Spatio-Temporal Surface Shear-Stress Variability in Live Plant Canopies and Cube Arrays


This study presents spatiotemporally-resolved measurements of surface shear-stress τ s in live plant canopies and rigid wooden cube arrays to identify the sheltering capability against sediment erosion of these different roughness elements. Live plants have highly irregular structures that can be extremely flexible and porous resulting in considerable changes to the drag and flow regimes relative to rigid imitations mainly used in other wind-tunnel studies. Mean velocity and kinematic Reynolds stress profiles show that well-developed natural boundary layers were generated above the 8 m long wind-tunnel test section covered with the roughness elements at four different roughness densities (λ = 0, 0.017, 0.08, 0.18). Speed-up around the cubes caused higher peak surface shear stress than in experiments with plants at all roughness densities, demonstrating the more effective sheltering ability of the plants. The sheltered areas in the lee of the plants are significantly narrower with higher surface shear stress than those found in the lee of the cubes, and are dependent on the wind speed due to the plants ability to streamline with the flow. This streamlining behaviour results in a decreasing sheltering effect at increasing wind speeds and in lower net turbulence production than in experiments with cubes. Turbulence intensity distributions suggest a suppression of horseshoe vortices in the plant case. Comparison of the surface shear-stress measurements with sediment erosion patterns shows that the fraction of time a threshold skin friction velocity is exceeded can be used to assess erosion of, and deposition on, that surface.

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A f :

Roughness element frontal area

D :

Average roughness element diameter

Q :

Mass transport rate

Re h = U δ h/ν :

Roughness element Reynolds number

S :

Ground area per roughness element

U δ :

Free-stream velocity

U s :

Wind speed at Irwin sensor-tube height

c 1 and c 2 :


d :

Sand grain diameter

f = 20 kHz:

Hot-film sampling frequency

f c :

Cut-off frequency

h :

Roughness element height

h s :

Irwin sensor tube height

m :

Parameter relating \({\tau_{\rm s}^{\prime \prime}}\) to τ s

Δp :

Pressure difference measured by Irwin sensor


Fluctuations over pressure signal

u :

Mean streamwise wind velocity


Fluctuations in mean streamwise wind velocity

\({u_\ast=(\tau/\rho)^{1/2}}\) :

Friction velocity

u *t :

Fluid threshold friction velocity

\({u_{\tau}=(\tau_{\rm s}/\rho)^{1/2}}\) :

Skin friction velocity

u τ t :

Fluid threshold skin friction velocity

\({\overline {{u}^{\prime}{w}^{\prime}}}\) :

Kinematic Reynolds stress


Fluctuations in mean vertical wind velocity

β c :

Irwin sensor calibration constant

β :

Ratio of roughness element to surface drag coefficient

λ :

Roughness density

ν :

Kinematic viscosity of air

\({\psi}\) :

Percentage of time that threshold skin friction velocity is exceeded

ρ :

Air density

σ :

Ratio of roughness element basal to frontal area

σ u :

Standard deviation of wind speed U s

\({\tau=\rho u_\ast^2 }\) :

Total shear stress averaged over whole canopy

τ R :

Shear stress acting on roughness elements

τ s(t, x, y):

Shear stress acting on surface

\({\tau_{\rm s}^{\prime \prime}}\) :

Spatial peak of temporally-averaged surface shear-stress distribution

τ s0 :

Spatiotemporally-averaged surface shear stress in the absence of roughness elements

\({\xi}\) :

Normalized turbulence intensity


  1. Anderson RS, Sorensen M (1991) A review of recent progress in our understanding of aeolian sediment transport. Acta Mech 1: 1–19

    Google Scholar 

  2. Bagnold R (1943) The physics of blown sand and desert dunes. Meghuen, London, 265 pp

  3. Brown S, Nickling WG, Gillies JA (2008) A wind-tunnel examination of shear-stress partitioning for an assortment of surface roughness distributions. J Geophys Res 113: F02S06

    Article  Google Scholar 

  4. Burri K (2011) Plants and mycorrhizal fungi on wind erosion control. Dissertation, ETH Zürich. doi:10.3929/ethz-a-006570793

  5. Burri K, Gromke C, Graf F (2011a) Mycorrhizal fungi protect the soil from wind erosion: a wind-tunnel study. Land Degrad Dev. doi:10.1002/ldr.1136

  6. Burri K, Gromke C, Lehning M, Graf F (2011b) Aeolian sediment transport over vegetation canopies: a wind-tunnel study with live plants. Aeolian Res 3: 205–213

    Article  Google Scholar 

  7. Clifton A, Lehning M (2008) Improvement and validation of a snow saltation model using wind-tunnel measurements. Earth Surf Process Landf 33: 2156–2173

    Article  Google Scholar 

  8. Crawley DM, Nickling WG (2003) Drag partition for regularly-arrayed rough surfaces. Boundary-Layer Meteorol 107: 445–468

    Article  Google Scholar 

  9. Gillette DA, Stockton PH (1989) The effect of nonerodible particles on wind erosion of erodible surfaces. J Geophys Res 94: 12885–12893

    Article  Google Scholar 

  10. Gillies JA, Nickling WG, King J (2002) Drag coefficient and plant form response to wind speed in three plant species: Burning Bush (Euonymus alatus), Colorado Blue Spruce (Picea pungens glauca), and Fountain Grass (Pennisetum setaceum). J Geophys Res. doi:10.1029/2001JD001259

  11. Gillies JA, Nickling WG, King J (2007) Shear-stress partitioning in large patches of roughness in the atmospheric inertial sublayer. Boundary-Layer Meteorol 122: 367–396

    Article  Google Scholar 

  12. Gromke C, Manes C, Walter B, Lehning M, Guala M (2011) Aerodynamic roughness length of fresh snow. Boundary-Layer Meteorol. doi:10.1007/s10546-011-9623-3

  13. Irwin HPAH (1981) A simple omnidirectional sensor for wind-tunnel studies of pedestrian-level winds. J Wind Eng Ind Aerodyn 7: 219–239

    Article  Google Scholar 

  14. Kim DS, Cho GH, White BR (2000) A wind-tunnel study of atmospheric boundary-layer flow over vegetated surfaces to suppress PM10 emission on Owens (dry) Lake. Boundary-Layer Meteorol 97: 309–329

    Article  Google Scholar 

  15. King J, Nickling WG, Gillies JA (2006) Aeolian shear-stress ratio measurements within mesquite-dominated landscapes of the Chihuahuan Desert, New Mexico, USA. Geomorphology 82: 229–244

    Article  Google Scholar 

  16. Lancaster N, Baas A (1998) Influence of vegetation cover on sand transport by wind: Field studies at Owens Lake, California. Earth Surf Proc Landf 23: 69–82

    Article  Google Scholar 

  17. Lyles L, Allison BE (1975) Wind erosion: Uniformly spacing nonerodible elements eliminates effects of wind direction variability. J Soil Water Conserv 30: 225–226

    Google Scholar 

  18. Marshall JK (1971) Drag measurements in roughness arrays of varying density and distribution. Agric Meteorol 8: 269–292

    Article  Google Scholar 

  19. Morris HM (1955) Flow in rough conduits. Am Soc Civil Eng 120: 373–398

    Google Scholar 

  20. Musick HB, Gillette DA (1990) Field evaluation of relationships between a vegetation structural parameter and sheltering against wind erosion. Land Deg Rehabil 2: 87–94

    Article  Google Scholar 

  21. Musick HB, Trujillo SM, Truman CR (1996) Wind-tunnel modelling of the influence of vegetation structure on saltation threshold. Earth Surf Proc Landf 21: 589–605

    Article  Google Scholar 

  22. Raupach MR (1992) Drag and drag partition on rough surfaces. Boundary-Layer Meteorol 60: 375–395

    Article  Google Scholar 

  23. Raupach MR, Gillette DA, Leys JF (1993) The effect of roughness elements on wind erosion threshold. Geophys Res 98: 3023–3029

    Article  Google Scholar 

  24. Schlichting H (1936) Experimental investigations of the problem of surface roughness. NASA Tech Memo 823 Washington

  25. Shao Y (2008) Physics and modelling of wind erosion. Springer. ISBN:978-1-4020-8894-0, 452 pp

  26. Sutton SLF, McKenna-Neumann C (2008) Variation in bed level shear-stress on surfaces sheltered by nonerodible roughness elements. J Geophys Res 113:F03016. doi:10.1029/2007JF000967

  27. Valyrakis M, Diplas P, Dancey C, Greer K, Celik AO (2010) Role of instantaneous force magnitude and duration on particle entrainment. J Geophys Res. doi:10.1029/2008JF001247

  28. Walter B, Gromke C, Lehning M (2009) The SLF Boundary Layer Wind-tunnel—An Experimental Facility for Aerodynamical Investigations of Living Plants. In: 2nd international conference “Wind effects on trees”, Freiburg, Germany, pp 31–37

  29. Walter B, Gromke C, Leonard K, Clifton A, Lehning M (2011) Measurements of surface shear-stress distribution in live plant canopies. In: 13th international conference on wind engineering, Amsterdam, The Netherlands

  30. Wolfe SA, Nickling WG (1996) Shear-stress partitioning in sparsely vegetated desert canopies. Earth Surf Proc Landf 21: 607–619

    Article  Google Scholar 

  31. Wooding RA, Bradley EF, Marshall JK (1973) Drag due to regular arrays of roughness elements of varying geometry. Boundary-Layer Meteorol 5: 285–308

    Article  Google Scholar 

  32. Wu H, Stathopoulos T. (1993) Further experiments on Irwin’s wind sensor. J Wind Eng Ind Aerodyn 53: 441–452

    Article  Google Scholar 

  33. Wyatt VE, Nickling WG (1997) Drag and shear-stress partitioning in sparse desert creosote communities. Can J Earth Sci 34: 1486–1498

    Article  Google Scholar 

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Correspondence to Benjamin Walter.

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Walter, B., Gromke, C., Leonard, K.C. et al. Spatio-Temporal Surface Shear-Stress Variability in Live Plant Canopies and Cube Arrays. Boundary-Layer Meteorol 143, 337–356 (2012).

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  • Aeolian processes
  • Drag partitioning
  • Particle erosion
  • Surface shear-stress
  • Turbulent boundary-layer
  • Vegetation aerodynamics