Drag coefficients, roughness length and zero-plane displacement height as disturbed by artificial standing vegetation

Abstract

Standing vegetation, for its disturbance to the near-surface airflow, is widely used in the arid and some semi-arid lands over the world to control wind erosion. Knowledge of drag coefficient (CD), roughness length (Z0) and displacement height (D) is essential to completely define the state of wind and the protective role of standing vegetation in wind erosion. Using standing sticks as model standing vegetation, detailed wind velocity distributions were measured above the vegetated surface in wind tunnel, a mass conservation method to estimate the zero-plane displacement height was tried, and the drag coefficients and roughness length were derived by a curve-fit method. Due to the disturbance of standing vegetation, the velocity distributions deviated from the logarithmic profile. The deviation increased with increasing vegetation density and height. The wind velocity profile disturbed by standing vegetation can be divided into three sections, and each section can be expressed by a logarithmic function. Displacement height is a significant parameter for tall and dense vegetation; choice of the value for displacement height greatly influences other parameters such as roughness length and drag coefficient, but its accurate value is not available yet. In the authors’ philosophy, drag coefficient and roughness length should be recommended. However, caution should also be taken in selecting the wind velocity measurements used to derive drag coefficient and roughness length. Both the height and density of standing vegetation influenced drag coefficient and roughness length, but their relative importance was different. A new parameter, effective lateral cover (Lec) was introduced to characterize the structure of standing vegetation. It was found that effective lateral cover and height/spacing ratio were better structural parameters than the others when the effects of standing vegetation on drag coefficient and roughness length were assessed. Good correlation existed between the derived roughness length and drag coefficient, implying that obtaining drag coefficient and roughness length of vegetated surface by curve fit method is reliable so long as caution is taken in selecting the appropriate measurements. Of the three parameters roughness length is the most sensitive to characterize the effects of roughness length on near-surface airflow.