Integrated Marine Informations System - IMIS

Within the framework of the Agenda for the Future defined by the Flanders-Dutch Scheidt Committee (VNSC), the Flanders Ministry of Public Works, department of Maritime Access (MOW-aMT) requested Deltares to carry out morphological research on the Westerscheide Mouth together with Waterbouwkundig Laboratorium Borgerhout (WL). In particular, this research concerns a Delft3D hindeast of the morphological developments of the Westerscheide mouth over the 1963-2011 period. This study thus provides an overiewof how process-based modeling efforts can contribute to predicting decadal time scale morphadynamie developments.

The Delft3D model is based on the 2D NeVla model earlier developed and includes a schematised wind wave climate and dredging activities. Further sensitivity analysis concerns 3D and 2 sediment fractions. Long run times (about 10 days for 2D and 70 days for 3D) limited extensive sensitivity analysis.

Overall, modelled erosion and sedimentation patterns reflect main developments in the mouth over the 1963-2011 period. Model performance is better in the (more confined) Westerscheide itself, on longer time scales and in 3D with two sand fractions compared to 2D with a single sand fraction. Sensitivity runs excluding waves or dredging activities lead to less performance and indicate where waves or dredging activities have a large impact. In general, dredging activities have a larger impact than wave action. Although model results differ from observations, model skill becomes better over time. The model reaches positive Brier skill scores after 50 years but only in the confined inner parts. A possible reason is that the mouth morphadynamies is governed by more complex processes (e.g. wave action and sediment composition) that requires more subtle process schematization.

Future studies may forecast developments beyond 2011. These predictions can be carried out including interventions such as the dredging of the Geul van de Walvisstaart into an access channel. A process-based modelling effort assessing the impact of such an intervention would be more convincing when the intervention measures are large compared to the autonomous development of the system. Starting from a hindcasted bathymetry or starting from a measured bathymetry would in such as case be of limited importance.