An Exploration of the Effects of Urban Block Design on the Outdoor Thermal Environment in Tropical Savannah Climate: Case Study of Nyamirambo Neighborhood of Kigali

Abstract

Urbanization is known to alter the microclimate, thus accelerating the effect of climate change. The built environment can have a positive and negative impact on local microclimates and especially at the neighborhood level. Microclimates at this level are created by items such as building form and geometry, street width, surface material types, soil types, trees, and vegetation types which represent urban block design characteristics. At street level, urban block settings have an impact on the overall air temperature, the surface temperature, the wind distribution, and on the solar radiation, and this will influence the outdoor thermal comfort. In this paper, we explore the impact of urban block design on the microclimate and its impact on the outdoor thermal environment by simulating the microclimate using ENVI-met and by assessing urban thermal comfort using UTCI in a carefully selected urban block fabric of the Nyamirambo neighborhood, which is one of the oldest mixed-used neighborhoods in Kigali presenting a particularly dense urban fabric in the fast-growing city of Kigali. While the region has been experiencing extensively high temperatures during the dry season in the recent past years, this paper tends to highlight urban block design strategies that can help to ease the effects of global warming by providing pedestrians with thermally comfortable conditions. We simulated the microclimate at street level of an urban block model in ENVI-met and analyzed its impact on Ta, Va, MRT, and UTCI at the current state and after introducing urban morphological techniques that have proved to enhance thermal conditions outdoors such as adding trees on the roadside, replacing dark and used concrete pavement with a light concrete pavement with a high albedo, and the creation of a small park to offset the positive impact of cool materials. We also analyzed the results based on the impact of the urban block’s building geometry represented by SVF and street orientation. Both strategies proved to have satisfactory results when treated separately. The addition of trees alone led to a reduction of Ta by 3.89 °C and MRT reduction by 1.1 °C at noon. Results of simulations based on SVF and street orientation impact on Ta, and MRT did not show a big difference in this particular urban fabric; however, SVF and street orientation proved to play an important role in the distribution of wind velocity. The best results, however, were obtained in the combination of all the mentioned strategies where the UTCI went from the state of “moderate thermal heat stress” to a state of “no thermal heat stress;” the highest MRT which corresponds to the high-angle sun between 12:00 and 13:00 went from 68 °C to 60.35 °C, especially in the area with added trees and regardless of the street orientation and SVF.