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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.