From Depth-Averaged Wave Models to Quasi-3D Transport: A Framework for Nearshore Microplastics

Understanding microplastics transport in the nearshore zone requires an accurate representation of wave-induced circulation and vertical mixing processes. This study presents a methodological framework for reconstructing continuity-consistent quasi-three-dimensional velocity fields from depth-averaged outputs of the Nonlinear Shallow Water (NSW) and Boussinesq equations. A benchmark configuration is first employed to verify the numerical consistency and physical plausibility of the model. The framework is then extended to compare shallow water and Boussinesq formulations over a simplified planar bathymetry, enabling the influence of individual equation terms on tracer evolution to be isolated and physically interpreted. The proposed technique offers a valuable tool for extending the utility of efficient two-dimensional models toward quasi-three-dimensional hydrodynamic analysis.