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A microclimatic and bioclimatic modelling assessment of the compact city morphology; a case study of Melbourne @ 5 million
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posted on 14.02.2017by D'Argent, Nadine Marie Josephe
The compact city morphology; Melbourne @ 5 million was released by the Victorian Government in 2002, as a planning framework to contain urban population growth and development within the Melbourne metropolitan region. Although Melbourne @ 5 million is guided by broad sustainability principles and goals, it has no recognition for the importance of the urban microclimate. This is particularly important because Melbourne @ 5 million promotes the widespread development of higher density urban morphologies that have the potential to modify the urban microclimate and induce physiological heat stress. This study aims to investigate the microclimatic and associated outdoor human thermal environment implications resulting from proposed changes in urban morphologies as highlighted in Melbourne @ 5 million. A three-dimensional microclimatic modelling tool ENVI-met, was used to evaluate the urban microclimate and outdoor human thermal environment across a range of urban configurations.
An initial validation assessment of ENVI-met was completed for two residential density developments to discern the ability of ENVI-met to simulate spatial and temporal patterns of microclimatic parameters (air temperature, relative humidity, wind speed and mean radiant temperature). This analysis identified that ENVI-met was able to adequately differentiate the relative spatial and temporal differences in microclimatic parameters across land uses, based on canyon geometry and surface and vegetative properties. A statistical analysis identified that model error and bias predominated. Limitations in the model were identified including; over/underestimation in the diurnal radiative energy budget, generic leaf area density profiles, simplified hydraulic and heat storage within the soil model and user specified boundary forcing conditions.
ENVI-met model simulations demonstrated that consolidating urban morphologies into higher density developments generated large spatial and temporal microclimatic variations dependent upon the urban canyon geometric form, vegetative cover and surface properties. Greater building mass and impervious surface cover associated with higher density developments, combined with insufficient vegetative cover and low soil moisture conditions, increased the radiative energy exchange between the urban dweller and canyon surfaces and increased mean radiant temperatures. Subsequently, throughout various periods of the day the Physiological Equivalent Temperature exceeded 50.0 ºC. During nocturnal conditions, the morphological form of higher density developments and lack of evaporative cooling simulated average magnitudes of mean radiant temperatures 4.0 ºC greater than low density developments that are characteristic of dispersed and shallower urban canyon geometries.
The need to alleviate unfavourable microclimatic conditions and moderate physiological heat stress is of major importance to support the continuous implementation of higher density developments. An assessment of urban heat island mitigation strategies (urban greening, irrigation and modifying canyon albedo) within an idealized urban canyon representative of various higher density developments revealed that urban greening coupled with irrigation was the most beneficial strategies to regulate the Physiological Equivalent Temperature by an average daytime magnitude between 2.0 to 12.4 ºC. However, the cooling effect of urban greening, irrigation and modifying canyon albedo within each urban canyon differed considerably. This emphasises that urban heat island mitigation strategies should be developed and implemented locally to cater for the heterogeneity of microclimatic conditions across urban canyons within higher density developments.
Overall, the outcomes of this study contributed to our understanding of how consolidating urban morphologies will modify the interaction between individual elements within the built environment, microclimatic processes and the human energy balance across higher density developments within the Melbourne metropolitan region. The recommendations outlined should be adopted in the implementation of Melbourne @ 5 million and could also assist in developing policies to include urban climate considerations in future planning frameworks. These recommendations could also improve Melbourne’s climate and offset the likely temperature and health impacts arising from consolidating urban morphologies and projected climate change impacts and extremes.