A Predictor of Turbulent Kinetic Energy for Oscillatory Flows Through
Submerged Aquatic Vegetation
Abstract
Aquatic vegetation modifies hydrodynamics, turbulence structure,
sediment transport, and ecological processes in marine ecosystems.
Recent turbulence models for vegetated flows have focused on open
channel unidirectional flows. However, the unsteadiness and turbulent
structure of oscillatory flows often prevent the direct application of
such models in wave-dominated environments. We investigate Turbulent
Kinetic Energy (TKE) connected to the flow structure in oscillatory
flows through aquatic vegetation. Using an oscillatory tunnel, we test
vegetation densities up to $\phi=0.10$ with wave
periods between 2.1-5.3 s and wave amplitudes between 2-10 cm. Our
measurements show a nonlinear relation between the TKE inside the canopy
and vegetation density due to the change from the stem- to canopy-scale
dominated regime. We observe that $ah\geq 0.8$ marks a
threshold for this transition: a reduction of wake TKE inside the canopy
and an increase of shear TKE at the top of the canopy. This transition
is characterized by increasing frequency and intensity of sweeps and
ejections near the bed and at the canopy top. We developed a
two-equation predictor for TKE at the top of the canopy using the
“short-cut” TKE transfer first proposed by
\citeA{finnigan2000turbulence} where canopy-scale
eddies convert TKE into stem-scale eddies via the work against
vegetation drag. For near-bed TKE, we adapt
\citeA{tanino2008lateral}’s model to predict the
maximum TKE values on oscillatory flows. These two predictors provide
easy-to-use tools suitable for wave-dominated environments to accurately
estimate TKE levels inside the canopy for estimating sediment transport
rates and mass exchange across the canopy.