On the consequence of a new tidal dock on the sedimentation regime in the Antwerpen area of the Lower Sea Scheldt

Abstract

Following the recent completion of the Deurganckdok (DGD) tidal dock in the Port of Antwerpen, Belgium, the Flemish government commissioned a programme of field surveys with the aim to identify potential changes in sediment properties. A significant feature of the Lower Sea Scheldt (LSS) is the presence of a turbidity maximum zone (TMZ) with depth-averaged suspended particulate matter (SPM) concentrations between 50 and 500 mg l −1. This paper highlights aspects of the findings of the suspended sediment properties measured during HCBS1 (conducted in February 2005 prior to DGD construction) and HCBS2 (September 2006 when the dock was open and in operation) surveys, including data comparison. Floc size ( D) and settling velocity ( W s) spectra were measured nominally 0.6 m above the estuary bed every 10–20 min (turbidity dependent), using derivatives of the INSSEV instrument. This instrument permitted the accurate calculation of the following floc properties: effective density, dry mass, porosity and mass settling flux (MSF). To characterise the corresponding near-bed hydrodynamics, the turbulence was measured by a 3-D Acoustic Doppler Velocimeter and the turbidity monitored by an array of Optical Backscatter Sensors. All measurements were conducted for runs of 8–10 h in duration. HCBS1 was conducted during neap tides in the winter, predominantly during the ebb phase, whilst HCBS2 experienced spring tides in the autumn with sampling generally on the flood. It is therefore important to note that only limited comparison between surveys and the different HCBS locations is possible. Even so, the survey revealed that, following construction of the DGD, turbidity was an order of magnitude higher in the DGD, than in the upper and lower Scheldt Estuary. It was also noted that that the DGD macrofloc fraction settled at W s macro =5.3 mm s −1 in the TMZ, which was 1.4 and 3.2 mm s −1 quicker than the fastest settling macrofloc population observed in the LSS during the winter at neap tides and late summer at spring tides, respectively. The HCBS surveys have highlighted the important role of low density macroflocs in the mass settling flux within the dock. At peak concentration in the DGD produced a MSF of 13.2 g m −2 s −1; over an order of magnitude greater than observed within the HCBS1 TMZ. Within DGD the time series MSF of 30,200 mg m −2 s −1, was five times the MSF observed at the dock entrance, and 19 times the MSF observed in the Scheldt estuary outside the dock (HCBS2_SS). Over 70% of the total MSF occurred during the TMZ passage through the dock on the flood. The weaker currents present in the dock, particularly on the ebb, when combined with a near continual abundance of fast settling macroflocs, will tend to trap sediment in the basin, whilst near-bed turbulence damping will reduce the level of bed erosion in DGD. The sedimentation in the dock is stimulated by a significantly less turbid supply of cohesive sediment present in the Scheldt Estuary. It is proposed that the construction of a passive structure, such as a current deflecting wall, may reduce sediment entering the open tidal dock.