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.@en

This paper presents the mud dynamics in the harbor basin of Zeebrugge in the Southern North Sea based on an analysis of field data. Mud is typically transported into and within the harbor basin through advection of suspended particulate matter (SPM). Three important timescales have been identified. On the intratidal timescale, sediment import occurs from 2 h before high water to high water. Flood currents in the North Sea (directed northeastward along the Belgian coast) drive the primary gyre in the harbor mouth which is advected into the basin during rising tide. This results in water inflow near the eastern breakwater and outflow near the western breakwater. Because of sediment settling in the harbor, this results in a net import of SPM. During spring tide, the SPM flux into the harbor basin is two to four times higher than during neap tide. However, the volume of sediment removed from the port by maintenance dredging is kept constant over the spring-neap cycle, causing the amount of mud in the harbor basin to grow around spring tide conditions. On the seasonal timescale, mud volume within the harbor basin is larger in winter and reaches a minimum at the beginning of autumn. Moreover, the measured densities within the deposited mud layers are lower in winter than in summer. The most shallow point of the 210-kHz reflector is also more shallow in winter. Finally, the profile of the interface of the mud layer in the sheltered Albert II dock is more horizontal in winter than in summer, suggesting seasonal variations in the strength of the mud layer. The question to what degree the seasonal variation of thickness and density of the fluid mud layer is related to differences in the suspended sediment input, to differences in the settling rates of suspended flocs, or to the mud consolidation rate remains open however. The data do not show a strong influence of meteorological conditions (waves, freshwater inflow) on siltation rates in the harbor basin.@en

The amount of sediments to be dredged and disposed depends to a large part on the suspended particulate matter (SPM) concentration. Tidal, meteorological, climatological, and seasonal forcings have an influence on the horizontal and vertical distribution of the SPM in the water column and on the bed and control the inflow of fine-grained sediments towards harbors and navigation channels. About 3 million tons (dry matter) per year of mainly fine-grained sediments is dredged in the port of Zeebrugge and is disposed on a nearby disposal site. The disposed sediments are quickly resuspended and transported away from the site. The hypothesis is that a significant part of the disposed sediments recirculates back to the dredging places and that a relocation of the disposal site to another location at equal distance to the dredging area would reduce this recirculation. In order to validate the hypothesis, a 1-year field study was set up in 2013–2014. During 1 month, the dredged material was disposed at a new site. Variations in SPM concentration were related to tides, storms, seasonal changes, and human impacts. In the high-turbidity Belgian near-shore area, the natural forcings are responsible for the major variability in the SPM concentration signal, while disposal has only a smaller influence. The conclusion from the measurements is that the SPM concentration decreases after relocation of the disposal site but indicate stronger (first half of field experiment) or weaker (second half of field experiment) effects that are, however, supported by the environmental conditions. The results of the field study may have consequences on the management of disposal operations as the effectiveness of the disposal site depends on environmental conditions, which are inherently associated with chaotic behavior.@en

The amount of sediments to be dredged and disposed depends to a large part on the suspended particulate matter (SPM) concentration. Tidal, meteorological, climatological, and seasonal forcings have an influence on the horizontal and vertical distribution of the SPM in the water column and on the bed and control the inflow of fine-grained sediments towards harbors and navigation channels. About 3 million tons (dry matter) per year of mainly fine-grained sediments is dredged in the port of Zeebrugge and is disposed on a nearby disposal site. The disposed sediments are quickly resuspended and transported away from the site. The hypothesis is that a significant part of the disposed sediments recirculates back to the dredging places and that a relocation of the disposal site to another location at equal distance to the dredging area would reduce this recirculation. In order to validate the hypothesis, a 1-year field study was set up in 2013–2014. During 1 month, the dredged material was disposed at a new site. Variations in SPM concentration were related to tides, storms, seasonal changes, and human impacts. In the high-turbidity Belgian near-shore area, the natural forcings are responsible for the major variability in the SPM concentration signal, while disposal has only a smaller influence. The conclusion from the measurements is that the SPM concentration decreases after relocation of the disposal site but indicate stronger (first half of field experiment) or weaker (second half of field experiment) effects that are, however, supported by the environmental conditions. The results of the field study may have consequences on the management of disposal operations as the effectiveness of the disposal site depends on environmental conditions, which are inherently associated with chaotic behavior.@en

One year of SPM concentration (SPMC) and Velocity data are analyzed to gain insight in the mud dynamics in the harbor of Zeebrugge. Seasonal dynamics are inferred from satellite images, depth soundings and SPMC data. @en