# Modelling waves and their impact on moored ships

Research output: ThesisDissertation (TU Delft)

The proposed model aims to be applicable at the scale of a realistic coastal or harbour region (say in the order of $1 \times 1$ km$^2$), while accounting for the relevant physical processes. This includes the processes that govern the nonlinear wave evolution over a varying bottom topography (e.g., the nonlinear interactions that excite infragravity waves), and the interactions between the waves and a moored ship (e.g., the scattering of waves by a fixed floating body). The approach is based on the recently developed non-hydrostatic wave-flow model SWASH, which has been successfully applied to simulate a range of wave related processes. This work pursues the development of a new modelling approach through a further development and evaluation of the SWASH model in (i) simulating the nonlinear wave dynamics in a coastal region, and (ii) simulating the interactions between waves and a restrained ship. The first crucial step in this development is to determine if the model can resolve the nonlinear wave field in a coastal environment. Previous studies showed that models like SWASH can resolve the short-wave dynamics in coastal waters. However, they did not address if such models can resolve the dynamics of the infragravity-wave field. Furthermore, most of these studies focussed on laboratory applications due to computational limitations, whereas field scale applications of non-hydrostatic models have been rarely reported. With the ever increasing computational capabilities, such scales are now within the reach of the state-of-the-art computer systems. To advance the capability of the non-hydrostatic approach towards such realistic applications, this work presents a thorough evaluation of the SWASH model in resolving the nonlinear wave dynamics at the scale of a realistic coastal region. Given the importance of infragravity waves with respect to the wave-induced response of a moored ship, this work particularly determines if the model can resolve their nearshore evolution. The model was validated using both laboratory and field experiments, covering a range of wave conditions (varying from bichromatic waves to short-crested sea states). A comparison between model predictions and laboratory measurements showed that the model captures the frequency dependent cross-shore evolution of infragravity waves with a coarse vertical resolution (2 layers), including their steepening and eventual breaking close to the shoreline. These results demonstrate that the model can efficiently resolve the dominant processes that affect their nearshore evolution (e.g., nonlinear interactions, shoreline reflections, and dissipation), permitting applications at the scale of a realistic harbour or coastal region. To determine the capability of the model at such scales, SWASH was applied to study the infragravity wave dynamics at a field site near Egmond aan Zee (the Netherlands), which is characterised by a complex bottom topography. The model was used to reproduce a total of six sea states (including mild and storm conditions), which were measured as part of a two month field campaign. For all conditions, the predicted wave field gave a good representation of the natural conditions, supporting a further study into the infragravity wave dynamics. A unique feature of these predictions is their extensive spatial coverage, allowing analyses of the wave dynamics at scales not easily covered by in-situ measurement devices.