Measuring wave transformation in a tidal inlet using Wave Resolving Drifters

Abstract

In the coastal defence of the Netherlands, sand nourishment is a common practice. Part of the Dutch coastal system, the Wadden Sea coast, is a very complicated coastal system in terms of hydrodynamics and morphodynamics. Many different processes are playing a role in the coastal dynamics in the Wadden Sea area. The execution of a mega nourishment in this area is intended. To make this possible, the dynamics of the system must be understood better than they are today.

Part of the coastal dynamic system is the intrawave sediment transport. To improve understanding of this phenomenon, research is done on the wave conditions in a tidal inlet of the Wadden Sea. In the accompanying field campaign use is made of unanchored WaveDroids, used as wave resolving drifter. This is the first time unanchored use of WaveDroids is carried out. The use of a moving measurement device gives rise to differences in measurement results compared to the known approach with fixed measurement devices.

This research shows the difficulties that must be overcome to process raw measurement data in such a way that the data becomes suitable for analysis and interpretation. The data is filtered on frequency and on wave height to give a useful representation of the wave field. The processed data is assessed on the energy density spectrum, the wave height distributions and the time series of wave heights and surface elevation. Subsequently, in the interpretation of the selected data, insight is given in the measured wave field.

From this research is concluded that the use of WRD’s is suitable to measure the vertical component of wave displacement. The effect of a Doppler shift, originating from the use of different frames of reference, is not significant is common sea states. The measurement of the horizontal component of wave displacement requires more research on correcting processing steps.