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. @en

Wave-supported gravity flows (WSGFs) generate rates of sediment flux far exceeding other cross-shelf transport processes, contributing disproportionately to shelf morphology and net cross-shelf fluxes of sediment in many regions worldwide. However, the conditions deemed necessary for the formation of WSGF limit them to a narrow set of shelf conditions; they have been observed exclusively in regions where the seabed consists of very fine-grained sediment and typically co-occur with nearby river flood events. Here we document the occurrence of a WSGF event on a predominantly sandy seabed and in the absence of a preceding river flood. Our measurements confirm that the dynamics are governed by the friction-buoyancy balance observed in other WSGF and that WSGF can form in mixed grain-size environments and transport high concentrations of sand. The occurrence of WSGF on a predominantly sandy seabed suggests that they may occur under a much wider range of conditions and, given the global prevalence of sandy shelves, they may be a more frequent and more ubiquitous feature of shelf dynamics than previously thought.

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The Sand Engine is a 21.5 million m³ experimental mega-nourishment project that was built in 2011 along the Dutch coast. This intervention created a discontinuity in the previous straight sandy coastline, altering the local hydrodynamics in a region that is influenced by the buoyant plume generated by the Rhine River. This work investigates the response of the cross-shore stratified tidal flow to the coastal protrusion created by the Sand Engine emplacement by using a 13 h velocity and density survey. Observations document the development of strong baroclinic-induced cross-shore exchange currents dictated by the intrusion of the river plume fronts as well as the classic tidal straining which are found to extend further into the nearshore (from 12 to 6m depth), otherwise believed to be a mixed zone. Estimates of the centrifugal acceleration directly after construction of the Sand Engine showed that the curvature effects were approximately 2 times stronger, suggesting that the Sand Engine might have played a role in controlling the cross-shore exchange currents during the first three years after the completion of the nourishment. Presently, the curvature effects are minute.

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We present a new mechanism for cross-shore transport of fine sediment from the nearshore to the inner shelf resulting from the onshore propagation of river plume fronts. Onshore frontal propagation is observed in moorings and radar images, which show that fronts penetrate onshore through the nearshore and surf zone, almost to the waterline. During frontal passage a two-layer counterrotating velocity field characteristic of tidal straining is immediately set up, generating a net offshore flow beneath the plume. The seaward flow at depth carries with it high suspended sediment concentrations, which appear to have been generated by wave resuspension in the nearshore region. These observations describe a mechanism by which vertical density stratification can drive exchange of material between the nearshore region and the inner shelf. To our knowledge these are the first observations of this frontal pumping mechanism, which is expected to play an important role in sediment transport near river mouths.

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This study presents the small scale bedform states found off the South-Holland coast during a 31+ days field observation of seabed acoustic imagery and near the bed velocities. Six main bed states were encountered: current ripples (C), wave ripples (W), combined wave-current ripples (WC), current ripples with subordinate wave ripples (Cw), wave ripples with subordinate current ripples (Wc) and poorly developed ripples (P). Direct visual detection of the bed state from the images showed good agreement with a simple predictor based on the mobility number. The most frequent type of bedform was C which is governed by the tidal currents. Wave ripple prevailed only during a storm with waves higher than 2 m. The combined Cw, WC and Wc types comprised 22% of the occurrences. Poorly developed ripples were associated with the neap tide during fair weather conditions.

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The South-Holland coast of the Netherlands undergoes the influence of the Rhine river plume released from the Rotterdam waterways. An experiment, STRAINS, was conducted to study the impact of the fresh water on the nearshore hydrodynamics and sand transport. As part of the experiment, an instrumented bottom frame measured the near-bed hydrodynamics at 12 m depth. The flow was decomposed in the tidal, wave and turbulent component. During moderate energetic wave conditions the cross-shore tidal flow was of similar magnitude as the wave orbital flow. The cross-shore tidal flow was asymmetric and larger in the seaward direction. The cross-shore tidal component may be generated by tidal straining due to the river plume.

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