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Nature's Drag Queens: Understanding Aquatic Flows Through Vegetation

Explore the impact of vegetation on aquatic flows in this symposium delving into the failures of current models to predict velocity profiles and mixing in canopies. Key questions include characterizing fluxes of nutrients, sediments, and gases, developing a general framework, and utilizing physical insights for prediction. Hydrodynamic features such as vortex structures and stratification are analyzed, along with experiments on vertical gradients and mixing events in vegetated environments. Learn how plant waving influences nutrient uptake and particle capture for a better understanding of these dynamic ecosystems.

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Nature's Drag Queens: Understanding Aquatic Flows Through Vegetation

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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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