Hydrodynamic Performance of a Top-Covered Semicircular Breakwater Under Regular Waves: An Experimental and Computational Study

Coastal protection structures are crucial for reducing wave impacts and protecting vulnerable shorelines and coastal ecosystems. The hydrodynamic performance of a Semicircular Breakwater with Cover (SBWC) under regular wave conditions was evaluated using physical flume experiments and computational fluid dynamics (CFD) modeling. A total of 216 regular-wave laboratory tests were conducted across four relative submergences (Rdh = d/h = 0.67, 1.00, 1.33, and 1.67), three wave steepness values (S1 = Hi/L= 0.02, 0.04, and 0.06), and 18 wave periods (T = 0.8–2.5 s), corresponding to relative breakwater widths (B/L = 0.17–1.22). CFD simulations were conducted in FLOW-3D using the Volume of Fluid (VOF) method and validated against physical flume measurements to assess the model’s ability to reproduce the hydrodynamic performance of the SBWC. The validated CFD model reproduced the measured transmission, reflection, and energy loss coefficients across all tested conditions, providing a basis for future investigations under other conditions within an appropriate range of applicability. Under emerged conditions (Rdh = 0.67), the SBWC functions primarily as a wave-reflecting structure, yielding low wave transmission (CT ≈ 0.07–0.37) and high reflection (CR ≈ 0.41–0.91). At the partially submerged condition (Rdh = 1.00), wave energy dissipation was maximized, with the energy loss coefficient (CL) reaching 0.93 due to wave breaking and turbulent shear across the SBWC. Non-linear multivariable empirical models incorporating second-order interaction terms were developed to predict CT, CR, and CL, with coefficients of determination of R2 = 0.84, 0.84 and 0.52, respectively (RMSE = 0.13, 0.10 and 0.12). Cubic polynomial models raised R2 to 0.91, 0.89 and 0.63. A comparison with the uncovered semicircular breakwater shows that the cover reduces wave transmission mainly when the still-water level is at the caisson crown. These findings quantify the hydrodynamic response of the SBWC and provide empirical equations for preliminary estimates of wave transmission, reflection and energy dissipation under regular waves, within the tested ranges.