University of Twente Student Theses


Modelling cross-shore transport of graded sediments under waves

Wardt, Willeke van de (2018) Modelling cross-shore transport of graded sediments under waves.

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Abstract:It is important that the development of the coastline is constantly monitored, and that the effects of interventions, such as nourishments, can be accurately predicted by morphological models. A widely used morphodynamic model by coastal engineers is DELFT3D (Lesser et al., 2004). Both the coastline and these nourishments contain sand with varying grain sizes (mixed sediment). Hence the model of DELFT3D needs to work with these mixed sediments to determine the evolution of the long-term morphodynamics of the beach profile. The objective of this thesis is to investigate the difference between modelled transport rates using a single-fraction approach and multi-fraction approach, and comparing these rates to wave flume data (Van der Zanden et al., 2017). This is done with DELFT3D, using formulations for bed-load transport by Van Rijn (2007c). First, two stand-alone MATLAB models for bed-load transport were used to compare the results of a single-fraction approach and multi-fraction approach to a database containing data from graded sediment transport experiments in oscillatory flow tunnels (Van der Werf et al., 2009). The bed-load transport models that were used were the bed-load transport formulations by Van Rijn (2007c) and the SANTOSS model (Van der A et al., 2013). The Van Rijn model gave comparable results for both the single-fraction and multi-fraction approach, giving only slightly better results for the multi-fraction approach. For the SANTOSS model, the multi-fraction approach evidently gave a better approximation of the measured bed-load transport rates. Additionally, the SANTOSS model gave the best results when compared to the database Before any analysis of the transport rates using DELFT3D took place, the hydrodynamics were recalibrated. Previously, Schnitzler (2015) already modified formulations in DELFT3D to obtain better results for regular breaking waves. Since the data were not processed till after these modifications, recalibration was required. Generally, DELFT3D replicated the wave height and undertow velocities accurately, with exception of the undertow velocities at two of the twelve locations. At these two locations the measurements were underestimated. Subsequently, DELFT3D was used to model both bed-load and suspended-load using a single-fraction and multi-fraction approach. When modelling the current-related suspended sediment transport and bed-load transport, little difference was noticed between the two approaches. The wave-related and total transport rates did show differences between the two approaches, where the single-fraction gave wave-related suspended sediment transport rates 3 times larger than the multi-fraction approach. It has not yet been discovered whether these differences can be attributed to grading effects or an error in DELFT3D. Based on the results of the bed-load transport rates and current-related suspended sediment transport rates, it does not really seem important whether a single-fraction or multi-fraction approach is used. The logical follow-up step would be to implement the SANTOSS bed-load transport formulations in DELFT3D, as this bed-load transport model showed larger differences between the single-fraction and multi-fraction approach.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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