This thesis examines the effects of the stratified tidal flow on the morphodynamics of the Dutch inner shelf. The south portion of the Dutch inner shelf is strongly influenced by the Rhine River ROFI (Region Of Freshwater Influence), which is generated by the discharge from the Rhine River through the Rotterdam waterways. Under stratified conditions, the three-dimensional structure of the tidal currents develops a strong cross-shore shear so that the bottom and surface currents become 180deg out of phase. The sheared flow created by stratification operates in the inner shelf and nearshore zones so that the flow asymmetries imparted by stratification are expected to impact the morphodynamics, however the role of the stratified tidal flow on the morphodynamics along the Dutch coast has been often neglected or oversimplified. In this context, this thesis aims to provide new insights on how the stratified tidal flow dictates the morphodynamics outside the surfzone. In the south portion of the Dutch coast is located the Sand Engine, a 21.5 million m3 experimental mega-nourishment that was built in 2011. This intervention created a discontinuity in the previous straight sandy coastline, altering the local hydrodynamics in a region that is influenced by the Rhine River ROFI. Estimates of the centrifugal acceleration directly after construction of the Sand Engine showed that its curved shape impacted the cross-shore flow, 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 owned to the morphodynamic evolution of the Sand Engine. 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 6 m depth), otherwise believed to be a mixed zone. In the inner shelf, shoaling waves are as effective in mobilizing sediment as the other co-existing flows. The influence of stratification on the hydrodynamics is translated into near-bed shear velocity in the layer immediately above the sea floor. The tide-induced bed shear stress is able to periodically agitate the bed near the peaks of flood and ebb cycles mostly during spring tides. Results from observations suggested that, under stratified conditions, relatively high values of bed shear stress are sustained for a prolonged period of time. The results also revealed that the non-tidal flow, such as the wind-induced flow, plays a role in controlling the bed mobility. However, wave-induced bed shear stress in general does not set sediment in motion during fair weather conditions and thus the stirring role of the waves is mostly important during storms. The co-exiting near-bed flows in the inner shelf are responsible for moulding the seafloor so that the resulting types of bedforms can reveal important information on the hydrodynamic forcings that dictate the sediment mobility. Observations showed that 56% of the ripples in the Dutch inner shelf are classified as current ripples. Wave ripples occur only during storm conditions, comprising 3%. The frequency of occurrence of transitional bed types composes 23% and poorly developed ripples is found to develop mostly during neap tides making up 15% of the observed bed types. The feedback of the different types of bedforms on the overlying boundary layer plays a fundamental role in the dynamics of the sediment load. The morphological response of the bed to the stratified and non-stratified tidal flow leads to differentiations of the ripple migration as well as the sediment transport modes (bedload and suspended load). The bedforms at the measurement site are strongly controlled by tides so that their behavior exhibits not only a spring-neap signature, but also a distinct semi-diurnal fluctuation. Under the influence of the Rhine ROFI, the bedform mean dimensions (ripple height and wavelength) are reduced, indicating that their development is affected by the stratified tidal flow. In the absence of (ambient) stratification, the tidal current ripples are more developed, attaining relatively larger dimensions. The net alongshore bedload transport is south-directed, whereas the net alongshore suspended load is north-directed regardless of stratification. Moreover, the net alongshore bedload transport is higher during stratified conditions but the net alongshore suspended transport is smaller. Regarding the cross-shore sediment transport, the findings show that ambient stratification promotes onshore-directed bed- and suspended load net transport. The gross suspended transport rates are 10 times greater than the gross bedload transport rates.@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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Extensive mud deposits are found off Cassino Beach, Brazil. The wave damping over the muddy bottom was studied using field measurements. By applying a technique of spectral analysis we showed that the wave attenuation occurred differently throughout the wave spectra. Field measurements revealed that the maximum wave energy dissipation took place over the deposit's depocenter and that lutocline height varied significantly in the order of days. The results indicated that short waves (from 3.75 to 6.25 s) underwent the greatest damping due to the interaction with fluid mud. An idealized 1-D model helped to explain the observations.@en

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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