The effects of currents on wave nonlinearities

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.SEAWAD: research into a sand engine for the Wadden IslandsCivil Engineering | Hydraulic Engineerin