Modelling sand-mud-bed interaction in the Scheldt estuary
Groenenboom, J. (2015). Modelling sand-mud-bed interaction in the Scheldt estuary. MSc Thesis. Civil Engineering and Geosciences: Delft. viii, 85 + appendices pp.
The tide-dominated Scheldt estuary is located in the southwest of the Netherlands. From the Dutch-Belgian border seawards, it consists of a system of channels and shoals and an ebb-tidal delta facing the North Sea. The behaviour of non-cohesive (sand) and cohesive (mud) sediment particles in the area are (until now) generally studied using separate numerical models. Better understanding of the physical processes in the area of interest can be obtained using process-based hydro-morphodynamic models.Based on a literature review, it is concluded that the mutual influence of sand-mud mixtures is of significant importance on the morphological development and should be accounted for when modelling multiple sediment fractions. Recently implemented modules in Delft3D can be used to improve the modelling of the cohesive fines and the interaction between the sand and mud fractions.An existing Delft3D sand-model of the Scheldt estuary is extended with a mud fraction based on a mud model of the same area. The model simulates the morphological development of one year using a representative spring-neap tidal cycle and a morphological acceleration factor of 25. The bed is modelled using a fluff layer, transport layer and base layer. Furthermore, a critical mud content is used to distinguish between a non-cohesive and cohesive regime. In each regime, different formulations are used to calculate the erosion fluxes of sand and mud. A calibration on the mud parameters is performed to improve the behaviour of the mud particles in the model.The performance of the model is assessed using a list of desiderata. The development in the model is subjected to a simulation loop in which the output is used as the input for the subsequent run in order to reduce the spin-up effects. The required period for the bed composition to develop into a state where it is in line with the boundary forcing is considered to be too long. Therefore, it is important that the model results are sufficiently stable so that the morphological development in the warmed-up model is better predictable.The computed mud concentrations in the water column and their spatial distribution are in good agreement with the results of the mud model and with observations. However, the model calculates an export of mud at the mouth of the estuary whereas observations show an import of mud. Spin-up effects are responsible for this flaw of the model as the mud transports are still influenced by the initial bed composition. The computed mud export gradually decreases over time and is expected to change to import when a significantly longer simulation period is used.Several model scenarios were set up to assess the importance of the hydrodynamic forcing mechanisms and sand-mud-bed interaction processes. By comparing the results of the model scenarios, the sensitivity of different components (tides, wind and waves) on the sediment transport and morphological development is investigated. In addition to the tidal forcing, the locally generated wind-waves have a significant impact on the morphological development of the Scheldt estuary.Based on the results of this study, it is concluded that the effect of the mud fraction on the large-scale net sediment transport is of minor importance. Studies on the large-scale sand-transport patterns between macro cells in the Scheldt estuary can therefore be done using a model that only contains a sand fraction. However, the morphological development of the fringes of the intertidal areas is significantly affected by the presence of mud since it prevents erosion of these areas. It is therefore recommended to take this interaction into account in morphodynamic simulations of the Scheldt estuary.
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