Suspended Sediment Modelling in the Port of Rotterdam

Abstract

This Master thesis presents an improved model for the transport of fine suspended sediment (SPM) in the Rhine region of fresh water influence (ROFI) and in the Port of Rotterdam. It is known that the transport of SPM in the Rotterdam harbor depends on marine as well as fluvial processes. SPM transport inside the harbor has been modelled in the past, but these studies did not take into account the dynamic nature of SPM concentrations at sea because they employed constant sediment boundary conditions and did not resolve wind waves. At the same time, numerous model studies of SPM transport in the Southern North Sea have been carried out, but none included the Rotterdam harbor as more than a discharge point. This MSc thesis is the first study to combine a North Sea SPM model with a detailed model of the Port of Rotterdam. By doing so, it incorporates the dynamic nature of sediment concentrations in the North Sea, and its effect on sediment transport in the Rotterdam harbor.

A new suspended sediment model of the Rotterdam harbor was set up with the
Delft3D-WAQ software package and the NSC-Course model grid already in use by Port of Rotterdam. A validated hydrodynamic model was available for this grid schematization, and has been used as model forcing. At the open sea boundaries, the validated ZUNO-DD model for SPM transport on the North Sea was used to compute suspended sediment concentrations, which were then applied as boundary conditions. In addition, a wave buoy assimilation technique was used to include the effect of wind
waves on the resuspension of sediment. A set of initial conditions was created by repeating a single spring-neap cycle with calm, virtually waveless conditions until sediment concentrations reached a dynamic equilibrium. A byproduct of this method was that it created a large sediment availability in the bed, which lead to higher than expected resuspension during storms. To investigate the response of the system to variable forcing conditions, three 14 day periods have been selected from measured environmental conditions of the year 2007, containing different combinations of storms and river discharges.

The model shows positive results, but does need improvement. Principally, the episodic nature of siltation in the harbor basins lining the mouth of the Rotterdam Waterway was reproduced. In line with observations, storms at sea correspond to high SPM concentrations at sea, large influxes through the harbor mouth, and increased siltation rates in the harbor basins. Furthermore, differential advection of the salinity structure and the trapping of SPM in the Rotterdam Waterway was observed. However, erosion was also observed, and harbor basins lining the Rotterdam Waterway showed a strong response to events at sea which contradicts observations. This is attributed to
the wave stress assimilation performed on NSC-Coarse grid which generates large wave stresses inside the harbor and causes unwanted resuspension. Three different forcing scenarios were chosen to highlight the differences between marine and fluvial processes, but a clear distinction could not be made. This is expected to be caused by the erroneous wave shear stresses which obfuscate the effects of different siltation mechanisms. It is recommended that this aspect of the model is improved and that the model is reassessed using measurements.