The Rhine region of freshwater influence (ROFI) is strongly stratified, rotational, relatively shallow and has large tides, resulting in a dynamic field of fronts that are formed by multiple processes. We use a 3D numerical model to obtain a conceptual picture of the frontal structure and the processes responsible for generating this multiple front structure in the Rhine ROFI. The horizontal salinity gradient and numerical tracers are used to identify three different types of fronts: outer, inner, tidal plume and relic tidal plume fronts. Tidal plume front (TPF) trajectories together with the tracers demonstrate that TPFs exist for longer than one tidal cycle. A Lagrangian frontogenesis analysis shows that the fronts are strengthened mainly as a result of increased convergence, which is observed to occur at times when tidal straining is large. Additionally the alongshore tidal excursion and the dominance of the tidal currents over the intrinsic frontal propagation speed, trap TPFs within 20 km from the river mouth. Trapping and re-strengthening maintain several fronts at a time in the mid-field region, resulting in a multi-frontal system. The observation of a complex river plume system is expected to be important for cross-shore exchange, transport and coastal ecology.@en

In this study we used field data and radar images to investigate the influence of winds and tides on the propagation of tidal plume fronts. The measurements were collected in a shallow shelf region off the Dutch coast, 10 km north of the Rhine River mouth, and they clearly show the passage of distinct freshwater lenses and associated fronts at the surface that propagate all the way to the coastline. These fronts are observed as a sudden drop in near-surface salinity, accompanied by high cross-shore shear with onshore velocities at the surface. We determined the arrival time to our measurement site, frontal propagation speed, and structure of the fronts by combining the in situ data and radar images. Frontal Froude numbers show a wide range of values, with an average of 0.44. Our results show that fronts during spring tides are thinner, more mixed, and move faster relative to the ground during calm spring tides when compared to calm neap tides. Downwelling winds during spring tides result in thicker and faster fronts; however, the intrinsic frontal propagation speed indicates that the wind and tide control the frontal propagation mainly due to advection rather than by changing the frontal structure. Strong return currents in the near-bed layer resulting from fast and thick fronts increase near-bed turbulence and bed stresses. These high stresses suggest that the passage of fronts in shallow coastal areas can initiate sediment resuspension and contribute to transport processes.@en

This study investigates the influence of tidal straining in the generation of turbidity maximum zones (TMZ), which are observed to extend for tens of kilometers along some shallow, open coastal seas. Idealized numerical simulations are conducted to reproduce the cross-shore dynamics and tidal straining in regions of freshwater influence (ROFIs), where elliptical current patterns are generated by the interaction between stratification and a tidal Kelvin wave. Model results show that tidal straining leads to cross-shore sediment convergence and the formation of a nearshore TMZ that is detached from the coastline. The subtidal landward sediment fluxes are created by asymmetries in vertical mixing between the stratifying and destratifying phases of the tidal cycle. This process is similar to the tidal straining mechanism that is observed in estuaries, except that in this case the convergence zone and TMZ are parallel to the shoreline and perpendicular to both the direction of the freshwater flux and the major axis of the tidal flow. We introduce the term minor axis tidal straining (MITS) to describe the tidal straining in these systems and to differentiate it from the tidal straining that occurs when the major axis of the tidal ellipse is aligned with the density gradient. The occurrence of tidal straining and the coastal TMZ is predicted in terms of the Simpson (Si) and Stokes (Stk) numbers, and top–bottom tidal ellipticity difference (∆ε). Based on our results, we find that SiStk2 > 3 and ∆ε > 0.5 provide a limiting condition for the required density gradients and latitudes for the occurrence of MITS and the generation of a TMZ.@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.@en

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.@en

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.@en

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.@en

Remotely sensed images document the occurrence of multiple packets of internal solitary waves (ISWs) in the Rhine River plume at the same time. We use a combination of field observations, and non-hydrostatic and hydrostatic modeling to understand the processes that lead to the generation and retention of multiple ISW packets within the Rhine plume. Previous numerical modeling shows that the tidal plume front is trapped in the mid-field plume for more than one tidal cycle due to tidal straining and recirculation within the plume, resulting in the presence of multiple fronts in the near-and mid-field plume regions. In this work, we show how variations in the strength of these fronts can lead to the release of ISW packets. We conclude that the retention of the fronts in the mid-field region of the plume and modulation in the strength of the fronts can explain the presence of multiple ISW packets. A frontal Froude number analysis shows that fronts generated during the previous ebb tide can release ISWs in addition to the newly released tidal plume front.@en

River plumes, also regions of freshwater influence , are important features to understand because of their impact on the current structure, stratification and the transport of fine sediments, nutrients and contaminants. One important river plume is the Rhine ROFI. Prior studies have sought to understand far-field dynamics where cross-shore straining is dominant (Simpson & Souza, 1995; Souza & Simpson,1995; De Boer et al., 2008). However, less is known about the mid-field region of this river plume, where fronts matter as well. Here we use field observations from a 6 week measurement campaign in fall 2014 to investigate the dynamics of the mid-field region of het Rhine ROFI. We will focus on the interaction between far-field processes, such as tidal straining, and near field processes, such as fronts. The Rhine ROFI is of interest because the Dutch coast has been modified by extending the Port of Rotterdam and the construction of the Sand Engine that extends into the southern North Sea. These perturbations might impact the currents, the ROFI and the distribution of fine sediment, nutrients and contaminants. Therefore, the understanding of this is of importance. During September and October 2014, a large field observational campaign was conducted off the Dutch coast close to the sand Engine, 10 km north of the river outflow. Measurements were made at two locations, 2 and 5.5 km offshore (see Figure 1). Moorings, with Conductivity Temperature Depth (CTD) and Optical Backscatter (OBS) instruments at different depths, were deployed to obtain vertical profiles of salinity and suspended sediment concentrations (SSC). In addition, at each location a bottom-mounted Acoustic Doppler Current Profiler (ADCP) measured vertical velocity profiles. Radar images of the area were used to gain surface information, specifically about frontal propagation in the vicinity of the measurement locations. The weather conditions were highly variable during the six week period. There were very calm periods, but also storms, which completely destroyed stratification. The wind direction changed during the campaign as well. In addition to the data, numerical modelling with a 3D hydrostatic model will be used to gain information of the entire mid-field plume. @en

We present measurements of along and across-shore sediment transport in a region of the Dutch coast 10 km north of the Rhine River mouth. This section of the coast is characterized by strong vertical density stratification because it is within the midfield region of the Rhine region of freshwater influence, where processes typical of the far-field, such as tidal straining, are modified by the passage of distinct freshwater lenses at the surface. The experiment captured two storms, and a wide range of wind, wave, tidal and stratification conditions. We focus primarily on the mechanisms leading to cross-shore sediment flux at a mooring location in 12 m of water, which are responsible for the exchange of sediment between the nearshore and the inner shelf. Net transport during storms was directed offshore and influenced by cross-shelf winds, while net transport during spring tides was determined by the mean state of stratification. Tidal straining dominated during neap tides; however, cross-shore transport was negligible due to small sediment concentrations. The passage of freshwater lenses manifested as strong pulses of offshore transport primarily during spring tides. We observe that both barotropic and baroclinic processes are relevant for cross-shore transport at depth and, since transport rates due to these competing processes were similar, the net transport direction will be determined by the frequency and sequencing of these modes of transport. Based on our observations, we find that wind and wave-driven transport during storms tends move fine sediment offshore, while calmer, more stratified conditions move it back onshore.@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.@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.@en

Prior studies have shown that freshwater fronts in the Rhine ROFI have a significant role of the cross-shore currents. Here we present results from the STRAINS project in which a major field campaign was carried out along the Dutch coast near the Sand Engine in 2014. Here we investigate the propagation and strength of freshwater fronts in the mid-field river plume by using field-data, radar images and numerical modelling. We used a unique dataset collected 10 km north of the river mouth at 2 and 6 km offshore. The dataset contains salinity, temperature and velocity data. In addition, high frequency velocity data just above the bed (0.25, 0.5 and 0.75 m) is available at the station 2km offshore, which has been used to calculate near bed Reynolds stresses. The data and radar images show the onshore propagation of freshwater fronts after high tide. The data shows that during spring tide with moderate downwelling winds (here SW) the fronts are thicker and propagate faster than during neap tides. The spring tide fronts coincide with a high cross-shore Reynolds stress near the seabed just after frontal arrival corresponding to high near bed velocities. A 3D hydrostatic numerical model is used to investigate the spatial structure of the plume. First results show the existence of multiple freshwater lenses within the entire plume, formed each tidal cycle. The data, radar images and numerical model results provide a detailed picture of plume and frontal dynamics within a shallow river plume area. @en