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Deurganckdok: High-Concentration Benthic Suspensions (HCBS)


The Lower Sea Scheldt is characterized by large horizontal salinity gradients and the presence of a turbidity maximum with depth-averaged concentrations ranging from 50 to 500 mg/l.

The salinity gradients generate significant density currents between the river and the entrance channels to the locks, causing large siltation rates. It was to be expected that Deurganckdok will suffer from such large siltation rates, which may double the amount of dredging material to be dumped in the Lower Sea Scheldt. Another observation during the last years is that the composition of the sediment dredged at the Sill of Zandvliet became more muddy.

To deal with these problems, and to facilitate the management of the Lower Sea Scheldt, more knowledge on the fine sediment dynamics was obtained from in-situ measurements and the development of an advanced numerical sediment transport model.

It was expected that temporary layers of soft mud may be formed in this lower part of the water column, which may move independently of the tidal water movement, in particular during slack water. They may contribute significantly to the siltation rate in the Deurganckdok. Therefore it needed to be established what the role of these soft mud layers on the sediment dynamics in the Lower Sea Scheldt, both from a qualitative and quantitative point of view.

The goal of the first HCBS study was threefold:

  1. To detect the occurrence of near-bed high-concentration mud suspensions (referred to as high-concentration benthic suspensions - HCBS), their dynamic behaviour and the conditions and locations of their occurrence;
  2. the second goal was to establish fluxes of fine sediment in the river with the purpose to calibrate a numerical 3D cohesive sediment transport model of the Lower Sea Scheldt;
  3.  the third goal is to establish the sediment properties required for the cohesive sediment transport model.

The second HCBS survey aims to complete the same HCBS and flux measurements after the opening of Deurganckdok in July 2005.

The analysis of the measurements has clearly proven that the variables measured vary in a very complex way with the tide, along neap tide-spring tide cycles and with the seasons.

The HCBS measurement campaigns and the numerical modelling studies have improved the understanding of the sediment dynamics in the Scheldt.

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