The Near-Bed Flow Structure and Bed Shear Stresses Within Emergent Vegetation

Abstract

The structure of the bottom boundary layer (BBL) in aquatic flows influences a range of biophysical processes, including sediment transport, hyporheic exchange, and biofilm formation. While the structure of BBL above bare sediment beds has been well studied, little is known about the complex near-bed flow structure within aquatic vegetation. In this study, we used high-resolution laboratory measurements and numerical Large Eddy Simulations to investigate the near-bed mean and turbulent flow properties within staggered-ordered emergent vegetation under a wide range of flow conditions and densities. There is strong spatial variability of key near-bed flow characteristics on the scale of the vegetation elements. Measurement locations that provide single-point flow characteristics closest to the spatially averaged values were identified. The spatially averaged BBL thickness is influenced strongly by vegetation density. This impact of vegetation density is engendered through its direct control of near-bed turbulent kinetic energy (TKE), which in turn is negatively correlated with BBL thickness, both locally in a given flow and across the range of flow conditions studied here. A model based on the near-bed TKE is developed to predict the BBL thickness and, ultimately, the bed shear stress. Model predictions were in close agreement with the experimental and numerical results. These findings provide new insights into the physical links between near-bed flow variables and therefore contribute to the understanding of some of the complex biophysical processes present in vegetated flows.