Towards an Improved Understanding and Numerical Simulation of Wave and Sediment Dynamics in the Shoaling Zone
Coastal regions worldwide are popular destinations for human beings to live, work and recreate, yet they are widely threatened by erosion and flood risks associated with sea rise and severe storms. The understanding and both short and long-term predictions with numerical simulations of nearshore hydro- and morphodynamics remain thus essential in helping the policy makers to ensure coastal safety and develop cost-effective protection strategies. Therefore, the dynamics of waves, currents and sediments in the nearshore regions has been the focus of many previous investigations, especially in the past three decades, yet they remain far from being solved and completely understood (Van Rijn et al., 2013).
The current study contributes in improving the understanding and numerical predictions for wave and sediment dynamics in the shoaling zone. The first aim is to investigate the bound infragravity (IG) wave behavior for a non-flat bottom. It is motivated by the errant estimation of long wave energy level following the current available method proposed under the local equilibrium hypothesis (Longuet-Higgins and Stewart, 1960) (LHS60) for a non-flat bottom.
The assumption fails to stand for a slope bed, which is typically found in a nearshore shoaling zone (Fiedler et al., 2018). In this study, we have applied the
semi-analytical solution proposed by Schäffer (1993), for which a new method to separate the bound and free long waves in the shoaling zone has been proposed, given the original solution merely offers the overlapped energy level of these. A number of 1344 sets of combination for near-shore conditions, including varying offshore water depth, bottom slope and incoming short wave parameters have been studied. For mild wave conditions (wave height ≤ 0.5m), the bound wave shoaling has been found to be correlated to the adimensional bed slope . Yet when the incoming wave height reaches around 1m, the mean primary wave frequency comes into play. No general shoaling law has been established, even for the simplest case of a constant slope. A detailed analysis confirmed an overestimation of the equilibrium solution by LHS60. This over-estimation has been found to be the most significant for incoming short waves with a small wave steepness in the offshore zone. In the parameterization study, a correction to the equilibrium solution of LHS60 has been derived, and its validity has been verified through a comparison to a high-resolution laboratory experiment under bichromatic wave conditions. Furthermore, the phase shift between the short wave groups and the IG waves has been discussed. At the innermost of the shoaling zone, the bound IG waves phases are still found to be strongly governed by the local forcing of short wave groups, as has been pointed out by Baldock (2006). For the near-resonant case, the bound IG waves are anti-phase to the short wave groups, and a larger phase shift in the shoaling zone might be the result of superposition of incoming free IG waves.
The second objective of the thesis is to investigate sheet flow under progressive waves and its enhanced wave bottom dissipation. Accurate predictions of crossshore sediment transport under storm conditions remain difficult, with non consensus has been reached for wave dominant coasts. In addition, the wave energy dissipation along its propagation has never been taken into account in the past studies. In this investigation, for the first time, the sheet flow under progressive waves and its enhanced wave energy dissipation are studied as a whole through the use of a multi-phase flow model (Ouda and Toorman, 2019). This process-based model not only resolves the vertical and horizontal structure of the intra-wave dynamics and the sand concentration fields, but also includes the complex mechanisms in terms of the particle-water, particle-turbulence and particle-particle interactions. The model has been previously validated under various scenarios varying from single sedimentation to turbulent sheet flow and wave-induced scour under a submarine pipeline (Ouda and Toorman, 2019). In this thesis, firstly a validation test case for oscillatory flows in U-tunnel has been shown, with the experimental sediment behavior, the erosion depth and the velocity profile well captured by the model. It is followed by a second validation case of sheet flow dynamics under progressive waves, which also serves for an in-depth analysis and parameterization study. It is found that the wave friction factor, which is used to evaluate the Shields parameter and further the sand transport, and the wave dissipation factor, which traces the waveaveraged hydraulic gradient, can not be assumed equal in the sheet flow regime. The former parameterizes the total shear stress for the sediment transport, and the later encompasses all the mechanisms contributing to the wave energy dissipation, including the particle collisions and particle-turbulence interaction. Under oscillatory flow, the sand particles are constantly being stirred up and resettling within one wave cycle, resulting in a non-negligible effect of particle collisions. The equivalent roughness height of Wilson (1989) (approximately 10d50), coupled to the sand flux of Ribberink (1998), is found to capture well the phase-dependent sand transport under waves. For computing the wave dissipation factor, a larger roughness height as in Grant and Madsen (1982) (approximately 500d50) needs to be exploited. This height was initially derive based on the wave energy measurements in oscillatory water tunnel (Carstens et al., 1969), implying a negligible impact by progressive wave streaming on energy dissipation.