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

Raul Flores Audibert, University of Washington, Department of Civil and Environmental Engineering, Seattle, United States; Universidad Técnica Federico Santa María, Departamento de Obras Civiles, Valparaíso, Chile, Sabine Rijnsburger, TU Delft, Delft, Netherlands, Alex R Horner-Devine, University of Washington Seattle Campus, Department of Civil & Environmental Engineering, Seattle, United States, Nirnimesh Kumar, University of Washington, Department of Civil & Environmental Engineering, Seattle, WA, United States, Alejandro J Souza, CINVESTAV-IPN, Mérida, YC, Mexico and Julie Pietrzak, Delft University of Technology, Environmental Fluid Mechanics, Delft, Netherlands
Abstract:
The occurrence of turbidity maximum zones (TMZs) is relevant for a wide variety of ecological processes and problems, as they are often related to the inhibition of primary production, hypoxia, the accumulation of contaminants and the impairment of the optimal functioning of coastal engineering projects. In the Rhine region of freshwater influence (ROFI), which is created by the freshwater discharge from the Rhine-Meuse system into the Southern North Sea, a persistent turbidity maximum zone has been observed between 1-4 km from the coast with peak sediment concentrations of 500-1000 mg/l (Figure 1a; van der Hout et al., 2015; van Alpen, 1990). This TMZ is thought to extend at least 80 km downstream of the mouth of the Rhine-Meuse estuary (Figure 1b), making it the one of the main features of the fine sediment dynamics along the Dutch coast. Here we summarize results from idealized numerical simulations that were conducted to investigate the role of the cross-shore dynamics and tidal straining that is observed along the Rhine ROFI on the formation of the coastal TMZ. The model qualitatively agrees with in situ observations, and accurately reproduces the cross-shore location of the TMZ. The sediment convergence is generated by subtidal landward sediment fluxes that are created by asymmetries in vertical mixing between the stratifying and de-stratifying phases of the tidal cycle. The magnitude of the modeled suspended sediment concentrations within the TMZ and its cross-shore extension depend on the particle size of the suspended sediments. Results are generalized by varying bottom slope, tidal forcing and density gradients, and the model output is analyzed in terms of Simpson, Stokes and Rouse nondimensional numbers. Based on these numbers, the possibility of occurrence of tidal straining and a coastal TMZ can be readily evaluated for other locations.

References

Van Alphen, J. S. L. J. (1990). A mud balance for Belgian-Dutch coastal waters between 1969 and 1986. Netherlands Journal of Sea Research, 25(1-2), 19-30.

van der Hout, C. M., Gerkema, T., Nauw, J. J., & Ridderinkhof, H. (2015). Observations of a narrow zone of high suspended particulate matter (SPM) concentrations along the Dutch coast. Continental Shelf Research, 95, 27-38.

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