Formation of the Zeebrugge coastal turbidity maximum: The role of uncertainty in near-bed exchange processes

Despite availability of a large amount of observational data and modelling studies, the mechanisms maintaining the Turbidity Maximum in the Belgian-Dutch coastal zone around the port of Zeebrugge (Belgium) are insufficiently understood. In order to better understand the dynamics of this turbidity maximum we examine the role of baroclinic (salinity and sediment-induced) processes and local sediment sources on the formation and persistence of the turbidity maximum through two different numerical model approaches. One model approach allows erosion of the highly compacted muddy seabed, serving as a sediment source, in line with observations of bed level change over several decades. The other approach reduces the exchange between the bed and the water column, to mimic the formation of highly concentrated near-bed suspensions with concentrations of several g/l observed around the port of Zeebrugge. Both model approaches are calibrated to various sources of available data (in situ sediment concentration observations, satellite image, bed level changes, mud content and dredging data), which they reproduce comparably well. However, reducing the water-bed exchange strengthens sediment convergence in the turbidity maximum, whereas the sediment source leads to sediment export. With the available data, it is difficult to determine which of the approaches is more realistic. Apparently, the lack of knowledge on near-bed exchange processes introduces an important source of uncertainty which cannot be adequately addressed with currently available observations. This work therefore shows that more quantitative knowledge on water-bed exchange processes in turbid marine environments is needed. It is further hypothesized that the large-scale erosion of the muddy seabed following the extension the port of Zeebrugge in the early 1980's brought such a large amount of sediment in suspension (50-100 million ton) that sediment convergence was strengthened. This increasing sediment convergence introduces a positive feedback mechanism that maintains sediment in the Turbidity Maximum, or even strengthens it. The high sediment concentrations observed today may therefore be a long-term effect of port construction carried out decades earlier.