Onshore sandbar migration: Processing PIV measurements to analyse wave driven sediment transport in the nearshore

This study investigates wave driven onshore sandbar migration. Observations show that sandbars move onshore during mild wave conditions. Literature describes different potential sediment mechanisms that cause this onshore migration, but the dominant cause is still under debate. The goal of the present study is to determine the dominant sediment mechanism in wave driven onshore sandbar migration. The study uses data collected in a mobile-bed wave flume. The study consists of two parts: (1) calculating sediment transport based on PIV data and (2) analysing wave driven sediment transport in the wave bottom boundary layer. The analysis distinguishes three transport mechanisms: current related, long wave related and short wave related sediment transport. The study further distinguishes wave properties corresponding to different intra-wave processes within the short wave related sediment transport. The findings show that short wave related sediment transport is dominant. The wave shape changes towards the bottom: asymmetry is transformed into skewness, which results in strongly skewed and backward pitched waves close to the bottom. Additionally, the results seem to indicate that the short wave related sediment transport mainly depends on near-bed skewness and not or slightly on the asymmetric pressure gradient. The implication is that sediment transport calculations should focus on short wave related transport, near-bed skewness and should include the wave shape development in the wave bottom boundary layer. In addition, the study shows that PIV data have potential to determine sediment transport without additional concentration measurements. Three essential steps in the method are: (1) placement of the PIV laser in front of the wave flume instead of the common position on top to illuminate both the flow and bottom sediment, (2) calibrate the intensity and concentration based on the mean bed and mean water intensity and (3) multiply the PIV sediment velocities with the concentration to obtain sediment transport. The results of this study contribute to improve the accuracy of coastal morphology predictions and the efficiency of beach nourishments and other coastal protection measures.Civil Engineering and GeosciencesHydraulic EngineeringFluid mechanics & Coastal Engineerin