The Influence of Tide and Wind on the Propagation of Fronts in a Shallow River Plume

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.Environmental Fluid Mechanic

The formation of turbidity maximum zones by minor axis tidal straining in regions of freshwater influence

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.Environmental Fluid Mechanic

The Evolution of Plume Fronts in the Rhine Region of Freshwater Influence

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.Environmental Fluid Mechanic

Observations of Multiple Internal Wave Packets in a Tidal River Plume

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.Environmental Fluid Mechanic