The Experts below are selected from a list of 312 Experts worldwide ranked by ideXlab platform
Francis C.k. Ting - One of the best experts on this subject based on the ideXlab platform.
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Laboratory study of wave and turbulence velocities in a broad-banded irregular wave surf zone
Coastal Engineering, 2001Co-Authors: Francis C.k. TingAbstract:The characteristics of wave and turbulence velocities created by a broad-banded irregular wave train breaking on a 1:35 slope were studied in a laboratory wave flume. Water particle velocities were measured simultaneously with wave elevations at three cross-shore locations inside the surf zone. The measured data were separated into low-frequency and high-frequency time series using a Fourier filter. The measured velocities were further separated into organized wave-induced velocities and turbulent velocity fluctuations by ensemble averaging. The broad-banded irregular waves created a wide surf zone that was dominated by spilling type breakers. A wave-by-wave analysis was carried out to obtain the probability distributions of individual wave heights, wave periods, peak wave velocities, and wave-averaged turbulent kinetic energies and Reynolds stresses. The results showed that there was a consistent increase in the kurtosis of the vertical velocity distribution from the surface to the bottom. The abnormally large downward velocities were produced by plunging breakers that occurred from time to time. It was found that the mean of the highest one-third wave-averaged turbulent kinetic energy values in the irregular waves was about the same as the time-averaged turbulent kinetic energy in a regular wave with similar deep-water wave height to wavelength ratio. It was also found that the correlation coefficient of the Reynolds stress varied strongly with turbulence intensity. Good correlation between u' and w' was obtained when the turbulence intensity was high; the correlation coefficient was about 0.3-0.5. The Reynolds stress correlation coefficient decreased over a wave cycle, and with distance from the water surface. Under the irregular breaking waves, turbulent kinetic energy was transported downward and landward by turbulent velocity fluctuations and wave velocities, and upward and seaward by the Undertow. The Undertow in the irregular waves was similar in vertical structure but lower in magnitude than in regular waves, and the horizontal velocity profiles under the low-frequency waves were approximately uniform. © 2001 Elsevier Science B.V. All rights reserved.
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LABORATORY STUDY OF SURF-ZONE TURBULENCE ON A BARRED BEACH
1995Co-Authors: Francis C.k. TingAbstract:Wave height, wave set-up, Undertow, and turbulent velocity on a plane beach and a barred beach were compared in the case of regular waves. The study showed that the presence of an offshore bar altered the turbulent flow in the surf zone by altering the characteristics of the broken waves. It was found that the magnitude of Undertow and turbulence intensity were smaller in the inner surf zone on the barred beach. These results suggest that it may be possible to reduce the erosive wave action on beaches by construction of underwater berms in the nearshore zone. Further studies are needed to determine the effects of water depth, berm width and crest elevation on surf-zone turbulence under different wave conditions in order to provide explicit design guidance.
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Observation of Undertow and turbulence in a laboratory surf zone
Coastal Engineering, 1994Co-Authors: Francis C.k. Ting, James T KirbyAbstract:Undertow and turbulence in the surf zone have been studied in a wave flume for a spilling breaker and a plunging breaker. Fluid velocities across a 1 on 35 sloped false bottom were measured using a fiber-optic laser-Doppler anemometer, and wave decay and set-up were measured using a capacitance wave gage. The characteristics of mean flow and turbulence in spilling versus plunging breakers were studied. The mean flow is the organized wave-induced flow defined as the phase average of the instantaneous velocity, while the turbulence is taken as the deviations from the phase average. It was found that under the plunging breaker turbulence levels are much higher and vertical variations of Undertow and turbulence intensity are much smaller in comparison with the spilling breaker. It was also found that turbulent kinetic energy is transported seaward under the spilling breaker and landward under the plunging breaker by the mean flow. The study indicates that there are fundamental differences in the dynamics of turbulence between spilling and plunging breakers, which can be related to the processes of wave breaking and turbulence production. It is suggested that the types of beach profile produced by storm and swell waves may be the results of different relationships between mean flow and turbulence in these waves. © 1994.
K A Rakha - One of the best experts on this subject based on the ideXlab platform.
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numerical study of different boundary conditions for Undertow models
Coastal Engineering Journal, 2003Co-Authors: K A RakhaAbstract:The Undertow current has an important effect on the cross-shore sediment transport rates. Several models for predicting the Undertow current have been developed over the last two decades. The closure of the Undertow model requires the specification of different boundary conditions. In this study, two main types of closures for Undertow models are studied using a phase-resolving and a phase-averaged wave model. These wave models are coupled with a phase-resolving hydrodynamic model for calculating the vertical distribution of the Undertow current.
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Wave-induced currents in the vicinity of a seawall
Coastal Engineering, 1997Co-Authors: K A Rakha, J. W. KamphuisAbstract:Wave-induced currents play an important role in the sediment motion. This papers provides a description of a wave-induced current module for the case of an infinite beach backed by a continuous seawall. Two formulations are proposed for the calculation of the depth-integrated currents. The first formulation includes the additional mixing induced by the interaction between the Undertow and the longshore current (UV). The second formulation assumes a higher eddy viscosity to replace this additional (UV) term. The vertical distribution for both the Undertow and the longshore current is also formulated. A sensitivity analysis and the verification of the numerical model is provided using tests performed in a wave flume and a wave basin. A description of a set of wave basin tests performed on a fixed beach is also provided. Both the Undertow and the longshore currents were measured for cases with and without reflection. The numerical and experimental results showed that the reflected wave reduced the Undertow, and had a small effect on the longshore current.
Nobuhisa Kobayashi - One of the best experts on this subject based on the ideXlab platform.
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application of an Undertow model to irregular waves on barred beaches
26th International Conference on Coastal Engineering, 1999Co-Authors: Douglas L Kennedy, Nobuhisa KobayashiAbstract:An Undertow model calibrated for regular waves on plane beaches is applied to predict the irregular wave induced Undertow for both plane and barred beaches and for both laboratory and field data sets. The model combines a logarithmic profile in the bottom boundary layer with a conventional parabolic profile in the interior. The height and period of the irregular waves are represented by the local root-mean-square wave height and spectral peak period, and the measured mean volume flux below trough level is used as input to the model. The model is capable of predicting the Undertow profiles both inside and outside the surf zone, provided that the empirical coefficient associated with the mean bottom shear stress is adjusted at each measuring line. The model appears to give reasonable predictions of the bottom boundary layer thickness and shear velocity, although these predictions could not be verified due to a lack of data. To develop a predictive Undertow model, a simple relationship with an adjustable coefficient is applied to predict the measured volume flux below trough level using the local wave height and water depth. The calibration coefficients involved in the predictive model are not universal among the lab and field conditions possibly due to the effects of wave directionality and longshore currents in the field measurements which are neglected in this paper.
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application of an Undertow model to irregular waves on plane and barred beaches
Journal of Coastal Research, 1998Co-Authors: Nobuhisa KobayashiAbstract:An Undertow model calibrated for regular waves on plane beaches is applied to predict the irregular wave induced Undertow for both plane and barred beaches and for both laboratory and field data sets. The model combines a logarithmic profile in the bottom boundary layer with a conventional parabolic profile in the interior. The height and period of the irregular waves are represented by the local root-mean-square wave height and spectral peak period, and the measured mean volume flux below trough level is used as input to the model. The model is capable of predicting the Undertow profiles both inside and outside the surf zone, provided that the empirical coefficient associated with the mean bottom shear stress is adjusted at each measuring line. The model appears to give reasonable predictions of the bottom boundary layer thickness and shear velocity, although these predictions could not be verified due to a lack of data. To develop a predictive Undertow model, a simple relationship with an adjustable coefficient is applied to predict the measured volume flux below trough level using the local wave height and water depth. The calibration coefficients involved in the predictive model are not universal among the lab and field conditions possibly due to the effects of wave directionality and longshore currents in the field measurements, which are neglected in this paper.
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Nearshore Wave and Circulation Modelling
1998Co-Authors: Robert A Dalrymple, Nobuhisa KobayashiAbstract:Abstract : Numerical models were developed to predict the propagation and transformation of the time-dependent wave field across the nearshore zone, nearshore circulation including longshore, Undertow and rip currents, and instantaneous and mean water levels including wave runup and swash on beaches. The inclusion of time-dependence permitted the study of evolving wave and current fields, including the generation of low frequency waves and shear instability of the currents. The developed two-dimensional and three-dimensional models were extensively validated by comparison to laboratory experiments conducted in this project and available field experiments. Equipment purchases and upgrades allowed the laboratory studies of breaking waves, and the associated turbulence, bottom friction, Undertow, swash, runup, overtopping, and overwash in the University of Delaware's Precision Wave Tank, Directional Wave Basin, and Sand Beach Wave Tank.
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kinematic Undertow model with logarithmic boundary layer
Journal of Waterway Port Coastal and Ocean Engineering-asce, 1997Co-Authors: Nobuhisa KobayashiAbstract:A new kinematic Undertow profile model is developed to relate the mean horizontal velocity, bottom shear stress, and boundary layer thickness in a simple but general manner. The model combines a logarithmic profile in the bottom boundary layer with a parabolic profile in the interior layer. Use of a logarithmic profile is justified using our laboratory measurements for regular waves spilling on a rough, impermeable slope. Two forms of the model are presented, each with one calibration coefficient associated with the mean bottom shear stress. By adjusting the calibration coefficient at each measuring line, the model is shown to be capable of predicting the measured Undertow profiles both inside and outside the surf zone for our rough slope case and for smooth slope cases from the literature. The model does not predict the overshoot in the bottom boundary layer for the rough slope case outside the surf zone. The predicted velocity profile for the smooth slope case in the bottom boundary could not be verifie...
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Undertow PROFILES IN THE BOTTOM BOUNDARY LAYER UNDER BREAKING WAVES
1997Co-Authors: Nobuhisa KobayashiAbstract:The vertical distribution of the mean shear stress inside the surf zone is compared to the terms in the time-averaged horizontal momentum equation using one set of laboratory measurements of the free surface elevations and fluid velocities u and w induced by regular waves spilling on a plane slope. The vertical distribution of the eddy viscosity is estimated directly from the measured mean shear stress and velocity. The shear stress distribution in the surf zone is shown to vary linearly with depth until the bottom boundary layer where it reached a nearly constant, negative value. The shear stress variation in the transition region differs distinctly from the inner surf zone. The vertical variation of uw is shown to be small outside the surf zone except near the bottom. Inside the surf zone, it is shown that the uw term of the horizontal momentum equation is likely to be important in the transition region and that its importance diminishes in the inner surf zone. The vertical distribution of the eddy viscosity has a form which is small near trough level, increases to a maximum value about one-third of the depth below trough level, and then decreases toward the bottom. The eddy viscosity in the middle of the bottom boundary layer is two orders of magnitude less than the eddy viscosity in the interior.
Akira Watanabe - One of the best experts on this subject based on the ideXlab platform.
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variations of orbital velocity and Undertow in the nearshore zone
27th International Conference on Coastal Engineering (ICCE), 2001Co-Authors: Akira Watanabe, Zakaria ElnaggarAbstract:A flow model is developed to predict the fluid velocity in the nearshore zone. The model is a combination of 1-D horizontal wave model with 1-D vertical model. A new combination is suggested at the trough level. A comparison with the experimental data is made to check the validity of the flow model. The model is used to study the orbital velocity variation for spilling and plunging breakers.
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field verification of a numerical model of beach topography change due to nearshore currents Undertow and waves
24th International Conference on Coastal Engineering, 1995Co-Authors: Takuzo Shimizu, Masahito Tsuru, Akira WatanabeAbstract:A practical method for estimating the Undertow velocity and its direction is developed and verified on the basis of field measurement data. It is found that a simple vector addition of nearshore current and Undertow gives a good approximation. The actual bottom topography change during a year is simulated by the 3-D beach evolution model taking into account the cross-shore sediment transport due to waves and Undertow as well as the transport due to nearshore currents. The results of both measurements and calculations show that the sediment transport due to nearshore current is predominant and the contribution of cross-shore sediment transports is cancelled for a long-term beach evolution.
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modeling op energy transfer and Undertow in the surf zone
22nd International Conference on Coastal Engineering, 1991Co-Authors: Akio Okayasu, Akira Watanabe, Masahiko IsobeAbstract:A model is presented to describe accurately the energy transfer under breaking waves. In the model, the energy of organized large vortexes as well as those of wave-induced motion and turbulence is taken into account. The model allows to estimate the dissipation rate and distribution of energy, and then the cross-shore two-dimensional distribution of an Undertow. The applicability of the model is confirmed by laboratory experiments.
Ib A Svendsen - One of the best experts on this subject based on the ideXlab platform.
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Modeling Wave Transformation in the Surf Zone
Coastal Engineering, 2001Co-Authors: Jayaram Veeramony, Ib A SvendsenAbstract:Boussinesq-type equations are used to model waves breaking in shallow water by including the effect of vorticity generated in the surf zone. The terms that describe breaking appear as corrections to the momentum equation. The model reproduces most of the dynamics in the surf zone. Non-linearity is important for wave height and set-up predictions close to breaking, both in the shoaling region and in the surf zone. The magnitude and the profile of the Undertow, especially in the surf zone, is predicted accurately by the model. Magnitude of eddy viscosity, which is used in the formulation for vorticity transport, is not critical except for Undertow predictions. Comparisons to wave group data show that the predictions of the wave shape and the wave height are very good. The model predicts a larger phase speed for the waves in the shoaling region, which is related to the assumption of weak dispersiveness of the waves. The model also predicts the generation of long waves in the surf zone.
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vertical structure of the Undertow outside the surf zone
Journal of Geophysical Research, 1993Co-Authors: Uday Putrevu, Ib A SvendsenAbstract:The vertical structure of the Undertow in the shoaling region outside the surf zone is quite different from that inside the surf zone. Inside the surf zone the Undertow typically has a large seaward directed velocity near the bottom and either a shoreward directed or a small seaward directed velocity at trough level. Measurements show that outside the surf zone the Undertow has a small seaward directed velocity near the bed and a large seaward directed velocity at trough level. In this paper we develop theoretical expressions for the Undertow outside the surf zone and show that the steady streaming from the bottom boundary layer which was earlier found to have a negligible effect in the surf zone (Svendsen et al., 1987) does have a significant influence on the vertical structure of the Undertow in that region.
