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.

Tropical cyclones have the ability to very quickly increase in strength. This process is called rapid intensification and as a result, tropical cyclones can transform into hurricanes. Rapid intensification is related to the availability of heat and the amount of negative feedback of the ocean on the tropical cyclone. Negative feedback results in the weakening of the tropical cyclone. Cyclones passing over a warm ocean anomaly have access to more heat and due to the relatively high temperatures, the amount of negative feedback is reduced considerably. A necessary condition for rapid intensification is therefore the presence of a warm ocean anomaly, often being warm core eddies. This paper relates the rapid intensification of tropical cyclone Matthew to the presence of warm core eddies in the track of Matthew. Results show that there is no extensive evidence found for the presence of a warm core eddy before rapid intensification took place. Although maps of the sea surface height and sea surface temperature indicate the possible existence of a warm core eddy, surface velocities do not show the characteristic rotation flow of an eddy. The enthalpy flux is considerably large just before the rapid intensification of Matthew indicating that the negative feedback by the ocean is reduced and heat is available for transport. The rapid intensification of Matthew might be linked to other physical mechanisms that have been overlooked. Possible mechanisms identified are the Amazon-Orinoco river plume and La Ni˜na. Further studies on the rapid intensification of tropical cyclone Matthew should therefore take into account these mechanisms and study their influence on rapid intensification.