Graduation of Tim Overgaauw

Because of an increasing pressure onto the coastal zones, the urge to perform long-term analysis of the coast­line evolution does exist. Nowadays, several types of models are available to simulate the dynamic behaviour of coastlines for engineering purposes. Each type has its own characteristics and contains specific pros and cons. Aims of the modelling effort, associated time and spatial scales and the site specific characteristics are one of the most important aspects while considering the model type to be used in a particular case. Recently, a new one-line model is developed to deal with complex sandy coasts but still being computational for short and larger time scales. This new model, named ShorelineS, is suitable for geometric complex coastlines and allows for the interaction between different coastline sections. Therefore, being capable of modelling spit formation, merging of islands and the formation of tombolo’s and salients.

However, (Elghandour, 2018), (Ghonim, 2019) and (Mudde, 2019), which contain recent ShorelineS model developments, all recommended separately to include the effect of wave diffraction behind shore normal structures in order to improve the model performance. Furthermore, applying the model to a study area sit­uated at Constanta, lying at the coast of Romania, initially did not lead to a numerical result matching the survey data. Based on the site specific characteristics, it was suggested to include the effect of wave diffrac­tion while modelling the shoreline evolution to retrieve a more representative shoreline shape. Therefore, is the core of this thesis focused on incorporating the effect of wave diffraction onto the shoreline evolution in the vicinity of a groyne. This study demonstrates that while accounting for wave diffraction effects, the numerical result of the Constanta case study is matching the observed coastline shape. The improved model succeeded in simulating an anti-clockwise rotation of the coastal cell.

While waves are propagating towards the shore and are interrupted by an obstacle like a groyne, they will turn around the tip into the sheltered region of the groyne. This sheltered region is called the shadow zone and contains a reduced wave climate. The turning of the waves is based on the lateral transfer of wave en­ergy along the wave crest, caused by a gradient in wave height. This process is called diffraction. Breaking wave heights and angles inside the shadow zone will be influenced significantly because of wave diffraction. Since variations in breaking wave height and/or angle are responsible for gradients in the alongshore sedi­ment transport, should the process of wave diffraction be taken into account while simulating the shoreline evolution in the vicinity of a groyne. Different methods were found to incorporate the effects of wave diffrac­tion inside a numerical model. This study applies the methods described by (Kraus, 1984), (Kamphuis, 1992), (Leont’Yev, 1999), (Hurst et al., 2015) and (Roelvink, 2018). A new function is implemented inside ShorelineS so that the effects of wave diffraction can be incorporated. The structure of this function and several param­eters are based on the method established by (Elghandour, 2018) to model wave diffraction in the case of an offshore breakwater. First, the breaking parameters without the effect of diffraction are calculated. Subse­quently, the influence area of diffraction is determined based on the wave characteristics at the groyne tip. In the influence zone a distinction is made between the shadow zone and transition zone. By definition, the breaking parameters at the edge of the transition zone are not subjected to diffraction effects anymore. Next, for each point of breaking inside the shadow or transition zone, the diffracted breaking wave height and an­gle are calculated following one of the methods listed earlier. Some of these methods needed to be modified in order to be applicable in this study. Finally, the alongshore sediment transport is determined using the diffracted breaking parameters. Based on gradients in this alongshore sediment transport the new coastline position is determined by the model. Additional to developing this function, a new approach regarding the boundary condition of the groyne is suggested to retrieve a better coastline response very close to the groyne. The key aspect of this approach is dividing the shoreline into two sections, containing the total updrift and downdrift area respectively.