Influence of harbour construction on mud accumulation in the Scheldt estuary (original) (raw)

Marine vs Fluvial Bottom Mud in the Scheldt Estuary

Estuarine, Coastal and Shelf Science, 2000

The ratio of marine to fluvial bottom mud in the Scheldt Estuary (south-west Netherlands) was calculated by applying factor analysis to a data set of 66 bottom samples. In each sample, concentrations of As, Cd, Cr, Cu, Hg, Ni, Pb and Zn were measured in the mud fraction (<63 m). Three linear independent processes were found to explain the variability of the elemental concentrations: (a) the supply of fluvial mud to the mixing zone, (b) chemical precipitation of Ni on insoluble manganese oxides in the transition area between the anoxic upper estuary and oxic lower estuary, and (c) the formation of insoluble Cd-, Cu-, Hg-and Zn-sulphides in the anoxic upper estuary and a mobilization of Cd, Cu, Hg and Zn in the oxic lower estuary. Scores of the first factor were used to calculate the marine fraction of bottom mud. In most of the upper estuary, the marine fraction is less than 10%. Between Lillo and Saeftinge, the marine fraction increases sharply from 10% to 75%. Further seaward, in the lower estuary, the marine fraction gradually increases from 75% to 95%. Possible influences of the Zeebrugge dumpsites on the mixing ratio was not revealed by the data set.

On the consequence of a new tidal dock on the sedimentation regime in the Antwerpen area of the Lower Sea Scheldt

Continental Shelf …, 2010

Following the recent completion of the Deurganckdok (DGD) tidal dock in the Port of Antwerpen, Belgium, the Flemish government commissioned a programme of field surveys with the aim to identify potential changes in sediment properties. A significant feature of the Lower Sea Scheldt (LSS) is the presence of a turbidity maximum zone (TMZ) with depth-averaged suspended particulate matter (SPM) concentrations between 50 and 500 mg l À 1 . This paper highlights aspects of the findings of the suspended sediment properties measured during HCBS1 (conducted in February 2005 prior to DGD construction) and HCBS2 (September 2006 when the dock was open and in operation) surveys, including data comparison.

Development of a mud transport model for the Scheldt estuary

Continental Shelf Research, 2011

a b s t r a c t A mud transport model for the Scheldt estuary is being developed. Its purpose is to support managers of the Scheldt estuary with the solution of a number of managerial issues. The model domain ranges from the tidal boundary at Gent down to the Belgian coastal cities of Nieuwpoort and Zeebrugge.

General description of the Scheldt estuary

Trace Metals in the Westerschelde Estuary: A Case-Study of a Polluted, Partially Anoxic Estuary, 1998

A general description of the Scheldt estuary, including the hydrology, the sediment transport, the productivity and the biodegradation with respect to their influence on the trace metal behaviour in the Scheldt estuary, is given. The river basin can be divided in several sections according to their morphological, hydrodynamical and sedimentary properties. The zone from km 78 to 55, which corresponds roughly with the salinity zone from 2 to 10 psu, is the zone of high turbidity, high sedimentation and of oxygen depletion, especially in the summer period. That area is called the geochemical filter because the solid/dissolved distribution of the trace metals is controlled by redox, adsorption/desorption, complexation and precipitation/coprecipitation processes. The sedimentation rate in that area is estimated at 280 Mkg y ,1. In the downstream estuary the phytoplankton activity increases due to the restoration of oxygen and to the much lower turbidity values. That area is called the biological filter because incorporation of trace metals by the plankton communities lowers the trace metal concentrations during the productivity period, while transformation of metal species, especially observed with mercury, occurs during that period too.

The mud deposits and the high turbidity in the Belgian–Dutch coastal zone, southern bight of the North Sea

Continental Shelf Research, 2003

The suspended sediment processes and the mudfields found in the Belgian/Dutch coastal area (Southern North Sea) are discussed by presenting an integrated data-modelling approach of the suspended sediment transport along the Belgian-Dutch coast, using a fine-grid coupled 2D hydrodynamic and sediment transport model and existing field and literature data. These mudfields and turbidity maxima are situated in a well-mixed, highly energetic hydrodynamic environment. In the past the occurrence of this high turbidity zone (more than a few hundreds mg/1 of suspended matter) was ascribed to a closed hydrodynamic system (gyre) in front of the coast. This study shows that the SPM input through the Strait of Dover, the shallowness of the considered area, the decreasing magnitude of the residual transport vectors from the French/Belgian border towards Zeebrugge and the specific hydrodynamic features are the main processes responsible for the presence of the turbidity maximum. The origin and the formation of these mud deposits in front of the coast are explained by the neap-spring tidal cycles and the presence of SPM sources (import of SPM through the Strait of Dover and through erosion of clay layers).

Baseline mapping of sediment distribution in the Western Scheldt estuary prior to tunnel boring

Large infrastructural construction projects may have a profound impact on the environment and ecology of environmentally sensitive areas. The construction of a tunnel below the Western Scheldt and the associated sediment dumping in the Western Scheldt Estuary may influence the sediment budget and the quality and quantity of pelagic and benthic life in the estuary. For the purpose of monitoring the effects of the dumping, it is essential to know the baseline situation, which is an inventory of a) the temporal and spatial dynamics and distribution of suspended matter in the estuary and b) the deposition of sediments on tidal flats. Because of the size of the potentially affected area and the dynamic character of the tidal water system it was decided to use remote sensing and mathematical models in addition to field measurements to determine the baseline conditions of the sediment distribution. The added value of an integration of modern techniques such as remote sensing and sediment transport modeling together with a sensible use of in-situ measurements was demonstrated. SPOT images rendered accurate synoptic distributions on suspended sediments, and were used for model calibration. Having established the baseline, the model was used to predict the distribution of dumped suspended matter in the Western Scheldt. The deposition of mud on tidal flats was analyzed both with models and from remote sensing. It was shown that dumped sediments will be best observable in spring and summer. Therefore, monitoring of the estuary during the tunnel construction with the present methodology is advised.

Long-term influence of maritime access works on the distribution of cohesive sediments: analysis of historical and recent data from the Belgian nearshore area ( …

Geo-Marine Letters, 2009

Long-term changes in the cohesive sediment distribution of the Belgian-Dutch nearshore zone (southern North Sea) are related to human activities (port construction, deepening of navigation channels, disposal of dredged sediments) and to natural variability, due to tides and meteorological effects. Results are based on the combined analyses of recent and historic (100 years ago) sediment sample information and bathymetric maps. Data processing was based mainly on field descriptions of the samples (consolidation, thickness) and on bathymetric maps of . Results indicate that the distribution of fresh mud and suspended sediment has changed during the last 100 years, due mainly to maritime access works. Most of the present deposition of thick layers of fresh mud (>30 cm) has anthropogenic causes. The results further indicate that erosion of older Holocene mud has increased in recent times and, as a consequence, higher amounts of fine-grained sediments are being released into the southern North Sea today.

Mud dynamics in the Port of Zeebrugge

Ocean Dynamics

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.

Long-term influence of maritime access works on the distribution of cohesive sediments: analysis of historical and recent data from the Belgian nearshore area (southern North Sea

Geo-marine Letters, 2009

Long-term changes in the cohesive sediment distribution of the Belgian—Dutch nearshore zone (southern North Sea) are related to human activities (port construction, deepening of navigation channels, disposal of dredged sediments) and to natural variability, due to tides and meteorological effects. Results are based on the combined analyses of recent and historic (100 years ago) sediment sample information and bathymetric maps. Data processing was based mainly on field descriptions of the samples (consolidation, thickness) and on bathymetric maps of 1866–1911. Results indicate that the distribution of fresh mud and suspended sediment has changed during the last 100 years, due mainly to maritime access works. Most of the present deposition of thick layers of fresh mud (>30 cm) has anthropogenic causes. The results further indicate that erosion of older Holocene mud has increased in recent times and, as a consequence, higher amounts of fine-grained sediments are being released into the southern North Sea today.

Suspended matter in the Scheldt estuary

Hydrobiologia, 2005

The Scheldt estuary is characterised by a specific energy pattern resulting from the interaction of wave energy, tidal energy and river energy. It divides the estuary into three parts and governs suspended matter transport and distribution pattern. Observation of suspended matter transport shows the existence of three estuarine turbidity maxima (ETM), a marine-dominated ETM in the lower estuary at the river mouth, a river-dominated ETM in the upper estuary with suspended matter concentration reaching up to 300 mg/l, and the most important tide-dominated ETM in the middle estuary with suspended matter concentrations from several hundred milligrams per litre up to a few grams per litre. Resuspension is the dominant phenomenon in this last ETM due to the tidal related bottom scour, which is initiated when a critical erosion velocity of 0.56 m/s is exceeded. An assessment of residual current along the axis of the estuary shows distinctive pattern between the surface water flow and the near bottom water flow. Also the local morphology of the river, natural or man-made, has a prominent effect on the orientation and strength of the residual currents flowing along either side of the river or river bend. Evaluation of suspended matter concentration in relation to the current flow shows no systematic correlation either because of phenomena as scour lag and settling lag mainly in the middle estuary, or because of the current independency character of uniform-suspension mainly in the upper and lower estuary. Quantification of suspended matter load exhibits a net downstream transport from the upper estuary, a near-equilibrium sustainable status in the middle estuary and a net upstream transport of suspended matter from the lower estuary. The characteristic of suspended matter is induced by and is a function of e.g. tidal phase, spring-neap tide, longitudinal and vertical distribution mechanisms, seasons, short and long terms of anthropogenic influence and/or estuarine maintenance. Suspended matter is dominated by complex and cohesive organo-mineral aggregates. It consists of a variable amount of an inorganic fraction (average of 89%) and an organic fraction and occurs largely as flocs, the size of which is remarkably larger in the upper estuary and smallest within the ETM in the middle estuary. Independent time series measurements (1990–2000) of suspended matter property show an increasing sand fraction, a decreasing organic matter content, a rise in δ13C as well as a decrease in water transparency. These independent measurements exhibit coherent consequences of estuarine maintenance operations. Maintenance dredging of the shipping channel and harbours and dumping operation in the Scheldt strengthen marine influence further landward, resulting in a sustained tidal range increment and upstream flow and transport of suspended matter.