Fronts and tidal straining in the mid-field plume of the Rhine ROFI
Sabine Rijnsburger, TU Delft, Delft, Netherlands, Raul P Flores, University of Washington, Department of Civil and Environmental Engineering, Seattle, WA, United States, Alejandro Jose Souza, CINVESTAV-IPN, Marine Resources, Merida, Mexico, Alexander Horner-Devine, University of Washington Seattle Campus, Department of Civil & Environmental Engineering, Seattle, WA, United States and Julie Pietrzak, Delft University of Technology, Environmental Fluid Mechanics, Delft, Netherlands
Abstract:
In this study we will present observations in the mid-field plume of the Rhine ROFI, focussing on the frontal dynamics. Early field studies were mainly focussed on the downstream plume (Simpson et al. ,1993; Simpson & Souza 1995; Souza & Simpson,1996), where cross-shore straining has a large contribution for the periodic stratification. De Ruijter (1997) showed the existence and advection of tidally pulsed freshwater lenses from Satellite images close to the river outflow.
During a six week period in September and October 2014, a large dataset was obtained off the Dutch coast, only 10 km downstream of the mouth of the Rotterdam Waterway. At two locations, 2 and 6 km offshore, moorings with CTD and OBS instruments at different locations over depth were deployed to obtain vertical profiles of density and suspended sediment concentrations (SSC). In addition ADCP instruments were used to get insight into the vertical velocity profiles. ADV instruments are used to measure the near bottom currents and turbulence. The weather conditions were varying from very calm weather to heavy storms during the six week period, resulting in a dataset with various conditions.
Preliminary results clearly show the presence of freshwater fronts at both locations. In addition stratification due to the cross-shore velocity shear is observed. The existence and strength of both mechanisms is dependent on the local conditions. Both show a different behaviour on spring and neap tide. The data shows the importance of the wind magnitude and direction on the behaviour of the water column as well. Further on, the development of a third density layer near the bottom is observed. It seems that this only happens during neap tide and very calm winds. This phenomenon is only observed 2 km offshore, which indicates the importance of the location. We expect with further study to get a more detailed picture of the hydrodynamics in this area.
