University of Twente Student Theses
Hydrodynamics and sand transport on the lower shoreface of the Ameland tidel inlet
Leummens, M. (2018) Hydrodynamics and sand transport on the lower shoreface of the Ameland tidel inlet.
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| Abstract: | The lower shoreface, part of the coastal profile located between 8 and 20 m water depth, corresponds to the deeper part of the so-called Coastal Foundation. This is defined as a coastal area between the first row of the dunes and 20 m depth contour and is one of the main concepts for the management of the Dutch coast. In order to provide the long-term safety given sea level rise, yearly nourishments are applied on the coast and its volume is determined based on the rate of yearly sea level rise and the area of the Coastal Foundation assuming there is no significant net sediment transport through the offshore boundary. This assumption, however, is not well substantiated and, because of that, the “Kustgenese 2.0” (“Coastal Genesis 2.0”, KG2) was started by Rijkswaterstaat along with the lower shoreface subproject at Deltares in order to investigate potential alternatives for the offshore boundary of the Coastal Foundation and required nourishment volume. For this purpose, Grasmeijer (2018) proposed a new “offline” approach, in which net annual sediment transport rates at any location can be calculated using simplified Van Rijn (2007a, b) formulas (TSAND model) with input currents from the DCSM-FM model (Zijl et al., 2018) and waves from the Wave Transformation Tool (de Fockert et al., 2011). The objective of this master project was to obtain a better understanding of the hydrodynamics and sediment transport on the lower shoreface of the Ameland tidal inlet and to validate the new sediment transport modelling approach. To meet these objectives, first, current measurements conducted in November and December of 2017 on the lower shoreface of the Ameland tidal inlet at 11, 16 and 20 m water depth were analysed in order to assess the effect of storms. The data analysis has shown that during storm events characterised by north-western wind and waves of more than 4 m there is a strong eastward and onshore residual flow on the lower shoreface increasing towards the shallow water, signs of which are also observed at 20 m water depth. Validation of the DCSM model, however, has shown that these currents are not captured by the model that does not include wave-driven currents. Comparing the results to the detailed Delft3D model of Nederhoff et al. (2018) with and without waves confirmed that the mismatch is not caused by the model resolution. Using the measured and the DCSM model currents as input for the TSAND model has shown that, even though statistically the model performs relatively well, the mismatch in currents during storms results in a similar mismatch in cross-shore and longshore sediment transport. Comparison of the transport rates predicted using measured current with the yearly transport rates calculated with the DCSM currents has shown that on a yearly time scale this mismatch can be significant even at 20 m water depth and, because of that, in its current state the “offline” approach cannot be applied for the analysis of the net annual sediment transport rates on the lower shoreface of the Ameland tidal inlet. Besides that, contribution of different sediment transport mechanisms to the net annual sediment transport was studied using the TSAND model with the DCSM model currents for the years 2013 to 2017 and the measured currents. This analysis has shown that the absence of wave-driven currents in the DCSM model also affects the calculated contributions of different mechanisms at shallow part of the lower shoreface, however, some qualitative conclusions still could be made there. The analysis has shown that wave asymmetry and near bed wave-induced streaming play very small role in the net annual suspended load and bedload sediment transport respectively. The additional transport due to Stokes drift can be significant, especially for the years that are characterized by large storm events, as well as wind-driven currents, which cause an increased eastward longshore and offshore transport. |
| Item Type: | Essay (Master) |
| Clients: | Deltares |
| Faculty: | ET: Engineering Technology |
| Subject: | 56 civil engineering |
| Programme: | Civil Engineering and Management MSc (60026) |
| Link to this item: | https://purl.utwente.nl/essays/77003 |
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