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Tim de Wilde

2 records found

Authored

Hydrodynamic and morphodynamic response of Perkpolder after a managed realignment

Master thesis (2022) - T. de Wilde, Bram van Prooijen, P.L.M. de Vet, D.S. van Maren, Zhengbing Wang

A managed realignment is the landward relocation of a flood defence to re-establish tidal exchange on formerly reclaimed land. In a managed realignment, the newly formed intertidal area acts together with the realigned dike as a nature-based flood defence system. In this thesis, the focus is on the managed realignment of Perkpolder, a former polder located in the south of Zeeland (The Netherlands). This realignment serves as nature compensation for the dredging activities in the Western Scheldt. The realigned area contains an intertidal flat which facilitates ecological services.

A combination of different data sources (sediment samples, flow velocities, turbidity measurements and bathymetric data), together with a hydrodynamic model of the Western Scheldt and a morphodynamic model of Perkpolder are used to interpret the hydrodynamic and morphodynamic response of Perkpolder after the managed realignment.

Bathymetric data composed of LIDAR, multibeam and single-beam measurements shows three main developments in Perkpolder: (1) erosion of the frontal entrance area, the inlet and the seaward side of the creeks, (2) infilling of the creeks landward of the first bifurcation and (3) sedimentation on the intertidal area and even more sedimentation of the pond. By comparing the temporal evolution of the bed levels of the frontal entrance area with measured concentrations at the inlet of Perkpolder, it can be concluded that the frontal entrance area acted as a finite source of sediment that eroded quickly in the first years and increased sedimentation within Perkpolder during these years.

New realignment projects can profit from the research at Perkpolder. Bed shear stresses of the initial proposed layout of Perkpolder gives a clear indication if the creeks are setup correctly. Furthermore, an estimate of the initial sedimentation can be based on the accretion rate of other intertidal areas within the estuary. Since realigned areas are often more sheltered and have a lower elevation, accretion rates may be higher. Moreover, not only the forcing of the estuary, but at the same time the availability of different sediment sources and the initial bathymetry shape the morphodynamic response of the realignment site. Therefore, it is recommended to measure the bathymetry for a sufficient long period (e.g. 10 years after the realignment) to get grip on the mechanisms shaping the morphodynamics at these areas.

Contributed

Analysing dispersal of sand nourishments using SedTRAILS

Evaluating the application of the SedTRAILS model on the Ameland ebb-tidal delta nourishment

Master thesis (2024) - M.H.V. Meijer, S.G. Pearson, Edwin P.L. Elias, K.B.J. Dunne, N. Pannozzo, T. de Wilde, L.B. (Laura) Brakenhoff

In 2019, a 5 million m³ sand nourishment was placed at the Ameland ebb-tidal delta. This pilot nourishment aimed to investigate the feasibility and effectiveness of ebb-tidal delta nourishments and to understand the dynamics in this system. To assess how this nourishment has dispersed through the ebb-tidal delta system, SedTRAILS was used. SedTRAILS is a particle-based sediment transport model currently in development. A new process for interaction with the bed and burial of sediment was recently added to the model. The impact of this process on dispersal and sediment pathways needed to be analysed, as not much is known about the effects of its inclusion on transport and dispersal.

Several schematisations of wave- and tidal forcing were tested. The wave forcing selection had little effect on the overall model results. Storm conditions included in the more detailed schematisation were underrepresented in the model results. The implementation of wave-driven bed velocity had little influence on result as well due to small magnitudes that were calculated for this. A significant difference was found between using a morphological tide and an artificial spring-neap tidal cycle however, requiring further investigation.

Investigation of the burial formulation gave insight into the response of the model to changes in the different parameters. Direct dependence of erosion probability on deposition probability presents a major limitation in the current implementation of burial. Additionally, deposition probability in its current form is positively dependent on the maximum bed shear stress. Inclusion of burial does provide significant room for calibration of results.

Through modelling these various configurations of SedTRAILS, results consistently showed a recirculation of sediment from the nourishment on the ebb-tidal delta together with bypassing around the edge of the ebb-tidal delta, as seen in Figure 1. However, the degree to which bypassing occurs depends heavily on the chosen burial configuration. The bypassing is largely a result of flow around the periphery of the ebb-tidal delta and differs from known bar migration and shoal attachment pathways. Transport into the Wadden Sea basin was not found, likely due to the underestimation of influence of storm conditions.

To improve dispersal modelling with SedTRAILS, the impact of storm conditions needs to be more accurately included in the model. An under-representation of energetic wave conditions leads to a consistent underestimation of wave-related transport. Additionally, more investigation into the effects of the spring-neap tidal cycle is required. The significant difference presented here is an early indication that this is an important factor for SedTRAILS. Validation of dispersal modelled with SedTRAILS can be achieved through quantification of transport, assigning a volume of sediment per particle.