In this study, we investigated the energy dissipation characteristics of stepped obstacles by using both the regular and irregular waves in a physical wave flume. The stepped obstacle used in this study is different from those of traditional stepped dikes with fixed horizontal and vertical stage surfaces; here, the angle of the stepped surface varies with the slope of the embankment surface, with various interaction angles between the incident waves and the rough stepped surfaces. The stepped embankments were composed of six types of stepped arrangements and were used to analyse the wave run-up heights and reflection coefficients; in addition, the variations in energy dissipation effects were also studied. The results identify two of the six roughness types that have better mitigation effects on the investigated the energy dissipation characteristics of stepped obstacles. The reflection coefficient is larger for a submerged dike with a rough surface than for one with a smooth surface.

The need for renewable energy has gained importance with growing concerns about climate change. Wave energy has attracted considerable attention owing to its sustainability potential. Reflection, refraction, diffraction, and shoaling of waves occur when waves propagate through a submerged structure. These mechanics, when properly utilized, can be employed to focus waves to a specific location and also to increase wave heights, by which wave energy is usually represented, for planning and designing wave farms. Wave focusing induced by a submerged crescent-shaped plate for different wave conditions, incident wave directions, and submerged depths mainly considering the potential applications of absorber wave-energy converters within the wave farm was investigated experimentally and numerically. All experimental regular wave conditions were controlled to be nonbreaking, and the numerical results were obtained by a 3D model, implemented through the boundary element method based on Airy wave theory. The results show that wave focusing appears behind the plate along the direction of the incident waves, and the locations of focused waves tend to be farther away from the plate for shorter-period waves. The maximum measured wave height can be 3.44 times higher than the incident wave height.

Wave attenuation for the purpose of coastal protection has been an important topic in coastal engineering. Wave attenuation in relation to the vortices induced by a solitary-like wave propagating over submerged breakwaters (BWs) is discussed in this paper. A series of hydraulic model experiments was conducted to investigate the occurrence of eddies, the types and combinations of submerged BWs, and related phenomena of the range expansion of vortices. The microscopic changes in the flow field, the variation of eddies, and the distributions of streamlines were analyzed using the particle image velocimetry (PIV) technique. The measured transmission and reflection coefficients, along with the concept of energy conservation, were also examined to support the results. The results indicate that the attenuated wave energy is related to the induced vortices, and show that the total relative vortex energy for rectangular submerged BWs is larger than that for undulating submerged BWs in both the single and composite sets. The magnitude of the maximum vorticity of the undulating BW sets is larger and more concentrated than that of the rectangular BW sets; however, the total vortex energy is slightly smaller owing to the narrower vortex area range.

Although viscous sediment environments along the coast strongly attenuate waves, the attenuation dynamics and physical mechanism governing the attenuation process remain relatively unknown. Extremely complex interactions between muddy seabed have become increasingly important for wave evolution studies pertaining to coastal areas. The coastal protection function of mangroves was confirmed during the 2004 South Asian tsunami. Nevertheless, most research has been limited to macro-qualitative analyses, including those on variations in the transmission coefficient K_t and reflection coefficient K_r, and subsequent comparisons. However, determining the micro-physical characteristics is challenging, similar to coastal vegetation analyses with respect to mangrove vegetation characteristics. This study aims to quantify the attenuation difference in the wave energy owing to the coastal vegetation structure, under different layout conditions and combinations. Particle image velocimetry (PIV) technology is used to explore the variations in the velocity field and velocity distribution during the interaction process and calculate the wave-induced kinetic energy before and after setting up the vegetation structure. The research results emphasize that the resistance and frictional effects generated by vegetation are inversely proportional to the size of the stem, and the variation of kinetic energy determined from the velocity distribution and the thickness of the vegetation stem is mainly due to the larger frictional resistance of dense vegetation, relative to the fast flow velocity above the vegetation. Different vegetation heights slightly affect the short-period waves; however, the impact on energy reduction was smaller. For long-period waves, vegetation height significantly reduces wave kinetic energy.

Most coral reef islands in the South China Sea are primary influenced by monsoons from the northeast in winter to the southwest in summer. Because the length scale of the islands ranges between O(100 m) and O(1000 m), nearshore waves and currents on the windward and leeward sides are modulated by seasonal wave directions as well as reef morphology. It is well known that long-period swell and infragravity waves play a vital role in dominating hydrodynamics and sediment dynamics on fringing reefs. However, spatial characteristics of long waves on the entire islands have not been well studied. In the present study, a phase-resolving Boussinesq-type wave model, is employed to investigate wave distributions around Taiping Island in the South China Sea during the monsoon. Wave transformations of refraction, diffraction, reflection and nonlinear wave interactions on the complex reef geometry are simulated using a high resolution grid. Model results will display spatial variability of significant wave height, infragravity waves and nearshore currents around the reef platform. The importance of long wave diffraction and resulting wave frequency components at the leeward side of the island will be examined. A small harbor located at the southern island often experiences harbor oscillations during the winter monsoon, even it located at the leeward side. Harbor oscillations forced by periods of wind-wave, swell and infragravity waves will be explored to provide critical indicators for the harbor management.