A., celebrada en su domicilio social a las nueve horas del doce

The INSSEV flocculation measurements (IMDC report 9) were conducted at three locations on three subsequent days, in September 2006. Measurements near the future CDW revealed three sediment concentration peaks: the first shortly after LW, the second halfway the flood, and the third around HW (Figure 6.17). Within these three peaks, the shear stresses were highest in the LW concentration peak (1.4 N/m2 versus 1.0 N/m2 in the other two). The resulting high floc break-up rates produced the lowest floc size and settling velocity. During the second peak, the shear stress was lower and therefore the floc size higher, but due to a dominance of microflocs, the average settling velocity was comparable. The shear stress and sediment concentration were similar in the third concentration peak shortly before HW. However, this peak was followed by a phase with lower concentration and low shear stresses, a large amount of macroflocs and therefore substantially higher settling velocities.

Figure 6.17: Time series of floc properties on 5 September 2006 (location CDW). a) shear stress and SPM, b) water depth, c) mean floc size and settling velocity, d) macrofloc and microfloc effective density, e) macrofloc and microfloc SPM distribution, and f) macrofloc and microfloc settling velocity. From IMDC

Figure 6.18: Time series of floc properties on 6 September 2006 (location sill). a) shear stress and SPM, b) water depth, c) mean floc size and settling velocity, d) macrofloc and microfloc effective density, e) macrofloc and microfloc SPM distribution, and f) macrofloc and microfloc settling velocity. From IMDC

Figure 6.19: Time series of floc properties on 7 September 2006 (location Deurganckdok). a) shear stress and SPM, b) water depth, c) mean floc size and settling velocity, d) macrofloc and microfloc effective density, e) macrofloc and microfloc SPM distribution, and f) macrofloc and microfloc settling velocity.

The shear stresses at the DGD sill on the right bank of the Deurganckdok (facing the Scheldt) on the following day were substantially lower, peaking at 0.8 N/m2. Only one distinct high sediment concentration peak was observed at the sill. This peak coincides with the third peak of the previous day, and is caused by an eddy deflected into the dock shortly before HW (see previous section). The settling velocity in this peak is high, and dominated by macroflocs (similar to the previous day) However, the concentration near the bed and near the surface is substantially higher than that measured the previous day. Therefore, it seems that this sediment patch is of different origin than that measured at the CDW location. The sediment patch measured on the sill is probably part of the main stream of the Scheldt, whereas sediment observed on the CDW location is probably locally resuspended.

The shear stress on the left bank of the DGD peaks at 0.5 N/m2. Sediment concentrations are substantially higher than during the other two surveys, ranging from several 100’s to over 2,000 mg/l. Macroflocs dominate the floc suspension throughout the tidal cycle, resulting in settling velocities of several mm/s. During a larger part of the ebb, inflow occurs (probably as density- driven near-bed inflow, see the section above) with a sediment concentration around several 100’s mg/l. Since the observations on the Scheldt and on the sill showed both a dominance of microflocs, the dominance of macroflocs even during inflow implies that microflocs rapidly evolve into macroflocs. High concentrations (> 1g/l) occur during the first half of the flood, during which the near-bed flow is directed out of the dock. During this time, a highly concentrated bottom current that must have been formed further away in the dock, is transported towards the Scheldt.

Comparing the INSSEV sediment concentration measurements carried out in the dock on 7 September 2006 (IMDC report 9) with sediment concentrations measured on the dock entrance on 26 September, especially the location Xa (IMDC report DGD 2.2) reveals that these surveys may not be very representative for the entire dock. Both locations were relatively close together, with the exception that the INSSEV measurements were done very close to the quay wall and the Xa survey was approximately 100 m from the quay wall. However, the sediment concentration at the INSSEV location was up to 2000 mg/l whereas that at Ka is up to only 100 mg/l. The ambient conditions (IMDC report 5.6) were, however, not substantially different. Although the sediment concentration at the beginning of September was slightly higher than at the end of September, the difference seems insufficient to explain the large variation in measured sediment concentrations. Also, the sediment concentration measured on the sill on the right hand side of the dock (facing the Scheldt) is more consistent with the 26 September survey: the sediment concentration peaks at 200 mg/l during the INSSEV survey, and at 100 mg/l on 26 September (location Zc, which is closest to the INSSEV location). Such a relatively small difference may be attributed to variations in the sediment concentration on the Scheldt. However, the difference of 100 mg/l and 2000 mg/l may not. Therefore the sediment concentration close to the quay walls appears to be substantially higher than further away from the walls. Hence, the INSSEV flocculation observations in the dock may not be entirely representative for the larger part of the dock.