8. A Semi-Empirical Model for Urban Trees Effects on the Wind Environment

High-density urban areas are often associated with poor outdoor natural ventilation. Given the growing call for more vegetation in the cities, it is important to study wind resistant of urban trees in order to address outdoor natural ventilation problem in landscape planning. Currently, Computational Fluid Dynamics (CFD) simulation and wind tunnel experiment can only model the simplified street canyon with roadside trees, at the expense of intensive technical support and high computational cost. Thus, their application is often restricted to research purpose only, and the impact of their research outputs on the landscape planning remains low due to the impracticality. In this chapter, we developed a semiempirical model to provide scientific understandings and practical tools for landscape planning practice. This new model was based on the balance between momentum flux and the drag force of both buildings and trees on airflow. Friction velocity \( \left( {u_{ *} } \right) \) was modeled and validated by existing CFD and wind tunnel data; effective frontal area density \( \left( {\lambda_{{{\text{f\_tree}}}} } \right) \) was estimated by the measured leaf area index. The impact of urban context density and trees (i.e., plant canopy density and typology) on wind environment was clarified. This research correlated the urban density and tree geometry indices with wind speed, thereby enabling planners to calculate trees’ effects on airflow using their in-house data. With such new practical tool and understandings, knowledge-based landscape planning can be established to introduce more trees into urban areas, and prevent from having negative effects of trees on outdoor wind environment at the same time.