Abstract
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.
Highlights
The Port of Zeebrugge is subject to high siltation rates of mainly mud, and as a result, huge maintenance dredging works are mandatory (Fettweis et al 2011, 2016)
This suggests that a slower settling fraction determines the sediment concentrations in the upper part of the water column. These results are consistent with the analysis of the flow exchange mechanisms between a harbor basin and the open sea, applied to Zeebrugge by Vanlede and Dujardin (2014). They concluded that horizontal exchange is the most important component of the sediment import at the harbor mouth of Zeebrugge and that most of the sediment import occurs from 2 h before high water to high water
The effect of freshwater inflow on sediment import into the harbor is negligible, and only a slight positive correlation was found between the peak significant wave height and the natural depth change of the mud-water interface
Summary
The Port of Zeebrugge is subject to high siltation rates of mainly mud, and as a result, huge maintenance dredging works are mandatory (Fettweis et al 2011, 2016). The amount to be dredged depends on the inflow of mud into the harbor basin. Maintenance depth and dredging strategy in the port are guided by the principle of nautical depth. Pianc (1997) defines the nautical depth as the level at which physical characteristics of the bottom reach a critical limit beyond which contact with a ship’s keel causes either damage or unacceptable effects on controllability and maneuverability. In Zeebrugge, the nautical bottom is defined as the density level of 1200 kg/m3. This article is part of the Topical Collection on the 14th International Conference on Cohesive Sediment Transport in Montevideo, Uruguay 13–17 November 2017
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