Effects of Leafy Flexible Vegetation and Bedforms on Turbulent Flow and Sediment Transport

Abstract

Recent studies have shown that vegetation can trigger sediment resuspension and facilitate sediment movement, highlighting the possibility of finding a linkage between turbulence and sediment transport rates in vegetated areas. This study investigates flow hydrodynamics, through a double-averaged analysis, focusing on data that were acquired in experiments with dunes and leafy flexible vegetation characterized by different Leaf Area Indices (LAI), which denotes the total one-sided leaf area $\left(A_L\right)$ per unit ground area $\left(A_B\right)$ ($LAI=A_L/A_B$). Flow velocity was measured under both fixed- and mobile-bed conditions, with the fixed-bed physical model representing the final topography from the mobile-bed experiments. The results suggest that double-averaged turbulent kinetic energy in mobile-bed conditions is approximately two to three times higher than that measured in fixed-bed conditions under comparable experimental conditions. Moreover, the spatially and depth-averaged turbulence intensity measured across various setups was correlated with a dimensionless factor derived from both dune height $\left(\Delta \right)$ and wavelength $\left(\lambda \right)$, ${\Delta }^2/\lambda H$ with $H$ being the water depth. This correlation remained effective in all tested setups, highlighting the dominant influence of dune geometry on turbulence compared to vegetation drag. The results show that applying turbulence-based models with depth-averaged turbulent kinetic energy leads to an underestimation of sediment transport in setups with denser vegetation, whereas near-bed values provide better agreement with measurements. Furthermore, the experimental data were used to update a turbulence-based bed-load transport model, incorporating the near-bed influence of both leafy vegetation and dunes.