Mud dynamics in the Belgian coastal zone and siltation in the harbor of Zeebrugge

Ports are important drivers for economic activity. For the Port of Zeebrugge, important sectors include cars, containers and liquefied natural gas (LNG). Due to significant siltation, frequent maintenance dredging is necessary in order to ensure the nautical accessibility. For Zeebrugge, that responsibility falls on the Flemish department of mobility and public works, at a yearly cost of about 70 millions euro. This thesis aims to contribute to the body of knowledge on the mud dynamics in the Belgian Coastal Zone, on the mechanisms behind the siltation of the harbor, and on the effects of the disposal of dredged material at sea. The cohesive sediment dynamics in the Belgian Coastal Zone (BCZ) are characterized by residual transport directed towards the northeast, and by the presence of a coastal turbidity maximum (CTM) that extends between Ostend and Zeebrugge. The resulting mud deposits are a persistent feature in the BCZ, at least since the beginning of the 20th century. Baroclinic effects, tidal asymmetry and local gradients in the residual current all play a role in trapping sediment in the CTM. In this thesis, the sediment dynamics are studied using a combination of data analysis and numerical modeling. First, a dataset is analysed that consists of 51 tripod deployments over nine years (2005-2013) at locations MOW1 and Blankenberge, kindly provided by the Royal Belgian Institute of Natural Sciences (RBINS). Tidal ensembles are derived of velocity and near-bed suspended sediment concentration (SSC). These ensembles are used to study the vertical gradient of SSC, the influence of waves, and the seasonal variation. Subsequently, a 1DV model is set up that computes the transient vertical distribution of a single fraction of SSC, and the mud content in the bed. This model is used to study the intratidal variation of the near-bed SSC observed at Blankenberge. It is shown that a two-fraction (coarse and fine) sediment model is necessary to model both the the ebb and the flood peak of SSC. Subsequently a 3D sediment transport model is set up. The settling velocity of the coarse and fine fraction are taken over from the 1DV model, as is the zero order resuspension constant. The set of measurements that is available for model calibration and validation is maximized by using both the comparable tide method and tidal ensembles. The model confirms that local hydrodynamic conditions trap sediment in the CTM, and it is used to study the role of salinity-driven baroclinic currents. A sediment balance is derived to better understand the sediment dynamics in the BCZ as an open system with some closed characteristics: even though the residual sediment transport through the Dover Strait is an important sediment supply to the BCZ, the relative importance of local erosion and deposition gives it some characteristics of a closed system, like a different clay mineralogical composition than English Channel mud.