Onshore sandbar migration in the nearshore

Martijn Henriquez

Research output: ThesisDissertation (TU Delft)

15 Downloads (Pure)

Abstract

This thesis is about the hydrodynamic processes that occur when a nearshore sandbar migrates towards the shore driven by the waves. These processes are relevant because process-based morphological models have difficulty simulating coastal accretion events in contrast to erosion events. Simulating accretion events is important for engineering purposes, for example, to optimize shoreface nourishment for the conservation of beach and dunes of the Netherlands. During onshore sandbar migration, sediment is dominantly transported at the bed where the orbital wave motion applies friction on the bed. This transport, known as bedload, may result in a cross-shore net transport due to various hydrodynamic processes. First off, the orbital motion under waves in the nearshore are not sinusoidal but asymmetric as waves have steep fronts and flat backs. The wave shape is reflected back in the orbital motion which has two consequences, first, the largest flow velocities are found under the wave crests, and second, the largest horizontal pressure gradients are found under the wave fronts. Numerical modeling results have shown that in the wave bottom boundary layer the shape of the orbital motion is altered due to flow deceleration. However, there have not been any observations to validate this. Another set of hydrodynamic processes are the time-averaged currents generated by the waves. In the wave bottom boundary layer these processes are ‘progressive wave streaming’ and ‘wave shape streaming’. In addition, mass flux by waves (Stokes drift) and wave breaking processes also generate currents that have an affect on the time-averaged bottom shear stress. Although the time-averaged bed shear stress is relatively small compared to the time-varying component it may have a large impact on the net sediment transport. The accumulative effect of the aforementioned processes on the time-averaged bed shear stress is not well understood.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Delft University of Technology
Supervisors/Advisors
  • Stive, M.J.F., Supervisor
  • Reniers, A.J.H.M., Supervisor
Award date5 Dec 2019
DOIs
Publication statusPublished - 2019

Keywords

  • nonlinear waves
  • wave bottom boundary layer
  • bottom shear stress
  • sediment transport

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