Abstract: Aquatic vegetation, as a key part of open-channel and lake ecosystems, plays critical ecological roles. It impacts flow turbulence and sediment suspension in a water body, thereby altering its turbidity—a basic indicator of aquatic ecosystem health. However, the mechanism of turbidity altered by natural aquatic vegetation remains complicated. This study conducts a flume experiment using Vallisneria natans arranged in a rectangular pattern, to observe vertical variations in the flow velocity and sediment concentration of water flows under different flowrates and densities of natural flexible submerged vegetation. Analysis on the measured vertical profiles shows the morphology of flexible vegetation markedly alters the flow velocity pattern and turbulence structure. Under different vegetation densities, the time-averaged velocity, lateral and vertical Reynolds stresses, and turbulent kinetic energy all vary significantly, particularly in the near-bed and canopy layers. A turning point of the mean velocity profile occurs at the canopy top; similar vertical patterns are observed in turbulent kinetic energy and Reynolds stresses, with the maximum values located near the top of the vegetation canopy. Compared with the flow on a bare bed, suspended sediment concentration is reduced significantly in the vegetation condition, and decreases further with the vegetation density increasing. Flow rate and density conditions impact the canopy layer remarkably. Thus, we demonstrate that the primary reason for the reduction of suspended sediment in the flow on a submerged vegetation riverbed is that vegetation lowers the time-averaged flow velocity within the canopy and thereby suppresses sediment suspension.

Key words: vegetation hydraulics, flexible submerged vegetation, turbulence structure, suspended sediment concentration, flume experiment