High resolution topo-bathymetric and flow measurements and 2D-hydrodynamic numerical modelling to evaluate the effects of the deepening of the navigation channel in the Western Scheldt

To guarantee optimal port accessibility, one of the projects within the scope of the LongTermVision for the Scheldt estuary to be executed, is the deepening of the navigation channel. In 2001 the Port of Antwerp Expert Team proposed a new disposal strategy, using dredged material to reshape eroded sandbar. A pilot study near the Walsoorden sandbar resulted in the positive evaluation of the feasibility of this new idea. Within the project of deepening of the navigation channel, this new idea was formulated in one of the alternatives. The environmental impact assessment and an appropriate assessment concluded that a new disposal strategy should be applied: the dredged material had to be disposed near sandbars at 4 locations, to create new ecological valuable habitats. To evaluate the success of this new disposal strategy an extensive monitoring programme was set up . Frequent (2-weekly up to two-monthly) topo-bathymetric surveys using multibeam echo sounding are performed in combination with seasonal sedimentation-erosion measurements using both RTK (intertidal) and singlebeam (subtidal). Also flow velocities on 10 transects, each consisting of 4 measurements points, in the shallow subtidal and the intertidal zone near the disposal locations are monitored using ADCP’s (Nortek – AquaDopp). The ADCP devices on the intertidal zone are dug in the sandbar and look upward, allowing maximal vertical resolution when flooded during the tidal cycle. Parallel with the monitoring, a validated 2D-numerical hydrodynamic model is used to evaluate the effects of the disposal of sediments. Every three months, a simulation is performed using the most recent topo-bathymetry. This allows an evaluation of the changes in flow velocities and flow patterns near the disposal areas. A special “protocol” (see Depreiter et al., 2012) with several criteria was developed to evaluate the monitoring results and the success of the new disposal strategy.