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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.