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”

A brief history…

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0 5 10 15 20 25 30

Velocity (cm/s)

Hei

ght

(cm

)

Slope 1:10000

4% plant volume

Vegetated

Bare

(taken from Defina and Bixio,

Water Resour. Res., 2005)

• Current models fail to predict in-canopy:

1.Velocity profile2.Vertical mixing

Diffusivity (m2/s)

Velocity profile

Mixing

A brief history…

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30

Velocity (cm/s)

Hei

ght

(cm

)

Slope 1:10000

4% plant volume

Vegetated

Bare

(taken from Defina and Bixio,

Water Resour. Res., 2005)

• Current models fail to predict in-canopy:

1.Velocity profile2.Vertical mixing

Diffusivity (m2/s)

Velocity profile

Mixing

“Model”

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 PHYSICALINSIGHT?• Can we develop a general, rather

than canopy specific, framework?

Understanding

Prediction(taken from piscoweb.org)

Experimental design

Model vegetation (7 m)

H = 47 cm

Velocity meters (acoustic Doppler)

Cylinder array

Flow straightener

Flow

• Canopy defined by its:

height: h

drag coefficient: CD

density: a

• Vertical transport dominated by coherent vortex structures

• Vortices generate strongly oscillatory flow and transport• Mixing is more rapid than above a flat bed

Salient hydrodynamic features:

1. The vortexVertical transport

High

Low

Canopy top

FlowFlow

• 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)

Salient hydrodynamic features:2. Hydrodynamic stratification

Exchange

Wake

Velocity profile

e/(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], Raupach et al. [1996], ◊ Brunet et al. [1994])

Extrapolation to other vegetated flows

e ≈ 0.2 / CDa

(i.e. less penetration into dense, drag-exerting canopies)

What do 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?

[NH4+] (M)

z (m

)

Flushing?

What don’t we understand ?

• How does plant waving impact nutrient uptake & particle capture ?