Hydrodynamics in the mid-field plume region of the Rhine ROFI

Abstract

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.