Understanding the morphological development of the Walsoorden sediment disposals

The objective of this study is to identify the mechanisms that drive the morphodynamic behaviour of the sediment disposals in the subtidal zone north of the intertidal shoal Walsoorden. The morphological development of the sediment in the subtidal zone near this shoal has been monitored and measured. The gathered bathymetric data show that one year after the sediment disposals had stopped, the sediment was spread out mainly in the flood direction. Little lateral spreading was found. After one year, 1.45 million m3 of the 3.72 million m3 sediment had disappeared from the control polygon. Sixty percent of this sediment disappeared during the execution of the disposals and 40 % can be assigned to the morphological development between September 2010 and October 2011. The new Delft3D Flexible Mesh (DFM) software was used to simulate the hydrodynamics and sediment transport around the disposed sediment. During the course of this study, the Delft3D Flexible Mesh software is still under development and not all functions are available yet. This is the reason why no simulations with a varying bed were performed and why a different model grid is used for the hydrodynamic calculations than for the sediment transport calculations. To include the seasonal variations in the water level, the hydrodynamic model simulates a full spring- neap tide cycle. The results are used to find the impact of the disposed sediment on the local water levels and flow conditions. Three model simulations with different bathymetries but the same initial and boundary conditions were produced. The first simulation is the reference situation and contains the bathymetry prior to the sediment disposals. The second contains the bathymetry just after the first sediment disposals. And the third contains the bathymetry just before new sediment disposals are executed. Because it was not possible to run a DFM model with sediment transport in parallel model during this study, a reduction of the grid was necessary to calculate the sediment transport with the used model. The simulations of the model that calculate sediment transport investigate a period of one tidal cycle. Similarly, as in the hydrodynamic simulations, the model that calculates the sediment transport was performed with three different bathymetries. For all three bathymetries this was executed for a spring tide and a neap tide. A discrepancy between the measured data and the modelled sediment transport was found. The model calculated a residual sediment transport into the balance polygon around the disposed sediment for all the sediment transport model simulations. This implies that the sediment volume within the balance polygon increases while a decrease of the sediment volume is obtained from the data. A conclusive reason for this has not been found. The migration of the sediment in the flood direction can be explained by the gradient of the residual sediment transport that is introduced by the disposed sediment. An increase of the flow velocity at the location of the disposed sediment and a decrease of the velocity at the downstream end of the disposed sediment during flood are responsible for this gradient. The impact of the disposed sediment turns out to be the largest on the peak flood velocities. During flood, the increased peak velocities at the location of the disposed sediment transport more sediment towards the shoal. In the downstream area of the disposed sediment the flow velocity decreases again and part of the transported sediment is deposited. The strongest gradient, and thus changes in bed level, occur during spring tide conditions.

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