Session: 06-02-02 Coastal Engineering - II

Submission Number: 174763

On Class Ii Bragg Reflection Induced by the Interaction Between Bichromatic Waves and Semi-Sinusoidal Ripple Beds 

The energy dissipation following wave breaking alters the local hydrodynamics in shallow water regions and indirectly influences onshore and offshore sediment transport. However, if this wave energy is not effectively dissipated, its cumulative long-term effects can disrupt the sediment transport mechanisms of the longshore current system, leading to shoreline retreat and beach erosion. Submerged breakwaters in series or periodically configured rippled seabeds represent a wave energy control strategy that offers low visual impact on the coastal landscape, frequency selectivity, and design flexibility. When the spacing of the rippled bed corresponds to specific wave conditions, the well-known Bragg resonance phenomenon can be triggered. This phenomenon enhances the reflection of waves at particular frequencies, thereby effectively mitigating the transmission of onshore wave energy into harbor basins and nearshore zones. In natural sea states, waves are typically composed of multiple frequencies. Among these, bichromatic waves, formed by the superposition of two regular waves with closely spaced frequencies, represent a fundamental form of a wave group. They are considered a principal model for multi-frequency waves, not only reflecting nonlinear wave-wave interactions but also providing an ideal condition for fundamental research into the physics of polychromatic waves. This research employs the non-hydrostatic numerical model SWASH (Simulating WAves till SHore) to investigate the third-order interactions of class II Bragg reflection between bichromatic waves and a semi-sinusoidal rippled bed, as well as their reflection characteristics. The four-point method is utilized to separate the reflection amplitudes and coefficients of the constituent wave components. The study further explores how the reflection characteristics of bichromatic waves are affected by variations in the ripple wavelength, number, spacing, and wave steepness. Preliminary results indicate that: (1) Significant wave reflection and Bragg reflection are induced when the wavelengths of the primary wave components satisfy the traditional class II Bragg reflection condition. (2) The individual reflections of free and bound waves resulting from nonlinear wave-wave interactions have a negligible effect on the overall wave reflection and the Bragg reflection, owing to their small wave steepness (3) Bragg reflection is not triggered when the wave group length satisfies the traditional Bragg reflection condition. (4) Variations in the amplitude, spacing, and wavelength of the rippled bed affect both the peak magnitude and the occurrence conditions of Class II Bragg reflection.

Presenting Author: TZU CHUN HUANG National Taiwan Ocean University

Presenting Author Biography: I received my B.S. degree in Harbor and River Engineering from National Taiwan Ocean University, where I currently serve as a research assistant at the Center of Ocean Engineering and Technology, National Taiwan Ocean University

Authors:

TZU CHUN HUANG National Taiwan Ocean University
Ting-Chieh Lin National Taiwan Ocean University
Tai-Wen Hsu National Taiwan Ocean University
Wei-Liang Chuang National Sun Yat-sen University
Chih-Min Hsieh National Kaohsiung University of Science and Technology
Chen-Wei Yang National Taiwan Ocean University