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Based on a re-analysis of the existing undertow models and the experimental results, a proper explicit model is proposed for computing undertow profile inside the surf zone. The model has been derived by using the eddy viscosity approach. The model is examined using published laboratory data from six sources covering small-scale and large-scale experiments, i.e. the experiments of Nadaoka et al. (1982), Hansen and Svendsen (1984), Okayasu et al. (1988), Cox et al. (1994), CRIEPI (Kajima et al., 1983) and SUPERTANK (Kraus and Smith, 1994). The present undertow model is considerably simpler than most of existing models. Although the model is simple, it shows good agreement with the experimental results above the bottom boundary layer. The calculation is so simple that the undertow profile can be calculated by using a pocket calculator.

Field measurements are important for understanding coastal processes, verifying and calibrating numerical and physical models, and as direct input to coastal designs. Many aspects of coastal engineering are hampered by the lack of high quality field data particularly during storms. This paper introduces the Sensor Insertion System (SIS), which uses a somewhat different approach for measurements that has proven useful at the US Army Corps of Engineers Field Research Facility (FRF). The SIS is a pier-mounted diverless instrument deployment and retrieval system that can be used to make measurements under calm or storm conditions anywhere across the surf zone. The SIS can operate in wave heights up to 5.6 m, with 20 m/s winds, and 2 m/s currents. The mobility of the SIS permits measurements to evolve with the morphology, thus avoiding many of the problems of traditional stationary instrument installations. The SIS approach is to use a single instrument array and reduce the cost and logistics of instrumenting the surf zone so that a long-term measurement capability can be maintained. This approach has helped overcome many of the obstacles of directly measuring storm longshore sediment transport processes at the FRF during the past six years. The utility of the SIS is demonstrated in this paper through examples of it's application for a variety of coastal science investigations.

An instantaneous dye-release experiment was conducted in a coastal field study. Dye was released into a longshore current field from the research pier HORS located in Hasaki, Japan. The release point of the dye was placed in a wave reforming zone which lay between a bar, where limited breaking occurred, and shorewards final surf zone, where all waves broke. Longshore current was present between the bar and shoreline. Deformation of the dye patch was observed efficiently and effectively with a moored video system. Some essential characteristics of the surf zone hydrodynamics and shear flow dispersion are explained from the results of video image analyses of the temporal variation of the dye patch distribution.

Many researchers have pointed out that the use of representative wave approach can give erroneous results in the computation of irregular wave height transformation. However, the representative wave approach seems to be an efficient tool incorporated into beach deformation models because of its simplicity and computational efficiency. It will be useful for practical work, if this approach can be used to compute the irregular wave height transformation. Therefore, this study is carried out to investigate the possibility of simulating irregular wave height transformation by using representative wave approach. A large amount and wide range of experimental conditions, covering small-scale, large-scale, and field experimental conditions, are used to calibrate and examine the model. The rms wave height transformation is computed from the energy flux conservation law. Various energy dissipation models of regular wave breaking are directly applied to the irregular wave model and test their applicability. Surprisingly, it is found that by using an appropriate energy dissipation model with new coefficients, the representative wave approach can be used to compute the rms wave height transformation with very good accuracy.

Organized eddies generated by wave breaking have an important role on sediment transport in surf zones. In order to clarify the characteristics of intermittent sediment clouds generated by vertically-oriented eddies, the Computed Tomography (CT) technique was applied for a laboratory experiment to capture instantaneous two-dimensional distributions of suspended sediment concentration in a near-bottom horizontal plane. The Extended Bayesian Method with a smoothing filter as the prior information model was applied to obtain the optimum solution of the distribution. A specification test showed that concentration distribution was reproduced fairly well. Successive images of concentration distribution due to wave breaking were obtained and movements of sediment clouds were investigated. Concentration at the center of the measuring area was simultaneously measured for verification by a conventional optical forward-transmission-type sedimentmeter. It was found that horizontal distribution of suspended sediment concentration in a surf zone is quite inhomogeneous and the high concentration regions are formed as they indicate separated sediment clouds. Distinctive patterns of sediment clouds were observed in a relatively small number of trials. The length scale of the clouds was found to be in the order of the water depth. The primary cause of sediment cloud formation can be considered sediment pickup by intermittent eddies generated by wave breaking.

The wave-induced pressure gradient, ∂p/∂x, at the bottom is related to fluid acceleration and sediment movement in the surf zone. Following similar large-scale laboratory work by Suzuki et al. [2008a], this paper deals with the observations and analysis of bottom pressure gradients on a natural sandy beach. The cross-correlation coefficients between ∂p/∂x and the water surface elevation are high even in the surf zone, and the coefficients are higher than the coefficients between ∂p/∂x and the vertical velocity component or ∂p/∂x and du/dt. The observed nonlinear characteristics of ∂p/∂x are weaker than the laboratory experimental data but extreme values of ∂p/∂x are larger than the experiments. The distributions of exceedance probability of ∂p/∂x are evaluated using the two-parameter Weibull distribution. The modulus of the Weibull distribution is evaluated as a function of local significant wave height normalized by the offshore significant wave height. The exceedance probability distributions of ∂p/∂x show a broader distribution for the field data compared to the laboratory, but are, nevertheless, predicted reasonably well with the Weibull distribution.

Nonuniform vertical discretization is investigated for use with a previously developed numerical model that solves the incompressible, nonhydrostatic, Navier–Stokes equations for free surface flow. The equations are vertically transformed to the sigma coordinate system and solved in a fractional step manner in which the pressure is computed implicitly by correcting the hydrostatic flow field to be divergence free. A specific discretization is proposed to attain greater accuracy and/or minimize computational effort when compared to a uniform vertical discretization. Numerical accuracy is assessed by comparison with experimental data for spilling and plunging breakers.

The breaking-limited (or depth-limited) approach is a traditional method to determine the maximum possible wave height for the design of coastal structures in the surf zone. It is well recognized that the maximum wave height in the surf zone is limited by wave breaking. The maximum possible wave height is usually determined from a breaker height formula. The present study was undertaken to examine the applicability of 14 existing breaker height formulas for computing the maximum possible wave heights. The existing breaker height formulas were examined against measured regular and irregular wave heights. A total of 17 863 data points from 30 sources of published experimental data were used to examine the formulas. The experiments cover a wide range of wave and bottom topography conditions including small-scale, large-scale, and field experiments. It was found that the errors of existing formulas for regular and irregular waves have the same tendency. The existing formulas give considerable underestimation of the maximum possible wave heights in shallow water. The top three formulas were modified by including a new form of relative depth into each formula. Overall, the modified formulas give a considerable better estimation than those of existing formulas.

Boulders numbering in the high hundreds/low thousands, and with masses up to $\sim$30 tonnes, were transported onshore by Super Typhoon Haiyan in Calicoan Island, Philippines to maximum ground elevations that could exceed 9m and terminal positions up to $\sim$180m inland. One-dimensional Boussinesq hindcasts of coastal boulder motion showed intermittent transport initiated at the fronts of infragravity swash bores. Transport distances were found to be highly sensitive to wave-height, enough so that observations of terminal positions may be a viable method of estimating rough paleostorm magnitudes. The large accelerations at bore fronts generated significant inertial forces, particularly for larger boulders, but drag forces had greater root-mean-square magnitudes in all simulations. Widely used relations to infer fluid velocities from boulder properties were tested using modeled boulders — inferred velocities at modeled terminal boulder positions were compared to maximum computed Boussinesq fluid velocities at these locations and found to be significantly lower. This underprediction of inferred velocities was greatest for smaller boulders that were strongly mobile. Inferred drag loads compared to modeled values were somewhat more accurate for large boulders when a Froude number of unity was assumed to estimate flow depths. Although these boulders were unequivocally transported by storm waves, their large sizes and distances traveled venture into what has been considered the tsunami range. Thus, care must be taken to interpret the provenance of coastal boulder fields with unknown origin for lower to mid-latitude regions.

The present paper is aimed to formulate a numerical model for calculating the time-dependent velocity field and the space and time distribution of sand concentration inside the bottom boundary layer in the surf zone. The numerical results are compared with recent laboratory data. The 2DV (two-dimensional vertical) numerical module for the velocity field is based on the Reynolds averaged form of the Navier-Stokes equations while the sand transport module uses the turbulent convection-diffusion equation. The equi-phase mean values for the horizontal and vertical velocities within the bottom boundary layer (the flow field) and the equi-phase mean values for the sand concentration are obtained. A transformation of the coordinate system is performed in order to enable the easy implementation of the numerical modelling. Details of the numerical modeling together with discussions on the boundary conditions are also presented. The comparison of numerical and laboratory results reveals relatively good agreement within certain limitations of the present model. The laboratory data, which are used to compare the hydrodynamic numerical model, have been obtained in the surf zone while the ones for verifying the sand concentration numerical model are obtained as a result of pure oscillatory movement.

The entrained air and turbulence characteristics under a breaking solitary wave on a 1:20 sloping beach are investigated through laboratory measurement. Free surface elevation is obtained from wave gauge measurements. Wave breaking process is captured in detail by a high-speed camera. The bubble image velocimetry (BIV) is used to measure the velocity and the fiber optic reflectometer (FOR) is used to capture instantaneous void fraction in the aerated region. The mean void fraction and velocities in the aerated region are obtained by ensemble averaging over 22 repetitions. Results show that the maximum mean void fraction is 0.6 in the collapsing cavity region and is 0.35 in the splash up region. The time series of the mean void fraction has good synchronization with the instantaneous images taken by high-speed camera. The maximum horizontal velocity occurs in the splash up region and reaches 1.17C shortly after the plunging jet hits the water surface, with C being the phase speed of the primary wave. The turbulence intensities over the entire aerated region are presented and discussed. The measured data can be used for the calibration and verification of the numerical model for aerated flows simulation under breaking waves in the surf zone.

This study investigates characteristics of wave deformation on a coral beach and behavior of coral gravels under such forward-leaning waves through two different laboratory experiments. All these experiments were captured by multiple video cameras mounted beside the glass wall of the flume and water surface fluctuations, topography change, nearbottom current velocities and gravel velocities were quantitatively extracted through the analysis of recorded images. Primary findings of the experiments are as follows: (i) deformation of nearshore waves and bed profiles dynamically interacted with each other and developed steeply sloping coral bed around the swash zone enhanced the formation of partial standing waves; (ii) initiation of motion of the gravels depends not only on the local bottom shear stress but also on the positions of each gravel relative to the others; (iii) after the initiation of motion, behavior of gravels were dominantly determined by the surrounding flow velocity but not by the bed conditions.

Waves entering the surf zone initially develop higher harmonics which disappear after two or three wave lengths. The default Lumped Triad Approximation (LTA) in the SWAN wave model generates only (persistent) second harmonics. An alternative approximation (DCTA) is suggested which is based on an analogy with quadruplet wave-wave interactions. It initially generates all (transient) higher harmonics and subsequently a smooth universal spectral tail, in agreement with observations. The DCTA is calibrated with 31 published laboratory cases and then applied to 20 other, one- and two-dimensional cases, including field cases. The conclusion is that using the DCTA instead of the LTA barely affects the significant wave height but improves the mean period and spectral tail of the computed spectra. Further improvements are expected with enhanced high-frequency dissipation.

This study aims at improving the computation of the wave-driven longshore currents in the surf zone. The vertical distribution of wave-driven currents often deviates from a logarithmic vertical distribution, due to the vertical mixing induced by wave breaking. 3D modeling of these currents provides the opportunity to take this vertical variation into account. The current method of computing the bed shear stress in the 3D approach of Delft3D is dependent on the thickness of the near-bed vertical computational layer: the thinner this layer, the larger the bed shear stress and the smaller the wave-driven longshore currents. Computing the bed shear stress using the velocity at the edge of the wave boundary layer avoids this layer dependency. With this method good agreement with measured velocity data from laboratory experiments and field experiments is obtained, except for very close to the shore. Although Delft3D in 2DH and 3D model have similar skill in simulating longshore currents, the 3D approach is recommended for wave-driven sediment transport related problems as it more realistically represents the cross-shore current and suspended concentration profile.

The development and evolution of crescentic patterns in double-barred systems is explored using a morphodynamic stability model. The description of the surf zone hydrodynamics is based on depth and wave averaged conditions while sediment transport is calculated using a total load formula. The linear stability analysis predicts that an initially rectilinear coast, subject to infinitesimal perturbations and under normal wave incidence, is unstable and can result in the development of crescentic shapes that can be coupled either in-phase (highs and lows of both sandbars are at the same alongshore position) or out-of-phase (highs and lows of one sandbar correspond to lows and highs of the other sandbar). Results of numerical simulations are sensitive to some of the parameterizations used in the description of hydrodynamics. Changes in the breaker index can have an effect not only on the spacing and growth rate of the pattern but also on the type of pattern that develops. An increase in the breaker index leads to a faster growth of the pattern but also to a smaller alongshore spacing. The role of parameterizations in lateral mixing and roughness length appear to be limited.