Flow structure and spectral characteristics in regions with highly flexible vegetation

Abstract

Although flexible submerged vegetation is widespread in natural rivers, previous models often overlooked its influence on flow dynamics and turbulence structure. Here, we report eight flume experiments conducted with Vallisneria nana under two depth-limited conditions (U = 0.1–0.3 m/s) to investigate these effects. High-definition video imagery captured vegetation motion, and a novel velocity distribution model was developed, achieving a predictive accuracy of R² value of 0.9844 for stratified velocity profiles. Quadrant analysis revealed that ejection (Q₂) and sweep (Q₄) events dominated momentum transfer, with Q₂ events strongly correlated with turbulence scales. This dominance highlights the canopy’s critical role in modulating energy exchange within the flow. Spectral analysis further identified shear-layer turbulence frequencies, with peak energy shifting from 0.18 Hz in deeper flow to 0.25 Hz in shallow conditions. This shift suggests a faster energy cascade under reduced depths, likely driven by increased interactions between canopy-induced vortices and the surrounding flow. Finally, the relationship between vegetation sway frequency and dominant flow frequency was quantified using a S_t number, demonstrating enhanced coupling with high-frequency turbulence. Together, these findings establish a robust framework for understanding the hydrodynamic mechanisms of vegetated flows, providing insights to advance ecological restoration strategies and optimize vegetation-based interventions in aquatic environments.