Urban exposure upstream fetch and its influence on the formulation of wind load provisions

Abstract With the rapid expansion of city area, concern has increased about the atmospheric boundary layer in urban area, which is influenced by the complex city morphology. This paper aims at providing a fundamental understanding of the influence of city upstream fetch length and exposure roughness on the mean velocity and turbulence intensity profiles. Wind tunnel tests of two real city models have been carried out. The results indicate that in urban areas mean velocity profiles are influenced by an upstream fetch length up to 750 m; and not influenced by the exposure morphology when the fetch length goes farther than 1250 m. Turbulence intensity profiles are more sensitive to upstream exposure and require longer upstream fetches to become steady. The evolution of mean velocity profiles of different fetch lengths and the influence of upstream roughness elements in the variation of turbulence intensity have been analyzed. The roughness length z0 of Kowloon, Hong Kong is determined, and issues related to the specification of minimum upstream fetch in current wind load provisions are tackled.