Wind Tunnel Studies of Shear Stress Partitioning in Live Plant Canopies

The earth's surface is permanently exposed to the atmosphere and accordingly to strong wind forces in many regions. Aerodynamic entrainment, transport and redeposition of sand, soil or snow are able to considerably reshape the surface morphology and influence the environment in areas ranging from deserts to polar regions. Even in moderate climate zones, entrainment and transport of dust, particulate matter and pollen or seeds by wind may have a strong impact on the local atmosphere and vegetation. Many of these processes exert negative influences on our sensitive natural environment. Land degradation, desertification or dust storms, increased particulate matter concentrations in the atmosphere or reduced accumulation of snow in arid regions are just a few examples of the impacts of wind erosion. Vegetation on the ground can provide an efficient sheltering effect against wind erosion. Plants influence sediment erosion mainly by the following four mechanisms: by reducing the surface exposed to the wind, by trapping particles in motion, by local stress concentration and by absorbing momentum from the flow. The latter results in lower surface shear stress on the ground beneath the plant canopies. The peak of the surface shear stress is responsible for the onset of erosion and the spatial mean is commonly used to estimate particle mass fluxes. To quantify the sheltering effect of vegetation, a method called shear stress partitioning has been extensively investigated in the past. This method determines the fraction of the total fluid stress on the entire canopy acting directly on the substrate surface. However, previous studies have limitations: they were either field-based, mainly using live plants, with the limitation that wind conditions could not be controlled, or from wind tunnels using rigid and non porous plant imitations, that poorly reflect the aerodynamic behaviour of live vegetation. This study takes a new approach, performing shear stress partitioning experiments in a controlled wind tunnel environment to systematically quantify the sheltering effect of live, flexible and porous plants. Subsequently, the data was used to test and improve a theoretical model that predicts the stress partition for vegetation canopies. This dissertation is divided into four sections. The main results of each section are discussed in this thesis and have also been published as one conference (Chapter 2) and three journal articles (Chapter 3-5). In Chapter 2, the flow conditions produced in the wind tunnel over live vegetation canopies were investigated to identify the suitability of the boundary-layer flow for these new investigations of shear stress partitioning. Flow characteristics like vertical Reynolds stress and integral length scale profiles and power spectral densities were determined from two-component hot-film anemometry measurements. The results were in good agreement with established literature, suggesting that well developed boundary-layers over li