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Wind flow and sedimentation in artificial vegetation: Field and wind tunnel experiments
- Hesp, Patrick A., Dong, Yuxiang, Cheng, Hong, Booth, Jennifer L.
- Geomorphology 2019 v.337 pp. 165-182
- arid lands, canopy, dowels, dunes, grasslands, hydrodynamics, leaves, plant density, sand, wind speed, wind tunnels
- The flow dynamics within and immediately above an artificial plastic vegetation canopy are measured in the field and wind tunnel. Artificial plants created from wooden dowels and cable (zip) ties were arranged in a variety of arrays on pegboards where plant cover (or lateral cover) could be accurately controlled. Plant covers examined ranged from ~12% to 60%. Wind velocity profiles and the bedforms (incipient dunes) that developed within and beyond the vegetation in a few hours were measured. The results indicate that flow deceleration and drag increases within the canopy as plant density increases. In general, there is a marked reduction and inflection in velocity at the canopy top, and a second major inflection in the velocity profiles occurs immediately below the base of the four arrays equal or >23% density. Tall vegetation allows flow to sweep under the canopy and local high speed maxima flows may develop leading to minimal sedimentation in those canopies, but deposition downwind beyond the canopies. Short vegetation, where the leaves fill the canopy close to the ground, display significantly greater drag and rapid rates of sand deposition. Flow velocities above the canopy increase as plant cover or density increases due to flow compression and acceleration. Bedform development is strongly correlated with plant cover, density or lateral cover and short plants such that shorter length, higher bedforms develop in higher density and/or shorter plants. The relationship between plant cover, density or lateral cover and within canopy velocity is non-linear, and a new tentative flow regime is proposed, canopy flow, which may occur between wake interference flow and skimming flow. This study further confirms that plant density plays a defining role in determining bedform and dune morphology. The results also apply to flow in semi-arid grasslands.