|Numerical modelling of wind wave energy dissipation at the bottom including ambient currents|
Padilla-Hernández, R. (2002). Numerical modelling of wind wave energy dissipation at the bottom including ambient currents. PhD Thesis. Katholieke Universiteit Leuven (KUL): Leuven. V, 171 pp.
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VLIZ: Open Repository 74805 [ OMA ]
|Document type: Dissertation|
Mathematical models; Simulation; Wave forecasting; Wave-current interaction; Wind waves; Marine
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The exploitation and preservation of coastal zones has put some pressure to understand the complex dynamical interactions between all physical processes that take part in that zone. Wind waves are the most eminent form of energy in the oceans and they play a key role in coastal hydrodynamics. The wind waves link most of the physical (and perhaps biological) processes in the coastal zone, by affecting the vertical profile of the ambient current at the bottom, by taking an active role in sediment transport, by oxygenating the water column due to air-sea gas exchange through the turbulence generated at the surface and at the bottom, by helping to determine sites for productive reef development and shaping reef morphology as well as community structure, among other processes. For the simulation, forecasting and understanding of the evolution of the waves one of the most useful tools is numerical modelling. In this thesis the research (carried out partially under the EU-PROMISE project) about wind waves begun with the adaptation of the WAM wave model making it able to simulate the evolution of the wind waves in shallow water areas with high spatial resolution. Two aspects were considered for its improvement: the computational efliciency and the representation of the physical processes involved in the evolution of waves in shallow water areas. The resultant wave model was named WAM-PRO. A number of simple or idealized example applications were done to illustrate some of the enhancements included in WAM-PRO. As a result of all the changes and additions, it has become feasible and economical to explore the wave spectra evolution in coastal areas with the WAMC4-P model. When the waves propagate from deep water to shallow water areas, they are mixed up in several mechanisms which modify the environment and in turn the wave properties are affected. One of these interactions takes place with the ambient currents (by tides and surges). A module was developed to study wave-current interactions. This module enabled combined modelling of tides, surges and waves in shallow water at the North Sea scale. Another important interaction in which the waves take part is the wave-bottom interaction. This interaction is responsible for drastic changes in the wave field and, in a way, it determines how the interactions between waves and other phenomena, including those between waves and current. In this study an attempt is made to search for evidence to determine which friction formulation, amongst four, performs best or is more consistent in shallow water regions. Some models for wave energy dissipation by bottom friction are very elaborate and have some very fine qualities. Most of them are function of a roughness height, which most of the time, at least in wave forecasting, remains unknown for a given wave, current and sediment characteristics. In order to remove the roughness height to be constant in the bottom friction formulations, a new ripple geometry predictor (MN81) has been proposed. lt is based on the model of Nielsen but including findings from other published investigations. The proposed model was coupled with the model of Christofferson and Jonsson (CJ85) for the wave-current bottom boundary layer. A set of measurements (waves, currents and ripples dimensions) in the field and in laboratory data (apparent roughness) were used to verify the ripple predictor MN81 and the WBBL model (CJ85). Without making specific assumptions for the different stages, the proposed MN81 model was able to fit the measurements in all ripple regimes. In turn the WBBL model (CJ85), using the ripple geometry from the MN81 model, reproduces the different flow regimes observed in the field and some parameters measured in laboratories by other researchers.