Abstract
A quasi-three-dimensional model (quasi-3D) has been developed through the implementation of an analytical 1DV flow model in existing depth-averaged shallow water equations. The model includes the effects of waves and wind on the vertical distribution of the horizontal velocities. Comparisons with data from both physical and field cases show that the quasi-3D approach is able to combine the effect of vertical structures with the efficiency of depth-averaged simulations.
 Inter-comparisons with three-dimensional simulations show that the quasi-3D approach can represent similar velocity profiles in the surf zone. Quasi-3D morphodynamic simulations show that the bed dynamics in the surf zone represent the
 relevant 3D effects in the surf zone much more than the depth-averaged computations. It was shown that the quasi-3D approach is computationally efficient as it only adds about 15-20% to the runtimes of a 2DH simulation which is minor compared to a run time increase of 250-800% when switching to a 3D simulation.
Highlights
PROBLEM DEFINITIONWhen considering the depth-averaged current field to be representative of the entire velocity pattern, one makes the implicit assumption of vertical similarity of the velocity profile, i.e. the velocity profile in every point in the horizontal has the same shape (e.g. logarithmic)
Over the last decade, process-based area models have become one of the major tools in coastal engineering
Quasi-3D approaches have been developed which primarily focused on improving the sediment transport predictions (Ranasinghe et al, 1999) without fully describing the vertical velocity profile
Summary
When considering the depth-averaged current field to be representative of the entire velocity pattern, one makes the implicit assumption of vertical similarity of the velocity profile, i.e. the velocity profile in every point in the horizontal has the same shape (e.g. logarithmic). A quasi-3D approach has been implemented to represent 3D processes in a depth-averaged model, while keeping the additional computational time limited. This approach is hereafter referred to as the quasi-3D model. The vertical velocity distribution in both longshore and cross-shore are computed with the analytical point model (1DV), which is computationally very efficient. The quasi-3D model was developed in which the analytical 1DV flow model of Reniers et al (2004) was coupled to the depth-averaged shallow water equations in which the effects of waves and wind on the vertical distribution of the horizontal velocities is accounted for (wave breaking induced currents, mass flux and boundary layer streaming). The suspended sediment transports are calculated using a depthaveraged advection-diffusion equation
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