Wave-induced swaying of Rhizophora mangle saplings and implications for hybrid coastal protection

Understanding the mechanical and morphological adaptation of mangrove seedlings under wave forcing is essential for effective coastal restoration. This study investigates the dynamic response and structural integrity of Rhizophora mangle saplings exposed to regular wave forcing (H = 4–10 cm; T = 0.8–1.4 s), both with and without protection by portable reef. A damping oscillator model incorporating stem elasticity, damping ratio, and wave kinematics accurately predicted tip displacement and higher-order vibrational modes. Among six saplings with co-measured stiffness, the Higher-Order Count (HOC) correlated significantly with the Keulegan Carpenter number (ρ = 0.51, p < 0.001), indicating dynamic response sensitivity to flow unsteadiness. Morphological assessments revealed no statistically significant differences in aboveground traits across treatments, even after prolonged exposure. However, root mechanical integrity was markedly affected by wave forcing. Saplings directly exposed to waves exhibited reduced uprooting strength (mean = 3.71 N), while those shielded by the portable reef maintained root strength equivalent to unforced controls (mean ≈ 8.9 N). These findings suggest that portable reefs preserve belowground anchorage without compromising structural development. These results underscore the value of integrating wave-exposure dynamics and protective structures into hybrid coastal protection strategies that support both structural integrity and adaptive flexibility in mangrove saplings.