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The effect of turbulence on modelling the sediment transport in the swash zone

Weeghel, L. van (2020) The effect of turbulence on modelling the sediment transport in the swash zone.

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Abstract:To better estimate whether coastal systems can withstand storms, it is important that the behaviour of the sand under the influence of the waves is understood in a higher extent. Especially for the part of the coast between the surf zone and the beach: the "swash zone" (Masselink & Puleo, 2006). How much and especially when is the sediment picked up by the water in the swash zone and when does it settle again. To model this behaviour the depth-averaged XBeach model is used. This is an open-source, depth-averaged model for hydrodynamics, sediment transport and morphology on and around the beach (Smit et al., 2009). There are three versions of XBeach, yet for this study the non-hydrostatic version was used. This research aims to investigate whether including effects of turbulence can improve the sediment transport as simulated by the model. In order to validate the model, the results were compared with measured data from the RESIST dataset. The RESIST dataset was obtained in the CIEM flume at the Universitat Politecnia de Catalunya in Barcelona (Eichentopf et al., 2019). The XBeach model used in this study is based on the characteristics of the RESIST dataset. This means including the full-scale, a 1:15 sloped bed, a D50 of 0,00025 m and bichromatic waves with a period of 3,7 s and a (maximum) amplitude of 0,320 m. Subsequently a turbulence data set has been made with a depth-dependent OpenFoam model that, unlike XBeach computes turbulence. In addition, the turbulence has also been estimated using an analytical expression (e.g., Reniers et al., 2013). Thereafter, a point-model has been made that models the sediment concentration using the Van Thiel-Van Rijn (VTVR) method (Van Thiel de Vries, 2009). In this model, both (depth-averaged) turbulent kinetic energy timeseries have been added to the hydrodynamics as modelled by XBeach. This was done by including the turbulent kinetic energy to the flow rate (Jongedijk, 2017). Finally, the new sediment concentrations have been translated into a sediment transport to see whether this modelled transport matches the measured transport to a higher extent. Subsequently, it was investigated whether some changes in the made assumptions could improve the results. The use of this point-model has some disadvantages, such as neglecting advection and using one turbulence value. Also, the fact that the VTVR method is not intended for intra-wave models (Ruffini et al., 2020) can cause deviations in the results. However, it can be concluded that XBeach is quite good at modelling hydrodynamics in deeper water and in the swash zone but that the modelled sediment transport is quite bad. Yet, adding turbulence does not provide significant improvements in modelling the sediment concentration although it does make some small changes. Also, the transport of sediment is not improved by the inclusion of turbulence. Therefore, based on these results, it can be said that including turbulence with this approach does not significantly improve modelling sediment concentration and transport. In order to better establish these results, further research could use a 2DH model to include advection and (horizontal) diffusion which tend to be important processes (Masselink & Puleo, 2006). Another recommendation is to use a dataset that has multiple measurements in the vertical plane so that a better sediment concentration profile and depth-depended turbulence can be used.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Civil Engineering and Management MSc (60026)
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