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Abstract

Theoretical considerations suggest that, while changes in hydrodynamic conditions are an important factor in mobilizing and resuspending sediments, seasonal shifts in spatial sediment distributions in shallow water environments of intermediate to high latitudes should be influenced by variations in settling velocities and critical shear strengths induced by changes in water temperature and fluctuations in salinity. In the Wadden Sea of the southern North Sea, water temperatures range from <4°C in January to almost 24°C in July, and salinities from 2.6% to 3.3%. In accordance, the kinematic viscosity of sea water varies from 1.5728–0.9096 cSt (centi-Stokes). Under these circumstances sediment particles respond to the changes in viscosity with corresponding changes in settling velocity and particles of a given size thus have as many settling velocities as there are viscosity changes. In hydraulic terms a particle can therefore have many effective grain sizes. To facilitate the calculation of critical shear velocity on the basis of settling velocity, the following equation was developed: U*cr = (0.482 [((δS – δf)/δf) v g]0.282) * (0.15 wforumla) + 0.61.

The back-barrier tidal basin investigated in this study is characterized by a shoreward-decreasing energy gradient, as documented by progressively decreasing grain sizes and the resulting landward-fining sequence of shore-parallel facies belts. Applying the viscosity principle, one should expect the facies belts to show a seasonal temperature adjustment. Time-series studies, however, have demonstrated that the sediment zonation pattern remains stable throughout the year; that is, a seasonal temperature-induced shift of the belts could not be confirmed.

This apparant contradiction can be explained if one assumes that the sediment distribution pattern is permanently adjusted to winter conditions at which the settling velocities are lowest (the sediment is hydraulically finest) and the energy input is highest. Subsequent summer conditions are unable to reverse the depositional pattern produced in winter because settling velocities are higher, making the sediment hydraulically coarser. This interpretation is supported by the observed seasonal dynamics of suspended matter. During winter, muddy sediments occupy relatively small areas adjacent to the mainland dike. In summer, on the other hand, mud contents of local sediments were found to increase and muddy sediments occupied much larger areas. This pattern is explained by the higher settling velocities of suspended matter in summer as required by the viscosity principle. Field data therefore support the theoretical considerations and demonstrate that the kinematic viscosity of the fluid plays an important role in depositional processes.

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