Flocculation and floc break-up related to tidally induced turbulent shear in a low-turbidity, microtidal estuary

Abstract

Flocculation and floc break-up dynamics were studied in two field campaigns with calm winds in the northernmost part of the Danish Wadden Sea. The studies were carried out using a LISST-100C together with CTD-instruments and a current meter. A quasi-Lagrangian profiling method was used to assess the dynamics on a short temporal scale (<tidal cycle) in the frame of the entire water column in the first campaign. In the second campaign the instruments were moored (Eulerian deployment) to get information on a longer temporal scale (>tidal cycle). The quasi-Lagrangian measurements showed strong flocculation and floc break-up dynamics in the lower part of the water column in the period around slack water. These dynamics were confirmed in the Eulerian deployments and were reoccurring in every tidal cycle. The dynamics were mostly governed by changes in turbulent shear. Strong microflocs with a lower mean threshold diameter of 50–60μm present at high turbulent shear flocculated to form fragile macroflocs with sizes of several hundred microns and mean diameters above 80μm around slack water periods. A hysteresis in floc break-up and flocculation was found at high water slack (HWS), as flocs formed at a root mean square velocity gradient (G) around 1s−1 were not broken up until the shear reached G>4s−1. As an effect, the flocculation time (time for flocculation from constituent microflocs to fragile macroflocs) was roughly 100min while the equivalent floc break-up time was roughly 200min. The asymmetrical, flood-dominated tidal wave showed how longer periods of low turbulent shear enhanced the hysteresis. The study demonstrates how the combination of the two deployment types will give a more adequate description of the flocculation dynamics over tidal cycles. Moreover, it shows the importance of incorporating specific flocculation aspects in numerical models, as a failure to incorporate the flocculation hysteresis results in an underestimation of the net import of sediment by about 30%.