Drag characteristics of dynamically submerged vegetation under combined wave-current action

Under combined tidal and wave effects, vegetation frequently experiences a complex dynamic submergence state. Traditional drag models ignore free-surface fluctuations and inaccurately estimate the effective resistance length. To address this, this study developed a 3-D numerical flume model coupling waves, currents, and vegetation based on the Reynolds Averaged Navier Stokes (RANS) equations. Various combinations of vegetation heights, wave heights, and wave-current conditions were simulated to reveal the wave propagation characteristics of dynamically submerged vegetation and build a drag force prediction model. The results show that current direction dominates wave attenuation and nonlinear evolution. Following currents trigger strong shear turbulence at the canopy top, leading to optimal wave dissipation, while opposing currents transfer primary frequency energy to higher harmonics and sharply increase wave skewness and asymmetry. Dynamic submergence also induces clear phase asymmetry in vegetation loads. To solve this, the Morison equation was modified for a more accurate drag coefficient ( C D ), and by introducing relative submergence ( S r ) as a dimensionless factor, the coefficient of determination ( R 2 ) of the fitting between C D and the Reynolds number ( Re ) as well as the Keulegan–Carpenter number ( KC ) were increased by 9% and 11% respectively compared to the traditional methods.