1. Nature’s drag queens: how vegetation impacts aquatic flows† Marco Ghisalberti Centre for Water Research, University of Western Australia DIALOG VII SYMPOSIUM † formerly known as “Momentum and scalar transport in vegetated shear flows”

2. (taken from Defina and Bixio, Water Resour. Res., 2005)
A brief history…
Velocity profile
Current models fail to predict in-canopy:
Velocity profile
Vertical mixing
Slope 1:10000
4% plant volume
Vegetated
Bare
Mixing
(taken from Defina and Bixio, Water Resour. Res., 2005)
Diffusivity (m2/s)

3. (taken from Defina and Bixio, Water Resour. Res., 2005)
A brief history…
Velocity profile
Current models fail to predict in-canopy:
Velocity profile
Vertical mixing
Slope 1:10000
4% plant volume
Vegetated
“Model”
Bare
Mixing
(taken from Defina and Bixio, Water Resour. Res., 2005)
Diffusivity (m2/s)

4. Questions that needed an answer
WHAT’S GOING ON IN THESE FLOWS?
Why does the traditional treatment yield such poor results?
HOW CAN WE CHARACTERIZE FLUXES
OF NUTRIENTS/SEDIMENT/GASES?
Why the sharp mixing gradient?
HOW CAN WE USE THIS PHYSICAL
INSIGHT?
Can we develop a general, rather than canopy specific, framework?
Understanding
Prediction
(taken from piscoweb.org)

5. Velocity meters (acoustic Doppler)
Experimental design
Velocity meters (acoustic Doppler)
Flow straightener
H = 47 cm
Flow
Cylinder array
Model vegetation (7 m)
Canopy defined by its:
height: h
drag coefficient: CD
density: a

6. Salient hydrodynamic features: 1. The vortex
Vertical transport dominated by coherent vortex structures
Vortices generate strongly oscillatory flow and transport
Mixing is more rapid than above a flat bed
Vertical transport
High
Canopy top
Low
Flow
Flow

7. Salient hydrodynamic features: 2. Hydrodynamic stratification
Vortices separate the canopy into two distinct zones.
Upper zone: “Exchange zone”
D ≈ 1/50 × vortex size × rotation
~ O(10 cm2/s)
Lower zone: “Wake zone”
D ≈ 1/400 × flow speed ×
stem diameter × % wakes.
~ O(0.1―1 cm2/s)
Velocity profile
Exchange
Wake

8. Extrapolation to other vegetated flows
de ≈ 0.2 / CDa
(i.e. less penetration into dense, drag-exerting canopies)
de/(CDa)-1
CDah
Closed symbols – Cylinders in water (♦ Ghisalberti and Nepf [2004], ● Vivoni [2000], ■ Dunn et al. [1996], ▲Tsujimoto et al. [1992])
Open symbols – Cylinders/strips in air (○ Seginer et al. [1976], D Raupach et al. [1996], ◊ Brunet et al. [1994])

9. What don’t we understand ?
What do we understand ?
What don’t we understand ?
Experiments have given us a much better idea of:
- Residence times and vertical gradients of passive tracers in canopies.
- Fluxes in/out of canopies
- Brief but intense nature of mixing events.
To what extent does the hydrodynamics control the chemistry & biology?
Flushing?
z (m)
[NH4+] (mM)
How does plant waving impact nutrient uptake & particle capture ?