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Title: Wave-group propagation and hydrodynamics in the inner surf and swash zones
Author: Padilla De La Torre, Enrique
ISNI:       0000 0004 7659 0852
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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This thesis concerns a fundamental study of grouping waves propagating over a dissipative beach slope (1:100) and the dynamics of the associated long waves. This thesis represents an important contribution to the understanding of the group modulation influence on high and low frequency motions during shoreward propagation. Two new experimental sets of bichromatic wave-groups are presented, where an effective wave generation up to second order is successfully achieved. The IBIMS-ICL data set explores the propagation of identical wave groups. The modulation is controlled by the group frequency, fg, which affects the energy transfer to high and low frequency components. The growth of the high frequency (hf) wave skewness increases when fg decreases. This is explained by nonlinear coupling between the primary frequencies, which results in a larger growth of hf components as fg decreases, causing the hf waves to break earlier. The breaking locations are very well described by the wave-height to effective-depth ratio (gamma). Due to the grouping structure, gamma increases with fg. Therefore, a modified Iribarren number is proposed leading to an improvement in reproducing the measured gamma-values. Within the surf zone, the behaviour of the incident long wave also depends on the group modulation. For low fg conditions, the lf wave decays only slightly by transferring energy back to the hf wave components. However, for high fg wave conditions, strong dissipation of low frequency (lf) components occurs, which is explained in terms of lf wave breaking. The DIFFREP-ICL data set investigates the generation and dynamics of longer waves than the wave-group structure induced by differences in the number of wave groups (Rp) within a repetition period. Consequently, an important energy content is measured at the repetition frequency fr. The cross-shore amplitude evolution at fr is partly explained by nonlinear energy transfers from the primary frequencies, and partly by a breakpoint forcing. When Rp increases, the energy transfer to fr reduces. When Rp >= 3, the amplitude of fr suddenly grows at the breakpoint displaying a node-antinode pattern within the surf zone. The observed dominance of the breakpoint forcing over the energy transfers is justified by the combination of steep-slope regime and steep-wave conditions. A new methodology is proposed to identify the amplitude and phase cross-shore evolution of the radiated and reflected components. When energy dissipation of the higher lf components occurs, the swash is dominated by wave motions occurring at fr.
Supervisor: Alsina, Jose M. ; Swan, Chris Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral