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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 - PowerPoint PPT Presentation

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  • Natures 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

    Slope 1:10000 4% plant volumeVegetatedBare(taken from Defina and Bixio, Water Resour. Res., 2005)Current models fail to predict in-canopy:Velocity profileVertical mixing

    Diffusivity (m2/s)Velocity profileMixing

  • A brief history

    Slope 1:10000 4% plant volumeVegetatedBare(taken from Defina and Bixio, Water Resour. Res., 2005)Current models fail to predict in-canopy:Velocity profileVertical mixing

    Diffusivity (m2/s)Velocity profileMixingModel

  • Questions that needed an answerWHATS GOING ON IN THESE FLOWS?Why does the traditional treatment yield such poor results?

    HOW CAN WE CHARACTERIZE FLUXESOF 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 designModel vegetation (7 m) H = 47 cm

    Velocity meters (acoustic Doppler)Cylinder array

    Flow straightenerFlow Canopy defined by its: height: h drag coefficient: CD density: a

  • Salient hydrodynamic features: 1. The vortexVertical transport dominated by coherent vortex structures

    Vortices generate strongly oscillatory flow and transportMixing is more rapid than above a flat bedVertical transportHighLowCanopy top FlowFlow

  • Salient hydrodynamic features:2. Hydrodynamic stratificationVortices 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(0.11 cm2/s)

    ExchangeWakeVelocity profile

  • Extrapolation to other vegetated flowsde/(CDa)-1CDahClosed 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]) de 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+] (mM)z (m)Flushing?What dont we understand ? How does plant waving impact nutrient uptake & particle capture ?