F. Francken

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.

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.

Seasonal variability of suspended particulate matter observed from SeaWiFS images near the Belgian coast

Suspended Particulate Matter (SPM) surface concentration maps in the Belgian/Dutch coastal zone are retrieved from SeaWiFS images and are corrected using in situ measurements to obtain depth-averaged SPM concentration maps. A spatial correlation analysis of the derived maps shows that the area could be divided into three subregions where the correlations between the SPM concentrations are higher than 70%. Examination of in situ SPM concentration measurements reveals that during about 1/3 of the tidal cycle the SPM concentration is significantly higher than during the rest of the cycle. Strong vertical gradients are sometimes observed during periods with increased SPM concentration. A satellite image taken during such a period would underestimate the depth-averaged SPM concentration. Images taken during other periods better represents (except for some small corrections) the averaged SPM concentration. The methodology for obtaining the depth-averaged SPM concentration maps from surface SPM distributions derived from SeaWiFS images is positive but can be further improved.

Suspended particulate matter mapping from multitemporal SeaWiFS imagery over the southern North Sea - SEBAB project

Suspended particulate matter concentration (SPM) maps at water-surface were retrieved from 172 SeaWiFS images using the MUMM turbid water extension to the SEADAS4.4 software and a regionally calibrated hydro-optical model. Examination of surface-SPM maps and the depth-averaged SPM derived from a 2D-hydrodynamic and sediment transport model over the BCZ shows interesting similarities. The effect of the neap-spring tide cycle on the satellite-derived SPM distribution is compared to the significant impact of this factor on modeled SPM distributions. Classification of the Belgian waters is carried out using the time-series of SeaWiFS-derived SPM maps and is explained in the light of the SPM transport modelling knowledge. This is preliminary step in this project to provide boundary conditions and initial data of SPM distribution to the coupled model.