The near-bed flow structure and bed shear stresses within emergent
vegetation canopies
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 canopies of
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 canopies under a wide range of flow and canopy conditions.
There is strong horizontal 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 canopy density. This impact of
canopy 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. The strong agreement between model predictions and experimental
data may explain why both TKE and bed shear stress may be seen as
drivers of sediment transport processes in vegetated flows. 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