MEAN FLOW AND SHEAR-STRESS DISTRIBUTIONS AS INFLUENCED BY VEGETATIVE WINDBREAK STRUCTURE

A major deterrent toward arriving at satisfactory estimates of shelter effect of vegetative windbreaks is the difficulty in quantifying permeability to air flow as a function of directly measured physical parameters such as porosity and plant surface area. This weakness seriously limits the accuracy of both empirical and numerical models in describing the effect of vegetative barriers without first knowing some aspects of the induced flow regime, An aerodynamic study of vegetative windbreaks was conducted in a wind tunnel and in the field to assess the effect of barrier structure upon windward and leeward reductions in near ground mean wind speed and surface shear stress. Results from this study show that decreasing windbreak porosities result in reduced or equivalent wind speeds at all leeward distances. An expression was derived to describe the near ground horizontal distribution of mean relative velocity as a function of the leeward minimum relative velocity, the position of this minimum, and surface roughness length. With the exception of the region in the near lee, this model also allows the estimation of the horizontal distribution of shear stress in the vicinity of a barrier. Minimum relative velocities were adequately described as non-linear functions of barrier porosity or projected plant surface area. However, the vertical distribution of porosity with height, largely controlled by plant form, had a major influence on these relationships.