J. PietrzakĀ¹*, M. Geraeds1, T. Wegman1 L. Keyzer1, A. Gerritsma1, A. Horner-Devine2, Dave Ralston3, H. Dijkstra4, M. Verlaan1, C. Katsman1, W. Kranenburg1,5

1 Delft University of Technology; 2 University of Washington; 3 Woods Hole Oceanographic Institute; 4 Utrecht University; 5 Deltares

*corresponding author: J. Pietrzak

Introduction

The Rhine River Plume forms one of the largest Regions of Freshwater Influence (ROFI) in Europe. In 2013 and 2014 the STRAINS (STRAtification Impacts Near-shore Sediment) field campaigns were carried out off the Dutch coast during typical river discharge conditions. Tidal plume fronts were found on every ebb tide as freshwater discharges into a strong tidal cross flow at the mouth of the Rotterdam Waterway, Rijnsburger et al. (2021). However, little is known about the impact of low river discharge conditions on the evolution of tidal plume fronts in Rhine River Plume and their downstream evolution. Here we use both data and numerical models to investigate low river discharge conditions and compare them to normal conditions.

Methods

During the summer drought of 2022 an extensive field campaign was carried out offshore of the mouth of the Rotterdam Waterway and within the estuary. The field campaign was part of the SALTISolutions research program, and was carried out together with Rijkswaterstaat. Here we focus on the data collected around the river mouth. We use the Dutch Continental Shelf model to simulate the flow and stratification in the downstream river plume, coupled with the RMM3D model to simulate the flow in the delta during the drought. We compare data collected during the drought with data collected during typical discharge conditions. We explore the role of tidal plume fronts on the near to mid field river plume on mixing and recirculation in the Rhine River Plume. Then we compare this data, with some of the data collected during the drought of 2022. We explore how extreme, low discharge conditions impact the Rhine River Plume.

Results and discussion

Under typical river discharge conditions tidal plume fronts are shown to play an important role in the mixing, downstream transport of freshwater and re-circulation in the mid to near field river plume. In contrast low river discharge conditions reduce the size and strength of the tidal plume fronts and the recirculation in the downstream river plume. We discuss the consequences for along and cross-shelf transport.

References

Rijnsburger, S., Flores, R. P., Pietrzak, J. D., Horner-Devine, A. R., Souza, A. J., & Zijl, F. (2021). The evolution of plume fronts in the Rhine region of freshwater influence. Journal of Geophysical Research: Oceans, 126. https://doi.org/10.1029/2019JC015927

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