Mangroves can function as a ‘bio-shield’ to protect coastal communities from harsh environments because of their strong ability to attenuate wave energy. However, as mangroves are usually oversimplified as rigid cylinders in antecedent studies, the effects of complex mangrove morphology on wave attenuation have not been well researched. Although increasing attention has been paid to the wave dissipation induced by varying mangrove morphologies, most of them focus on the bottom trunk and root components of mature mangrove trees. There are few investigations about the contributions of the canopies of young saplings and/or short species to wave attenuation. To bridge this knowledge gap, a series of laboratory experiments under regular waves were conducted to examine the hydrodynamic variations affected by varying mangrove morphology configurations. Three water depths were considered to explore the influences of the vertical-varying submerged volume of mangroves when the artificial mangrove models are submerged, nearly emergent, and fully emergent. The mangrove forest model is 2 m long at a 1:10 scale. Three mangrove configurations, i.e. with no canopy, sparse canopy, and dense canopy were applied and compared to isolate the wave attenuation contributed by mangrove canopies. The results highlight the wave energy attenuation attributed to the canopy density. A linear correlation is found between the wave damping factor and a new variable named hydraulic submerged volume index (HSVI). The bulk drag coefficient, including canopy effects, was calculated to characterize mangrove-induced wave attenuation when the mangrove canopy is submerged. The relationships between the bulk drag coefficient C_D and the characteristic hydraulic numbers (i.e., Reynolds number, Keulegan–Carpenter number, Ursell number) are discussed in detail. Consequently, new generic formulas of C_D were deduced considering the effects of the submerged canopy. The employment of new C_D formulas improves the reliability of the prediction of the wave attenuation ability by mangroves since the canopy effects are incorporated.

@en

Coastal permeable groins have been used to protect beaches from erosion for centuries. However, the hydraulic functioning of permeable groins has not been fully understood and their design heavily depends on engineering experiences. In this study, numerical experiments were executed to investigate the effects of layout configurations of a permeable groin system on longshore currents. The non-hydrostatic SWASH (Simulating WAve till SHore) model was employed to carry out the numerical simulations. Two data sets obtained from physical laboratory experiments with different permeable groin layouts on different slopes are used to validate the accuracy of the model. Then, the longshore current reduction by the permeable groin system with varying configuration parameters (e.g., groin spacing, groin length) was numerically investigated under different environmental conditions (e.g., a slight or a moderate wave climate). From the calculation results of numerical experiments, it is indicated that permeable groins function efficiently to reduce the maximal longshore current velocity under the condition that the groin length ranges from 84% and 109% of the wave breaker zone width. The longshore current reduction rate monotonously decreases with the increase in groin spacing; permeable pile groin functions best to reduce longshore current with the minimal groin spacing-groin length ratio 1:1 among the range between 1:1 and 2:1. When the groin spacing–groin length ratios are 1:1 and 1.5:1, the longshore current reduction is not sensitive to the investigated wave conditions in this study. When the spatial ratio is 2:1, the permeable pile groin system functions worse under a moderate wave climate than under a slight wave climate, from the view of longshore current reduction.@en

Beach erosion, the loss of sand from a beach due to longshore and/or cross-shore sediment transport mechanisms, is a challenging problem. In order to stabilize the beach and to slow down the rate of beach erosion, the construction of hard hydraulic structures is a traditional option. Groins are one of the oldest man-made hydraulic structures designed to intercept the longshore sediment transport and to stimulate sediment deposition within the groin compartments. However, erosion is likely to appear at the downdrift beach stretch of a groin system, due to lack of sufficient sand feeding from the updrift groined beach reaching the downdrift beach. To alleviate sand starvation at a downdrift beach of groins, groins are suggested to be gradually shorter and more permeable approaching the downdrift terminal groin. The primary advantage of permeable groins, compared to impermeable groins, is they do not entirely block longshore currents. The large openings of permeable groins allow littoral drift to flow through. The shoreline response to permeable groins is comparable to a straight line, other than a zig-zag shape response to impermeable groins. Nevertheless, even though the benefits of permeable groins seem obvious, the research on the subject of the hydrodynamics of permeable groins in coastal waters is limited. @en

This paper focuses on a specific form of groins, Permeable Pile Groins (PPGs), consisting of a single or double rows of wooden piles. With only few experiments and simulations available to study the hydraulic functioning of PPGs, the correlation between the effectiveness and the characteristics of the groin system has yet to be fully understood. This paper presents the application of SWASH, a non-hydrostatic wave-flow model to simulate flow fields affected by PPGs. The SWASH model was calibrated to correctly reproduce longshore current fields in the laboratory. Then, introducing PPGs in the model, the simulation results are compared with available experimental measurements to investigate current-PPG interaction. The simulation results, which generally agree well with the measurements, show that PPGs hardly attenuate wave energy but considerably retard longshore currents within the groin fields. Rip currents are predicted to develop at both flank sides of each pile groin, due to local positive water level gradients toward the pile groins.@en

In this paper, the ability of the numerical phase resolving model SWASH (Simulating WAves till SHore) to hindcast wave-induced longshore currents is evaluated. Using default settings for all processes modelled, highly accurate results are found for wave heights, mean water levels and longshore currents. While wave current interaction is intrinsically modelled, insights into the spatial variation of wave driven longshore currents are found. Additionally, vertical variations of modelled longshore currents have been compared. Depth uniform profiles of longshore current within surf zone are noted on plane beaches under regular waves, except for minor deviations near the shoreline. The apparent validity of a depth-uniform longshore current encourages the use of a depth-averaged moment balance equation to compute the longshore current. A simpler model is shown to also be able to predict a proper magnitude of longshore current, although the cross-shore distribution – in contrast with SWASH - needs tuning for the eddy viscosity and the bottom friction coefficient, since the distribution of the wave-induced longshore current heavily depends on lateral mixing.@en