erosion and beach evolution. In the past decades, several empirical formulas or parametric models have been proposed for predicting the gross LSTR as a function of the breaking wave characteristics, bed materials and beach slope. The main downside with these formulas is that they give wide range of different rate of predictions; consequently their reliability under the changing hydrodynamic conditions is very uncertain. In the present study, an alternative approach based on the Regression Trees (M5') was applied to present a new formula for prediction of LSTR in terms of the longshore sediment transport coefficient (e). Several high-quality data sets were employed, which comprise of the wave parameters, sediment characterist...

WAVE PROPAGATION ON A FLUME: PHYSICAL MODELLING

The knowledge of wave transformation and breaking characteristics near coastline is essential for the nearshore hydrodynamics and the design of coastal structures. This paper describes a wide range of wave flume tests performed at the National Laboratory for Civil Engineering (LNEC), located in Lisbon (Portugal), which main objective was to study wave shoaling and breaking over a set of different gentle slopes for several incident waves and thus to contribute for a better understand of the hydrodynamics of wave transformation.The experimental conditions, the measurement equipment, the incident wave characteristics, the type of measurements performed (free surface elevation and particle velocity) and the data obtained are described. Time and spectral analysis based upon the measured data are also performed and presented. For a regular wave with a period of 1.5s and a height of 0.1m are presented and discussed the following results: free surface elevation at selected sections along the flume; the spectral analysis; the significant wave height and average period along the flume; the particle velocity components at different locations along the flume; the average, maximum and minimum values of the longitudinal component of the velocity along the flume; the two dimensional distribution of the three components of the velocity; and longitudinal velocity component vertical profiles.

A corrected 3-D SPH method for breaking tsunami wave modelling

A 3-D large eddy simulation model that was first transformed to smoothed particle hydrodynamics (LES-SPH)-based model was employed to study breaking tsunami waves in this paper. LES-SPH is a gridless (or mesh-free), purely Lagrangian particle approach which is capable of tracking the free surface of violent deformation with fragmentation in an easy and accurate way. The Smagorinsky closure model is used to simulate the turbulence due to its simplicity and effectiveness. The Sub-Particle Scale scheme, plus the link-list algorithm, is applied to reduce the demand of computational power. The computational results show that the 3-D LES-SPH model can capture well the breaking wave characteristics. Spatial evolution features of breaking wave are presented and visualized. The detailed mechanisms of turbulence transport and vorticity dynamics are demonstrated as well. This application also presents an example to validate the SPH model.

Prediction of Wave Breaking on a Gravel Beach by an Artificial Neural Network

Abstract A great many studies on wave breaking have been carried out, and much experimental and field data have been documented. Moreover, on the basis of various data sets, many empirical formulas based primarily on regression analysis have been proposed to quantitatively estimate wave breaking for engineering applications. However, wave breaking has an inherent variability, which suggests that a linear statistical approach such as regression analysis might be inadequate. This study presents an alternative nonlinear method using an artificial neural network (ANN), one of the soft computing methods, for predicting breaking-wave heights and water depths. Using data from laboratory experiments showing that wave breaking characteristics on a gravel beach are different from those on a sandy beach, we developed a three-layered feed-forward type of network to obtain the output of wave-breaking heights and water depths using deepwater heights, wave periods, and seabed conditions as inputs. In particular, the eff...

Large‐scale laboratory observations of wave breaking turbulence over an evolving beach

Wave breaking turbulence over an evolving beach was observed in a large‐scale laboratory flume, as part of the CROss‐Shore Sediment Transport EXperiment (CROSSTEX). The data set included comprehensive measurements of water surface elevation, fluid velocity, and morphology for irregular waves under erosive and accretive wave conditions. For the both conditions, the beach reached a quasi‐equilibrium state, defined as when the bar shape was stable. Wave breaking characteristics, such as wave heights, average rate of energy dissipation by bores, and surf similarity parameter, were investigated in response to morphodynamics of the bar. Turbulent kinetic energy (TKE) was estimated using the method by Shaw and Trowbridge (2001). As the beach evolved, a less amount of TKE was observed at the trough for the erosive case, while more TKE was observed at the trough for the accretive case. It was also found that the temporal variation of the time‐averaged TKE were closely associated with the average rate of energy dissipation by bores. Comparing with the bar trough, the vertical distribution of nondimensionalized time‐averaged TKE and turbulence dissipation rate at the bar crest showed a large increase near the bottom, probably due to a strong cross‐shore RMS velocity. Finally, in the quasi‐equilibrium state, time‐averaged TKE, and turbulence dissipation rate at the bar trough were smaller than those inside the surf zone. Inside the surf zone, significant turbulence intensities were observed due to a second breaking on a shallow water depth.

The Impact of ENSO on Wave Breaking and Southern Annular Mode Events

Abstract This study examines the relationship between intraseasonal southern annular mode (SAM) events and the El Niño–Southern Oscillation (ENSO) using daily 40-yr ECMWF Re-Analysis (ERA-40) data. The data coverage spans the years 1979–2002, for the austral spring and summer seasons. The focus of this study is on the question of why positive SAM events dominate during La Niña and negative SAM events during El Niño. A composite analysis is performed on the zonal-mean zonal wind, Eliassen–Palm fluxes, and two diagnostic variables: the meridional potential vorticity gradient, a zonal-mean quantity that is used to estimate the likelihood of wave breaking, and the wave breaking index (WBI), which is used to evaluate the strength of the wave breaking. The results of this investigation suggest that the background zonal-mean flow associated with La Niña (El Niño) is preconditioned for strong (weak) anticyclonic wave breaking on the equatorward side of the eddy-driven jet, the type of wave breaking that is found to drive positive (negative) SAM events. A probability density function analysis of the WBI, for both La Niña and El Niño, indicates that strong anticyclonic wave breaking takes place much more frequently during La Niña and weak anticyclonic wave breaking during El Niño. It is suggested that these wave breaking characteristics, and their dependency on the background flow, can explain the strong preference for SAM events of one phase during ENSO. The analysis also shows that austral spring SAM events that coincide with ENSO are preceded by strong stratospheric SAM anomalies and then are followed by a prolonged period of wave breaking that lasts for approximately 30 days. These findings suggest that the ENSO background flow also plays a role in the excitation of stratospheric SAM anomalies and that the presence of these stratospheric SAM anomalies in turn excites and then maintains the tropospheric SAM anomalies via a positive eddy feedback.

A note on mixing due to surface wave breaking

Breaking of surface gravity waves generates turbulence and causes the entrainment of air at the sea surface, which again may lead to a temperature increase in the breaking region. These processes influence the near‐surface buoyancy and generate a thin layer close to the surface which is statically stable. Dynamically, the wave breaking itself (the loss of mean wave momentum) induces an Eulerian mean current near the sea surface through the action of the virtual wave stress. This current is instrumental in mixing the turbulence induced by breaking downward. The mixing is affected by the fact that slightly denser particles are entrained into the stable top layer from below during this process. Utilizing the experimental result that the overall Richardson number is constant in similar mixing problems, we investigate theoretically the growth of the mixed layer due to breaking waves. The results show that the mixing due to a single breaking event penetrates down to a depth which is comparable to the wave height. At this depth the entrainment velocity practically vanishes. This occurs on a timescale that is of the order of the breaking duration time. For multiple breaking events in a steady homogeneous sea state, the breaking frequency is so small that the turbulence due to breaking will not diffuse further into the fluid. Associating the thickness of the breaking mixed layer with a typical roughness length, we find that the roughness length can be expressed in terms of the breaking wave characteristics.

Aeration and bubble measurements of coastal breaking waves

The air entrainment process of surf zone waves is studied experimentally to understand as a first step of two-phase characteristics of surf zone breaking waves. A set of laboratory experiments of free surface elevation, water velocity, void fraction and bubble distribution is conducted simultaneously for regular gravity wave breaking on a plane slope. The in situ data are collected by using a measurement array of wave gages, a dual-tip resistivity void fraction probe and an acoustic Doppler velocimeter. Two-dimensional projected bubble size measurements are conduced by the high speed camera with imaging technique. The experimental data show a linear relationship between the void fraction and turbulent intensity. In addition, the bubble size distributions are proportional to the bubble size to the power of - 1 and - 3.4 independent of the distance from breaking point and water depth. The length scale separating two power laws is the Hinze scale which corresponds to the wave energy dissipation scale. This result will be important for modeling and implication for the study of surf zone dynamics.

Correlation of Breaking Wave Characteristics with Energy Dissipation

Correlation of Breaking Wave Characteristics with Energy Dissipation

Post-tsunami changes in the littoral environment along the southeast coast of India

The 26th December 2004 Indian Ocean tsunami devastated coastal regions of the Indian subcontinent. Andaman and Nicobar Islands, coastal stretches of Tamil Nadu, Andhra Pradesh and Kerala were the most affected regions of India. Changes in the beach profiles, long shore currents, breaking wave characteristics in the surf zone at selected locations along the Tamil Nadu coast were studied during January, April, October 2005 and January 2006. Long shore sediment transport rates were estimated from the observed parameters. Studies were carried out earlier (1995–1996 and 1998) to understand the coastal environment over a one-year cycle in the study region. The post-tsunami observations were compared with the earlier studies to establish the variations in the littoral environment and to ascertain the normalcy of the littoral environment in the post-tsunami scenario. From the changes in the beach profiles, the shoreline was observed to recede by about 20 m and built-up of backshore by about 0.5 m at most locations. Observations from the field investigations and comparisons with earlier studies along this stretch of the coastline indicate that the coastline is yet to return completely to normalcy.

Characteristics of the wind drift layer and microscale breaking waves

An experimental study, investigating the mean flow and turbulence in the wind drift layer formed beneath short wind waves was conducted. The degree to which these flows resemble the flows that occur in boundary layers adjacent to solid walls (i.e. wall-layers) was examined. Simultaneous DPIV (digital particle image velocimetry) and infrared imagery were used to investigate these near-surface flows at a fetch of 5.5 m and wind speeds from 4.5 to 11 m s−1. These conditions produced short steep waves with dominant wavelengths from 6 cm to 18 cm. The mean velocity profiles in the wind drift layer were found to be logarithmic and the flow was hydrodynamically smooth at all wind speeds. The rate of dissipation of turbulent kinetic energy was determined to be significantly greater in magnitude than would occur in a comparable wall-layer. Microscale breaking waves were detected using the DPIV data and the characteristics of breaking and non-breaking waves were compared. The percentage of microscale breaking waves increased abruptly from 11% to 80% as the wind speed increased from 4.5 to 7.4 m s−and then gradually increased to 90% as the wind speed increased to 11 m s−. At a depth of 1 mm, the rate of dissipation was 1.7 to 3.2 times greater beneath microscale breaking waves compared to non-breaking waves. In the crest–trough region beneath microscale breaking waves, 40% to 50% of the dissipation was associated with wave breaking. These results demonstrated that the enhanced near-surface turbulence in the wind drift layer was the result of microscale wave breaking. It was determined that the rate of dissipation of turbulent kinetic energy due to wave breaking is a function of depth, friction velocity, wave height and phase speed as proposed by Terrayet al. (1996). Vertical profiles of the rate of dissipation showed that beneath microscale breaking waves there were two distinct layers. Immediately beneath the surface, the dissipation decayed as ζ−0.7and below this in the second layer it decayed as ζ−2. The enhanced turbulence associated with microscale wave breaking was found to extend to a depth of approximately one significant wave height. The only similarity between the flows in these wind drift layers and wall-layers is that in both cases the mean velocity profiles are logarithmic. The fact that microscale breaking waves were responsible for 40%–50% of the near-surface turbulence supports the premise that microscale breaking waves play a significant role in enhancing the transfer of gas and heat across the air–sea interface.

Are the North Atlantic Oscillation and the Northern Annular Mode Distinguishable?

Abstract This study addresses the question of whether persistent events of the North Atlantic Oscillation (NAO) and the Northern Annular Mode (NAM) teleconnection patterns are distinguishable from each other. Standard daily index time series are used to specify the amplitude of the NAO and NAM patterns. The above question is examined with composites of sea level pressure, and 300- and 40-hPa streamfunction, along with tests of field significance. A null hypothesis is specified that the NAO and NAM persistent events are indistinguishable. This null hypothesis is evaluated by calculating the difference between time-averaged NAO and NAM composites. It is found that the null hypothesis cannot be rejected even at the 80% confidence level. The wave-breaking characteristics during the NAM life cycle are also examined. Both the positive and negative NAM phases yield the same wave-breaking properties as those for the NAO. The results suggest that not only are the NAO and NAM persistent events indistinguishable, but that the NAO/NAM events are neither confined to the North Atlantic, nor are they annular.

Simulation of breaking wave by SPH method coupled with k-ε model

The paper employs a Reynolds-averaged Navier–Stokes (RANS) approach to investigate the time-dependent wave breaking processes. The numerical model is the smoothed particle hydrodynamic (SPH) method. It is a mesh-free particle approach which is capable of tracking the free surfaces of large deformation in an easy and accurate way. The widely used two-equation k-ε model is chosen as the turbulence model to couple with the incompressible SPH scheme. The numerical model is employed to reproduce cnoidal wave breaking on a slope under two different breaking conditions–spilling and plunging. The computed free surface displacements, turbulence intensities and undertow profiles are in good agreement with the experimental data and other numerical results. According to the computations, the breaking wave characteristics are presented and discussed. It is shown that the SPH method provides a useful tool to investigate the surf zone dynamics.

Synoptic View of the North Atlantic Oscillation

This article investigates the synoptic characteristics of individual North Atlantic Oscillation (NAO) events by examining the daily evolution of the potential temperature field on the nominal tropopause (the 2-PVU surface). This quantity is obtained from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis dataset for the winter season. For both phases, the NAO is found to originate from synoptic-scale waves. As these waves evolve into the low-frequency NAO pattern, they break anticyclonically for the positive phase and cyclonically for the negative phase. The results of this analysis suggest that it is the remnants of these breaking waves that form the physical entity of the NAO. Throughout the NAO events, for both phases, the NAO is maintained by the successive breaking of upstream synoptic-scale waves. When synoptic-scale disturbances are no longer present, mixing processes play an important role in the NAO decay. As in other recent studies of the NAO, it is found that these individual NAO events complete their life cycle in a time period of about two weeks. Additional differences between the wave breaking characteristics of the two NAO phases are found. For the positive NAO phase, anticyclonic wave breaking takes place in two regions: one over the North Atlantic and the other near the North American west coast. For the negative NAO phase, on the other hand, there is a single breaking wave confined to the North Atlantic. An explanation based on kinematics is given to account for this difference.

Rossby Wave Breaking in the Southern Hemisphere Wintertime Upper Troposphere

Abstract The characteristics of Rossby wave propagation and breaking in the Southern Hemisphere upper troposphere during winter are examined. Although the Southern Hemisphere subtropical jet is more zonally symmetric than that of the Northern Hemisphere, there are still significant zonal variations in the upper-tropospheric flow. In particular, the flow within a given sector (≈120° longitude) can generally be characterized into one of four different configurations: (i) a single jet, (ii) a “broken” subtropical jet, (iii) a polar jet at the upstream end of the subtropical jet, or (iv) a polar jet at the downstream end of the subtropical jet. Using “potential vorticity thinking” and barotropic wind shear arguments, it is argued that the characteristics of the Rossby wave propagation and breaking will differ between each flow configuration. Consistent with these arguments, examination of potential vorticity maps and contour advection calculations show differing wave-breaking characteristics. In particular, t...