DIRECT FIELD MEASUREMENT OF WIND DRAG ON VEGETATION FOR APPLICATION TO WINDBREAK DESIGN AND MODELING

A field instrument was designed and field tested for measuring the app lied wind load on trees and surface-mounted obstacles in a natural bou ndary layer. Using this instrument, the effect of vegetation porosity on the drag coefficient of small conifer trees (h = 1.4 m) was determi ned directly in the field. Drag coefficients for two simple solid geom etric forms (cone and cylinder) having approximately the same size(hei ght and diameter) as the conifer trees were also measured over a relat ively wide range of Reynolds numbers and the results compared to publi shed drag curves for these shapes. The field study found that the poro us element had a higher drag coefficient than a solid element, both fo r the solid element tested and for the drag coefficient suggested for surface-mounted solid obstacles. The drag coefficient changed on a con tinuum with porosity, rising initially from the value measured for the element as a solid, reaching a peak at an intermediate value and even tually falling to zero when the element was removed. This peak in the drag coefficient versus porosity curve corresponds to reports that she lterbelt efficiency peaks at medium-porosities, and is an important re lationship in terms of modelling momentum extraction of vegetation, on e which has not been shown previously in the literature. Findings of t his study have direct application to the modelling of shelterbelts and windbreaks and the assessment of the amount of vegetation cover requi red to suppress wind erosion in rangeland vegetation communities. (C) 1998 John Wiley & Sons, Ltd.