Influence of Porous Column-Type Breakwater on the Regular Wave Hydrodynamic Characteristics of Uneven Reefs

Coral reefs, which are among the oldest and most biodiverse ocean ecosystems, play irreplaceable roles in sustaining global fishery resources and protecting coastal areas and biological habitats. However, with climate change and human activities threatening their survival, replacing traditional rigid coastal protection structures with ecofriendly alternatives is of urgent importance. In this study, we focused on the actual uneven topographies of coral reefs, which can more accurately simulate the impact of complex seabed morphologies on wave propagation than traditional simplified topographies can. Through physical modeling experiments, the hydrodynamic effects of a new porous column-type breakwater on regular wave propagation were explored. Currently, research on the influence of such structures (particularly new porous and permeable structures) on the wave propagation characteristics in complex uneven topographies is insufficient and requires further supplementation. The experiments were performed in a wave tank of 45 m length, 0.8 m width, and 1.0 m height, following a Froude similarity scale of 1:40. The breakwater was made up of 3D-printed cylindrical units, with adjustable wall perforation rates (n = 0 to 0.4), lateral spacing (D = 0.25 to 1.0 times d1), and positioning along the reef flat (l = 0 to 0.75 times the Lp). The testing parameters were the incident wave height (H = 0.04 to 0.1 m), reef flat water depth (hr = 0 to 0.075 m), and wave period (T = 1.0 to 1.75 s). Eighteen wave height gauges were used to record free-surface elevations, and the data on skewness, asymmetry, spectral energy, average wave height, and average water depth characteristics were analyzed. The following results were found: (1) According to the observed spatial variation in wave spectral energy along the reef, the wave energy was mainly concentrated at the primary frequency (f = 0.667 Hz) before reaching the reef, with second harmonic energy being slightly present. As waves propagated from the open sea toward the shore and reached the reef, they underwent shallow water transformation, nonlinear enhancement, and increased high-order harmonic energy. After the waves broke, the wave energy decayed rapidly, and the energy near the primary frequency continually decreased owing to wave-breaking losses. Compared with that in the no-breakwater scenario, the wave energy near the primary frequency in the nearshore region with the column-type breakwater was significantly decreased. (2) At the end of the reef flat, the average wave height in the scenario with the breakwater was slightly larger than that without the structures when the incident wave height was small. However, when the incident wave height was large, the average wave height was smaller with the breakwater present. In the no-breakwater scenario, the average water level on the reef flat was higher than that with the breakwater present. (3) When the reef flat depth was small, the presence of the breakwater had little effect on the average wave height. However, when the reef flat depth was 0.075 m, the average wave height fluctuated noticeably on the reef flat. At the end of the reef flat, the average wave height with the breakwater present was smaller than that without it for the same reef flat depth. At measurement point G18, the average water level without the breakwater was greater than that with the structures present, and the differences between the two at the reef flat end gradually decreased with increasing reef flat depth. (4) Compared with the other wave parameters, the wave period had a less regular impact on the average wave height and water level. On the reef flat, the average water level was generally lower for shorter periods than for longer periods, and the average wave height and water level fluctuations with the breakwater were more concentrated. (5) The different arrangement methods and wall perforation rates significantly affected the hydrodynamic characteristics along the reef flat, whereas the wall perforation rate had a limited impact on overall wave energy transmission.