This study investigates nonlinear wave shape and the near-bed velocity influenced in the vicinity of a tidal inlet for the SEdiment supply At the WAdden Sea ebb-tidal Delta project. Wave nonlinearities occur when waves encounter a flow. An opposing flow leads to waves with higher peaks and reduced wave length. During a following flow, the waves tend to flatten and the wave length increases. High peak and flat troughs is a condition referred to as wave skewness. Wave asymmetry entails a pitched forward wave shape, with a steep front face and a gentle rear face. These wave nonlinearities contribute to sediment transport. Literature shows that skewness and asymmetry are widely studied in the nearshore zone. However, nonlinearities in deeper water and under influence of flow are not well understood. The goal of the present study is to determine whether a flow contributes to higher wave nonlinearities in the ebb-tidal delta northwest of Ameland.


The study uses data collected during the SEAWAD field campaign. Over the course of 42 days, five frames and eight pressure sensors were deployed in the ebb-tidal delta northwest of Ameland. The analysis examines data from Acoustic Doppler Velocimeter (ADV) and Acoustic Doppler Current Profiler (ADCP) on frames in 5 m and 8 m water depth, and pressure sensors located around the frames.


The results consists of three parts; 1) a general overview of the surface elevation skewness and velocity skewness over the measured period; 2) the correlation between the sea-surface elevation skewness and the near-bed velocity non-linearities; and 3) the contribution of a flow to the sea surface elevation non-linearities and near-bed velocity nonlinearities.

Findings show that wave nonlinearities occur in the vicinity of an ebb-tidal delta. The variations in the velocity skewness seem to vary with the tide. Only the driving mechanism behind this variation is not clear. The wave asymmetry shows little variation over each tidal cycle. Larger significant wave height and peak period contributes to higher skewness and higher asymmetry.
Second, there is a significant correlation between the sea-surface elevation skewness and the near-bed velocity skewness. The observed correlation increases with an increasing wave period.
Wave skewness is affect by a flow. In a following flow, the sea surface elevation skewness values are larger than in an opposing flow for the same Ursell number. The velocity skewness shows the same behavior, only this trend is less distinct.

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.