Dries Van den Eynde

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

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/l 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)

Monitoring the effects of disposal of fine sediments from maintenance dredging on suspended particulate matter concentration in the Belgian nearshore area (southern North Sea)

The impact of continuous disposal of fine-grained sediments from maintenance dredging works on the suspended particulate matter concentration in a shallow nearshore turbidity maximum was investigated during dredging experiment (port of Zeebrugge, southern North Sea). Before, during and after the experiment monitoring of SPM concentration using OBS and ADV altimetry was carried out at a location 5 km west of the disposal site. A statistical analysis, based on the concept of populations and sub-sampling, was applied to evaluate the effect. The data revealed that the SPM concentration near the bed was on average more than two times higher during the dredging experiment. The disposed material was mainly transported in the benthic layer and resulted in a long-term increase of SPM concentration and formation of fluid mud layers. The study shows that SPM concentration can be used as an indicator of environmental changes if representative time series are available.

Seasonality of floc strength in the southern North Sea

The suspended particulate matter (SPM) concentration in the high turbidity zones of the southern North Sea is inversely correlated with chlorophyll (Chl) concentration. During winter, SPM concentration is high and Chl concentration is low and vice versa during summer. This seasonality has often been associated with the seasonal pattern in wind forcing. However, the decrease in SPM concentration corresponds well with the spring algal bloom. Does the decrease of SPM concentration caused by changing wind conditions cause the start of algae bloom, or does the algae bloom decrease SPM concentrations through enhanced flocculation and deposition? To answer the question, measurements from 2011 of particle size distribution (PSD), SPM, and Chl concentrations from the southern North Sea have been analyzed. The results indicate that the frequency of occurrence of macroflocs has a seasonal signal, while seasonality has little impact upon floc size. The data from a highly turbid coastal zone suggest that the maximum size of the macroflocs is controlled by turbulence and the available flocculation time during a tidal cycle, but the strength of the macroflocs is controlled by the availability of sticky organic substances associated with enhanced primary production during spring and summer. The results highlight the shift from mainly microflocs and flocculi in winter toward more muddy marine snow with larger amounts of macroflocs in spring and summer. The macroflocs will reduce the SPM concentrations in the turbidity maximum area as they settle faster. Consequently, the SPM concentration decreases and the light condition increases in the surface layer enhancing algae growth further.

Recommendations for the sustainable exploitation of tidal sandbanks

A basic requirement for allowing marine aggregate (sand) extraction on the Belgian Continental Shelf (which takes place on sandbanks) is that it should not result in major environmental changes. However, a tidal sandbank (Kwinte Bank, Flemish Banks), exploited intensively since the 1970’s, has shown evidence of significant morphological changes with the development of a 5 m deep depression in its middle section; thus, since February 2003, sand extraction has ceased in this area in order to study the environmental impacts and the regeneration potential of the seabed. The present contribution synthesises the results of the multidisciplinary research, which has taken place in the area and, on the basis of these findings, considers the need for an efficient management framework, in both the planning and monitoring stages of the extraction. The investigation has shown that extraction has had significant impacts on the seabed sedimentary character and ecology and the local hydro-and sediment dynamic regime. Under these conditions, regeneration of the seabed is not likely in the short-term and, although modelling exercises have indicated possible recovery in the medium- and long-term, this is likely to be inhibited by the lack of appropriate sediments in the area. The results have provided the basis of the identification of a ‘suite’ of criteria, which can assist in the strategic planning/design of marine aggregate concession zones, the efficient management of marine aggregate extraction and the planning of effective environmental monitoring; these criteria are related to considerations on resource location, the nature/thickness of the targeted deposits, morphodynamics and sediment dynamics, biology and ecology and extraction practices. The Kwinte Bank investigation has demonstrated also the need for intensive monitoring schemes in order to identify the morphological, sedimentary and ecological impacts, related to the dredging activities. A critical part of these schemes should be the evaluation of the dredging-related effects, against the background of the natural dynamics of the seabed; thus, baseline information is crucial, as, in its absence, impact assessments are likely to remain inconclusive.

Modelling the Effects of Sand Extraction, on Sediment Transport due to Tides, on the Kwinte Bank

In recent years, the exploitation of marine aggregates is increasing. As an example, on the Belgian continental shelf, one particular sandbank (the Kwinte Bank) is exploited extensively; this has led to the creation of a 5 m deep depression along its central part. In the present contribution, the influence of these bathymetric changes, on erosion and sedimentation patterns are studied, using numerical modelling, in order to obtain an initial impression of the effect of such intense sand extraction on the stability of the sandbank. Different numerical models are utilised. Twodimensional and three-dimensional hydrodynamic models have been used to derive currents, whilst third generation wave models have been used to simulate the waves. Two different models are presented, which calculate the total load sediment transport as a function of the local currents and waves. These models have been used to investigate the erosional and depositional patterns. The use of two different sediment transport models has some advantages, since the results of sediment transport models are still subject to some important uncertainties. The hydrodynamic model results are validated using ADCP current data, confirming the good performance of the models. Likewise the wave models provide good results, comparing their results with data from a buoy. The sediment transport model results were compared to the residual transport patterns, derived from the asymmetry of dunes. The results obtained seem to be in general agreement with these observations. The numerical models are used to simulate the response of the sediment transport to extensive sand extraction from the sandbank. One ‘worst-case’ scenario and two more realistic scenarios were simulated, whilst the effect of these bathymetric changes on sediment transport was studied. The results show that the intense sand extraction does not seem to influence extensively the stability of the sandbank, but that, as a consequence, there is less erosion and deposition. The model results show, for all of the scenarios, a small amount of deposition on the top of the sandbank; this could be an indication of a regeneration mechanism. A trench, created perpendicular to the crest of the sandbank, could be slowly refilled again. The time-scale of this regeneration and the influence of storms remain uncertain. Although the main emphasis of the paper relates to tidal forcing, a brief discussion is included on the influence of wave action, on sediment transport.