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A brief history…

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…. Velocity profile. Slope 1:10000

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A brief history…

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

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

  3. A brief history… Velocity profile Slope 1:10000 4% plant volume Vegetated • Current models fail to predict in-canopy: • Velocity profile • Vertical mixing “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. Experimental design Velocity meters (acoustic Doppler) Flow straightener H = 47 cm Flow Model vegetation (7 m) Cylinder array • 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 do we understand ? What don’t we understand ? • To what extent does the hydrodynamics control the chemistry & biology? • 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. Flushing? z (m) [NH4+] (mM) • How does plant waving impact nutrient uptake & particle capture ?

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