Wave attenuation by mixed mangroves in hybrid Mangrove–Breakwater systems using a modified Ergun equation

Coastal hybrid protection schemes integrating mangrove vegetation with vertical impermeable breakwaters can effectively mitigate coastal inundation and seabed erosion triggered by storm surges. Nevertheless, most existing numerical models overlook the vertical stratification of mixed mangrove communities, which leads to systematic underestimation of flow resistance within vegetated coastal zones. In this study, a two-dimensional numerical wave flume was established in ANSYS Fluent 2022 R2, and a vertically layered porous medium model embedded with calibrated modified Ergun coefficients (viscous correction factor β = 1.238, inertial correction factor γ = 1.029). This framework was adopted to investigate the non-Darcy hydrodynamic behaviors and wave attenuation performance of mixed mangrove stands consisting of Kandelia candel (KC) and Acanthus ilicifolius (AI). Numerical outputs for JONSWAP irregular waves were validated against laboratory experimental measurements, yielding a coefficient of determination R 2 > 0.92 and a root mean square error (RMSE) below 0.03 m. The numerical outputs indicate that the mixed mangrove belt achieves a 26%–30% wave height reduction. Planting density of AI is identified as the dominant factor governing wave energy dissipation. Raising the height of KC improves wave attenuation efficiency by 4%, whereas expanding its stem diameter increases near-bed turbulence intensity and dike overtopping discharge by 18% and 12%, respectively. Under the simplified assumptions of an ideal flat bed, static constant water depth, and 2D rigid mangrove canopy, the balanced configuration with an AI density of no less than 192 plants/100 m 2 can provide a conditional reference for subtropical integrated ecological-rigid coastal defense systems. It should be emphasized that this parameter scheme is only applicable to laboratory 2D water conditions, and field calibration is mandatory prior to practical engineering deployment.