Tsunami-Seabed Interactions

Tsunamis are long waves commonly caused by sudden motions of the seabed e.g. due to landslides or earthquakes. The potentially catastrophic impact of a tsunami is well known, and have recently been experienced, with the 2004 Indian Ocean tsunami and the 2011 Tohoku tsunami in Japan, responsible for 230,000 and 20,000 deaths, respectively. These tsunamis also caused severe damages to structures and buildings and eroded entire coastal regions.While the run-up, inundation, and destructive potential of tsunami events have received considerable attention in the literature, the associated interaction with the sea bed i.e. boundary layer dynamics, induced sediment transport, and resultant sea bed morphology, have received comparably little specific attention. Such issues and processes are important, however, both in assessing potential larger scale deposition and erosion in affected coastal regions, as well as in understanding smaller scale erosion, such as tsunami-induced local scour around coastal and offshore structures. Furthermore, even though the run-up has received considerable attention in the past, detailed descriptions and understanding of how the tsunamis run-up is still lacking. Such an understanding can prove useful when evaluating potential tsunami hazards.The lack of studies and understanding of the processes are probably due to, the long scales involved, which make it hard study experimentally. Furthermore, commonly used potential flow models do not resolve enough of the physics to provide the necessary insights.Computational Fluid Dynamics models can, in principle, naturally handle wave propagation and dispersion, resolve both boundary layer dynamics and wave break-ing, and could thus be used to study such processes. This present thesis aims at investigating, how primarily numerical but also experimental methods can be improved and used to ultimately increase fundamental knowledge about tsunami-seabed interactions. This involves local scour, run-up behaviour, tsunami-induced boundary layers, bed shear stresses and implications for the resulting sediment transport. The present thesis further aims at using the gained knowledge to improve the prediction of such processes, either through simple empirical relations or the adap-tation in ”simpler” potential flow models.The tsunami-induced scour process around offshore monopiles is studied both numer-ically and experimentally. The tsunamis were represented by time varying currents, which enabled the use of a rigid lid in the numerical simulations and a pump to drive the flow in the experiments. This approach thereby saved computational time and also made reasonably scaled experiments possible. Based on both the simulated and experimental results, details of the scour process is discussed, and a novel engi-neering approach, for predicting tsunami-induced scour around offshore monopiles is proposed.To be able to simulate the run-up of tsunamis methodological developments are necessary. The widely used solver interFoam, is shown to have big challenges in accurately simulating free-surface waves. The effects of the temporal and spatial resolution on the solution is discussed, and the effects of the discretization practises and iterative solver settings are likewise documented. It is shown how these can be changed to improve the solution.A previously described, though not well recognised, instability problem of two-equation turbulence closures is further analysed. It is shown that when this type of model is applied to potential flow waves, the instability problem cause the turbulent kinetic energy and eddy viscosity to increase exponentially. This has polluted many simulations of free-surface waves in the past, causing the waves to un-physically decay or arrive at the surf zone already polluted. It is then shown how two-equation turbulence models can be formally stabilized thereby solving this long standing problem.Numerical simulations of full-scale tsunamis propagating on a flat bed before run-ning up a constant slope region are presented. Both wave shapes and slopes are systematically varied, and the implications on the run-up heights are assessed. Fur-thermore, detailed descriptions of the run-up sequence for different scenarios are given, and it is discussed when the different run-up scenarios might occur. The importance of the shorter waves, sometimes riding at the front of the main tsunami wave, on the run-up height, inundation speed and local flow velocities, is likewise assessed.From the same numerical simulations, detailed boundary layer dynamics, resulting shear stresses and implications for sediment transport are described and discussed.The increased understanding of the tsunami-induced boundary layers, leads to the proposal of engineering approaches for predicting both boundary layer thickness and bed shear stresses beneath tsunami waves. These approaches are formulated such that they may easily be implemented in simpler numerical models, potentially improving their sediment transport predictive capabilities.