Effect of Submerged Vegetation on Water Surface Geometry and Air–Water Momentum Transfer

Understanding how submerged vegetation modifies the water surface is crucial for modeling momentum exchange between shallow waters and the atmosphere. In particular, quantifying its impact on the equivalent aerodynamic roughness of the water surface is essential for improved boundary-layer parameterization in oceanic and atmospheric models. In this Letter, we present fully resolved multiphase simulations of gravity-driven flow over a fully submerged vegetated bed, capturing the coupled dynamics of air, water, and individual plant stems, under quasi-realistic conditions (the air/water viscosity ratio is real, while the density ratio is reduced tenfold). Our results show that vegetation submerged for four times its height regularizes the water surface suppressing strong deformations and homogenizing streamwise-propagating wave fronts along the transversal direction. Despite these alterations, the equivalent roughness perceived by the overlying air flow remains unchanged. These findings clarify vegetation–surface interactions and provide quantitative insights for nature-based wave mitigation strategies and atmospheric boundary-layer modeling.