Processes affecting suspended sediment transport in the mid-field plume region of the Rhine River, Netherlands.

Raul P Flores, University of Washington, Department of Civil and Environmental Engineering, Seattle, WA, United States; Universidad Técnica Federico Santa María, Departamento de Obras Civiles, Valparaíso, Chile, Sabine Rijnsburger, TU Delft, Delft, Netherlands, Alexander Horner-Devine, University of Washington Seattle Campus, Department of Civil & Environmental Engineering, Seattle, WA, United States, Alejandro Jose Souza, CINVESTAV-IPN, Marine Resources, Merida, Mexico and Julie Pietrzak, Delft University of Technology, Environmental Fluid Mechanics, Delft, Netherlands
Abstract:
This work will describe dominant processes affecting suspended sediment transport along the Dutch coast, in the mid-field plume region of the Rhine River. We will present field observations from two long-term deployments conducted in the vicinity of the Sand Engine, a mega-nourishment experiment located 10 km north of the Rhine river mouth.

To investigate the role of density stratification, winds, tides, waves and river plume processes on sediment transport, frames and moorings were deployed within the excursion of the tidal plume front generated by the freshwater outflow from the Rhine River for 4 and 6 weeks during years 2013 and 2014, respectively. The moorings were designed to measure vertical profiles of suspended sediment concentration (SSC) and salinity, using arrays of CTDs and OBS sensors. Mean tidal velocities were measured using bottom-mounted ADCPs. The near-bed dynamics and the near-bottom sediment concentrations were measured as well using a set of synchronized ADVs and OBSs.

By combining the two deployments we observe hydrodynamics and suspended sediment dynamics under a wide range of forcing conditions. Preliminary observations indicate that stratification is highly dependent on wind magnitude and direction, and its role is primarily identified as to induce significant cross-shore sediment transport product of the generation of cross-shore velocities due to the modification of the tidal ellipses and the passage of the surface plume front. The passage of the surface plume front generates strong offshore currents near the bottom, producing transport events that can be similar in magnitude to the dominant alongshore transport. Preliminary results also indicate that storms play an important role in alongshore transport primarily by wave-induced sediment resuspension, but as stratification is suppressed due to the enhancement of mixing processes, no significant cross-shore transport is observed during very energetic conditions.

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