Floc Size Distribution Over The Tidal Cycle

Let us next examine how flocculation affects the sediment dynamics in a turbidity maximum. For this purpose, we analyze the field measurements carried out in the Ems estuary at the border of The Netherlands and Germany (Van Leussen5) with the use of a IDV-model. Contrary to Sections 12.2 and 12.3, we now include all physical processes in our analysis, such as settling and turbulent mixing, the effects of sediment-fluid interaction and flocculation. For details on this model, the reader is referred to Winterwerp.9

These measurements were carried out in June 1990 along five stretches of the river. At each stretch, two anchor stations at a mutual distance of approximately 1 km were deployed to measure the flow velocity with an Ott meter, the salinity from conductivity and temperature measurements, and the suspended sediment concentration with a Partech turbidity meter, or by taking water samples at high suspended sediment concentrations. During 13 h these parameters were measured every 1 h at 0.3, 0.5, 1,2 m, etc. above the bed, till 1.5 m below the water level. Along these stretches, measurements were also taken from a moving vessel, determining the relative flow velocity, salinity, and suspended sediment concentration with identical instruments, and the floc size and settling velocity with a video system (VIS). A summary of the measuring locations is presented in Figure 12.10, from Van Leussen.5

FIGURE 12.10 Locations of field measurements in the Ems estuary. (After Van Leussen, PhD thesis, University of Utrecht, The Netherlands, 1994.)

The river discharge during these measurements was about 10 to 25 m3/sec, which is very low for the Ems river. As a result, the turbidity maximum is located around stretch B, about 15 km upstream of its usual location in the Emder Fahrwasser. Because of this low river discharge, vertical salinity gradients were virtually absent during the entire measuring campaign. Moreover, the salinity at Station 1 and 2 remains almost constant at S « 0.2 ppt.

The water level and flow velocity at Stations 1 and 2 were almost identical, and h and U at both stations were also almost in phase. The suspended sediment concentration c at Station 1 was also almost identical to that at Station 2, though during maximum flow velocity, the measured values at Station 1 were somewhat larger than at Station 2. This indicates that, though advection certainly played a role, horizontal gradients in sediment concentration were small. Hence these data are suitable for analysis with a 1DV-model. Maximum values of the depth-mean concentration C varied from about 0.7 to 1.0 g/l during maximum flood and maximum ebb velocity (MEV) down to about 0.3 g/l during slack water, for example, Van Leussen5 and Winterwerp.9 Figure 12.11, presenting the measurements in the form of isolutals (i.e. lines of equal suspended sediment concentration), shows that during flood, the sediment was almost homogeneously mixed over the water depth, whereas during ebb, more stratified conditions occurred. Around high water slack (HWS), the suspended sediment concentration dropped rapidly. The very large concentrations around t = 1000 min are attributed to instabilities of the steep mud banks, supplying large amounts of mud to the river (Van Leussen23), and will be ignored.

The measurements in Station 1 and 2 have been simulated with a 1DV-model, including the effects of sediment-induced buoyancy effects and turbulence-induced flocculation (Winterwerp9,20). The measured variation in water level and depth-mean flow velocity is prescribed, and the measured depth-mean suspended sediment concentration is set at C0 = 0.61 g/l, according to the data. All sediment remains in

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