Parametrization of irregularity of urban morphologies for designing better pedestrian wind environment in high-density cities – A wind tunnel study

Abstract

Breezeways are crucial for horizontal urban ventilation. However, their morphologies, which are highly irregular, still lack investigation. This study aims to gain a categorical understanding of how irregular breezeway morphologies affect pedestrian-level wind performance in high-density cities. Firstly, we extracted breezeway areas and centerlines by adopting different definitions of breezeways: Method 1 defining all near-ground open-spaces (i.e., roads, other non-building areas, and low-rise-building areas) as breezeways; Method 2 defining roads as breezeways. Secondly, multiple morphological parameters were developed to describe irregularity (i.e., fragmentation, angularity, and sinuosity) and permeability of 19 typical urban sites in Hong Kong. Thirdly, linear regression analysis was conducted to correlate the parameters with wind tunnel data. The results show that in a dense built environment, urban ventilation relies on morphologies of all near-ground open-spaces rather than only roads. The regression analysis reveals a negative correlation between pedestrian-level wind performance and urban irregularity, where fragmentation has the largest impact due to the increase of building wall surfaces and viscous friction. The regression analysis reveals a positive correlation between pedestrian-level wind performance and urban permeability, where the newly-defined parameter (i.e., open-space width), which incorporates fragmentation into calculation, achieves higher coefficient of determination than the widely-used parameter (i.e., open-space coverage ratio). The newly-developed regression model, which considers both urban permeability and irregularity, predicts wind speed with a simple mathematical manipulation and a comparable accuracy to models in previous studies. More importantly, it provides applicable spatial information of urban irregularity for design of open-spaces and improvement of urban ventilation.