Low frequency waves in the near beach zone: case study at the Belgian West Coast
Vandevoorde, J. (2016). Low frequency waves in the near beach zone: case study at the Belgian West Coast. MSc Thesis. KU Leuven. Faculteit Ingenieurswetenschappen: Leuven. xiv, 116 pp.
Coastal protection has gained significant importance as a result of sea-level rise and climate change, in combination with growing urbanization in coastal areas. The probability of flooding is influenced by the manner in which a beach erodes. Erosion is the sediment transport as a consequence of the interaction between currents and waves with the seabed. The correct measurement and simulation of these waves is therefore essential to better understand the beach and near beach morphology. In the literature study the mechanism behind the generation of infragravi1y waves is discussed. Infragravity waves have low frequencies estimated between 0.004 and 0.04 Hz. Wave groups, consisting of high frequency waves, induce the formation of these infragravity waves. Despite the low energy content ofinfragravity waves, they strongly influence the sediment transport within the near beach zone. An analysis is performed on wave measurements in front of the Belgian West Coast, taken from during the 'Sinterklaasstorm' which occurred early December 2013. Wave groups are distinctly observed in these measurements by: (i) a spectral approach and (ii) a discrete counting scheme. Infragravity waves are harder to observe in these measurements, due to limitations of the measurement equipment To simulate these low frequency infragravity waves, a wave spectrum is derived from the wave measurements and used as wave input in the XBeach software. The down side of using a wave spectrum as input is that information regarding wave groups is lost. As a test case, a one dimensional (1D) simulation is made with a five kilometer long bathymetric profile, containing the two shoals called Trapegeer and Broers Bank The result of that test case shows that significant infragravity wave activity is to be expected. Finally, a comparison of the wave measurements with the simulation is made. The simulation correctly simulates the wave energy at the wave boundary. When comparing the wave group parameters, the simulated wave group lengths are much shorter than the measured ones. Therefore, further validation of the XBeach model is required, preferably using wave measurements as input which also includes low frequency waves.
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