Barrier coast systems connect oceanic and inland waters and support essential ecological functions. Their shallow subtidal and intertidal zones provide key ecosystem services, making it crucial to understand the nonlinear mechanisms driving water motion in these regions, particularly in terms of overtides, mean sea surface elevation, and residual flow. This study introduces a new interpretation framework for analyzing nonlinear processes in tidal systems with shallow regions, extending the established framework by Parker (1991) to account for wetting and drying dynamics. To illustrate this framework, it is applied to a model of the Marsdiep-Vlie double-inlet system in the Wadden Sea, governed by the cross-sectionally averaged shallow water equations, including the Defina (2000) approach to account for wetting and drying. The one-dimensional model bathymetry and planform feature a topographical high and varying channel widths, respectively. We assess contributions of nonlinear forcings to overtides and residual flow, and conduct sensitivity analyses on the effects of longitudinal variations in bottom elevation and channel width. Results confirm that Parker’s findings hold in deeper systems with minimal width variation; however, in systems with intertidal areas, distinct nonlinear processes emerge. Specifically, in these shallow regions, the Water Storage term, Nonlinear Continuity term, and Elevation Effect of the Friction term contribute greatly to \(M_4\) generation, while the Nonlinear Continuity term strongly contributes to \(M_6\) generation. For mean sea surface elevation, several nonlinear terms are equally important, while residual velocities are predominantly driven by the Nonlinear Continuity term.
