Combined wave-current flow is defined as flow in which waves and currents are present together. One of its occurrences in nature is at a tidal inlet system, where incoming waves meet either a current in the same (following) or opposite (opposing) direction. This type of flow is different when compared to wave- or current-only flows. As a result, different effects are expected on, amongst others, the (suspended) sediment transport. In this study, we are modelling combined wave-current flow with the non-hydrostatic wave-flow model SWASH. The goal was to extend the present SWASH model to facilitate the modelling of combined wave-current flow, validate model results with experimental data and use the new model to analyse waveforms in combined wave-current flow. Model results for spatially uniform and non-uniform currents showed that the evolution of the wave height over the domain was approximately constant. Validation results of the vertical structure of the flow were in general poor except at the boundary. However, the model is capable of producing qualitatively the same trends as observed in measurements of the horizontal structure of the flow. In particular, the spatial evolution of the skewness and asymmetry was captured fairly accurate by the model. It is recommended to expand the newly developed methods to 3D domains for further validation and analysis.

Drifters equipped with GPS-trackers have been widely used to investigate surface velocities in a wide range of coastal and oceanic settings. A detailed description of a new, cell-phone based, budget drifter system is given, along with an overview of the potential error sources and the magnitude of these sources. Drag related “windage” errors were found to be significant. As a part of the SEAWAD research project, the surface velocities on the ebb tidal delta of the
Amelander Zeegat (the Netherlands) are measured over a period of two weeks. The drifters performed well in the field (no drifter losses) but the results showed relatively frequent data gaps and filtering resulted in significant data loss. The signal quality in the area appeared to significantly influence the data loss.
The velocities on the ebb tidal delta vary around 0.7 [m/s], depending on location, depth, tidal phase and wind. The spatial structure of the velocities shows generally a uniform flow structure. Different wind conditions caused the most significant changes in the spatial distribution of the velocities, followed by the effect of the tidal range. Waves were found to have an insignificant effect
on the flow velocities. A number different deployment configurations were investigated in order to shorten the measurement interval as much as possible. Due to the absence of spatial structure, spatial “resolution” can be exchanged for a shortened measurement interval by deploying the drifters in two or more groups across the research area. Methods for measurement verification using nearby ADCP-equipped measurement points are presented but not performed.
Overall, the described drifter measurement method provides a cheap way of measuring the spatial distribution of velocity in an area. However, the accuracy of the measurement has not been verified. Until then, the accuracy of the results is uncertain.