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This paper presents experimental observations as well as theoretical formulations of the flow structure in the turbulent shear layer in spilling breakers. The theoretical model is developed within the framework of a three-layer system, with the top layer being the two-phase (air/water) turbulent surface layer, and the bottom layer comprising the single phase (water) irrotational flow. The model development in the single phase (water) turbulent shear layer is guided by and validated with PIV flow measurements of a turbulent hydraulic jump. The breaker shear layer is seen to resemble a classical mixing layer with entrainment of fluid into the reverse flow region localized about the mean surface. Preliminary results are reported on interfacial boundary conditions which account for the contributions from the surface layer as well as the evolution of turbulent and mean quantities in the breaker shear layer. Such interfacial boundary conditions can be incorporated into Reynolds-averaged Navier-Stokes numerical models for simulating the spatio-temporal evolution of the turbulent and mean flow in spilling breakers.

The horizontal mixing on nearshore flows characterized by non-uniform topographies is studied with specific focus on the interpretation of the statistics of passive tracers. Two different data sets have been used and the discrepancies/similarities between the same statistics derived from the two experiments have been explained in relation to the specific experimental conditions. The main sources of discrepancy seem to be the different seeding procedure (leads to changes in the ballistic regime), the varying confinement effects due to the basin boundaries and to the different still-water free board in use (both lead to differences in the asymptotic value of the eddy diffusivity). Notwithstanding these differences use of both data sets leads to an improved formulation for the eddy viscosity (equation 6).

The paper analyzes the mixing features in numerically generated shear waves by simulating the circulation during SANDYDUCK experiments. Different wave resolving Boussinesq type models and a wave averaged model have been compared. They produce different velocity/vorticity fields and in turn different mixing properties. Lagrangian statistics have been used to study this aspect. Differences in absolute and relative dispersion and diffusion have been pointed out. All the models seem to follow a Richardson-like intermediate regime for the longshore relative dispersion even though some results seem to suggest a dependency from particles separation weaker than the classical 4/3 law.

The generation of large-scale, horizontal eddies in correspondence of a submerged breakwater and their evolution in the region shoreward of the breakwater are studied by numerical and analytical means. Such eddies are thought of significantly influencing both flow patterns and sediment transport in the region of the very shallow waters near to the shoreline. Preliminary results reveal the main stages of possible scenarios for the generation and evolution of eddies at the breakwater location. The main characteristics of the transport of passive tracers and bottom sediments associated with the vortices motion are also illustrated.

Approximate shoreline boundary conditions for wave-averaged models of nearshore hydrodynamics are derived on the basis of the swash zone equations of Brocchini and Peregrine (1996). They allow for explicit computation of a non-zero mean water depth at the mean shoreline. This is computed in terms of the local height of the short waves. Implementation issues are also discussed.

Recent progress in the development of a Morphodynamics Shoreline Boundary Condition (MSBC) for use in wave-averaged nearshore circulation models are illustrated. The MSBC is derived from the integral, wave-averaged form of the sediment mass conservation equation. Available field data and suitably-computed numerical data allow for a preliminary assessment both of the theoretical analyses and of the balance of the simplified MSBC at long-period scale.

A simple but effective method is proposed for the mitigation of the impact of macrovortices generated by differential wave breaking in correspondence of submerged breakwaters. Inspection of the analytical expression which gives the intensity of such vortical flow features suggests that a reduction of the breakwater lateral slopes induces a reduction of the lateral gradients of the local wave height field. Numerical simulations, run with an available, shock-capturing Nonlinear Shallow Water Solver, support our conjecture and provide the data used for a quantitative assessment of the vortex intensity decrease as function of the parameters controlling the flow. Major findings are that: i) the macrovortex strength is directly proportional to the wavelength, ii) the breakwater lateral slope more strongly controls the strength than the beach slope, iii) the reduction of the lateral breakwater slopes more efficiently mitigates the vortex strength when the breakwater is placed on steep beaches.

The present study shows new experimental data aimed to analyze the influence of the swash zone processes on the overall beach dynamics with special emphasis on offshore bar migration events. The presented results were obtained in a large-scale flume facility in which two sets of experiments characterized by two different beach-face morphology conditions were conducted. The results are compared in terms of bar behavior with previous similar experiments. It is shown how flattening the emerged beach-face promotes a modification in the offshore outer bar migration which almost stops moving off-shoreward. It is suggested that more dissipative swash conditions result in a variation in the wave-swash interactions reducing the amount of sediment carried off-shoreward to the surf zone. It is also suggested that a dissipative beach-face reduces the shoreline long-wave reflection and promotes variations in the wave breaking intensity though the latter is not noticeable in the computed significant wave height due to limitations in the available number of sensors.