Introduction: In this study we investigate the Suspended Particulate Matter (SPM) source and dynamics in terms of resuspension and advection in the mid field region of Rhine Region Of Freshwater Influence (Rhine-ROFI). In this area of the Rhine-ROFI, the sediment transport mechanisms are governed by the Rhine freshwater plume originating from the Rhine-Meuse estuary and propagating towards the coast in northward direction. Methods: The SPM near the bottom at a mooring located at 12m of water depth is analyzed in terms of concentration, particle size and shape in correlation with frontal dynamics and weather conditions for two seasons of winter 2013 (12 February - 07 March) and autumn 2014 (17 September - 06 October). Results and discussion: The freshwater front transports organic matter (such as microalgae strains and other organic matter) from the estuary into the coastal area. In calm weather conditions in autumn, most particles in suspension are of low density and high anisotropy. These particles are recognized as elongated algae strains with some organic matter-clay aggregates (flocs), giving trimodal Particle Size Distributions (PSD). During the neap tides strong salinity stratification and low turbulence result in SPM accumulation at the bed forming a fluff layer. At spring tides a fast switch between stratified and well mixed water column conditions caused by tidal mixing results in resuspension of SPM. During spring tides, the PSD’s are multimodal at low bed stress (predominance of microalgae) and monomodal at high bed stress (predominance of mineral sediment). At the storm initiation in autumn, the organic-matter rich fluff layer is depleted in a matter of hours, which is reflected in the change in modality of the PSD’s. Once the resuspended material is dominated by the mineral clay fraction, the PSD turns sharply monomodal. During winter monomodal PSD’s are recorded during calm weather conditions. The particles in suspension are then relatively spherical flocs of low density. During the winter storm, the fluff layer, which is much thinner than in autumn, is depleted very fast. This study shows the importance of organic matter in the transport of mineral sediment particles in coastal areas. The dynamic composition of the fluff layer of the bed should be accounted for in erosion models.

We present direct measurements of seafloor ripple dimensions, near-bed mean flow Reynolds stresses and near-bed turbulent sediment fluxes on a sandy inner shelf subjected to strong wave and tidal current forcing. The measurements of ripple dimensions (height, wavelength) and Reynolds stresses are used to evaluate the performance of a methodology for the incorporation of non-equilibrium ripple dynamics into the calculations of the drag exerted by the bed on the overlying flow (i.e., the bed stress) using a boundary layer model for wave–current interaction. The methodology is based on the simultaneous use of existing models for the time-dependent evolution of ripple geometry and for the wave–current boundary layer that enable a continuous feedback between bottom drag and small-scale seabed morphology, which determines seabed roughness. The model-data comparison shows good agreement between modeled and measured bed stresses and bedform dimensions. Moreover, the proposed methodology is shown to give better results than combining the wave–current interaction model and standard equilibrium ripple predictors, both in terms of bed stresses and ripple dimensions. The near-bed turbulent vertical sediment fluxes show good correlation with the combined wave–current stresses and are used as a proxy for the resuspension of fine sediments (d < 64 μm) from the sandy seabed matrix. Implications for the modeling of the resuspension processes and erosional fluxes are discussed in light of our findings.

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.

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.

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.

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.

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.

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.

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.