Direct field measurement of wind drag on vegetation for application to windbreak design and modelling

Abstract

A field instrument was designed and field tested for measuring the applied wind load on trees and surface-mounted obstacles in a natural boundary layer. Using this instrument, the effect of vegetation porosity on the drag coefficient of small conifer trees (h=1·4 m) was determined directly in the field. Drag coefficients for two simple solid geometric forms (cone and cylinder) having approximately the same size (height and diameter) as the conifer trees were also measured over a relatively wide range of Reynolds numbers and the results compared to published drag curves for these shapes. The field study found that the porous element had a higher drag coefficient than a solid element, both for the solid element tested and for the drag coefficient suggested for surface-mounted solid obstacles. The drag coefficient changed on a continuum with porosity, rising initially from the value measured for the element as a solid, reaching a peak at an intermediate value and eventually falling to zero when the element was removed. This peak in the drag coefficient versus porosity curve corresponds to reports that shelterbelt efficiency peaks at medium-porosities, and is an important relationship in terms of modelling momentum extraction of vegetation, one which has not been shown previously in the literature. Findings of this study have direct application to the modelling of shelterbelts and windbreaks and the assessment of the amount of vegetation cover required to suppress wind erosion in rangeland vegetation communities. © 1998 John Wiley & Sons, Ltd.