City Cooling - Mitigation of the Urban Heat Island (UHI) Effect

 

Australian heatwaves are getting hotter and more frequent, with the largest impacts occurring in cities due to the urban heat island (UHI) effect.

The term UHI refers to how built up areas have a greater capacity to absorb, hold and emit the sun’s heat compared to rural areas. This absorption of energy can increase day and night time temperatures in cities by several degrees. The effect can increase summertime peak energy demands, air conditioning costs, and heat-related illness and mortality. This challenge is driving a new form of urban planning called ‘climate sensitive urban design’. This involves the creation of thermally comfortable, resilient, attractive and sustainable outdoor urban environments by enhancing positive natural and man-made features through architecture, planning and landscape design.

Thermal image taken in a heatwave show the impact of urban heat islands in Melbourne. Source: City of Melbourne

Thermal image taken in a heatwave show the impact of urban heat islands in Melbourne. Source: City of Melbourne

Thermal images taken in a January 2017 heatwave show the impact of urban heat islands in Melbourne. Source: City of Melbourne

Growing evidence is showing an effective method of delivering human thermal comfort is via introducing shade and moist soil environments (i.e. irrigated vegetation). This suggests the UHI effect could be mitigated through the establishment of healthy tree canopy supported by water sensitive urban design (WSUD) approaches. The specific connection of WSUD ensures tree canopies are large, healthy and actively release water vapour via transpiration. The CRC for Water Sensitive Cities (CRCWSC) have demonstrated that urban heat impacts are mitigated by means of passive watering practices and green infrastructure solutions (such as irrigated trees) and these practices can lower the Urban Thermal Climate Index by up to 10 °C.

This method is beginning to translate into policy for many Victorian cities with state green-blue infrastructure guidelines now available and the adoption of urban forestry strategies and targets of up to 40% tree canopy cover by 2040 committed to by progressive municipalities (such as, City of Melbourne and City of Ballarat).

Land surface temperature showing cooler areas around irrigated ovals and tree canopy on a day with a maximum temperature of 37 deg C. Source: CRC Water Sensitive Cities Land surface temperature showing cooler areas around irrigated ovals and tree canopy on a day with a maximum temperature of 37 deg C. Source: CRC Water Sensitive Cities

Research by the CRC for Water Sensitive Cities was able to show the clear physical urban cooling benefits of different levels and types of urban greening, and importantly the role of water in cooling. It showed that irrigated open spaces and trees can lower local temperatures and significantly reduce the health impacts of heat waves. The CRC created the Scenario Tool to allow users to model the impact of development on urban heat and how this could be reduced through the use of water sensitive initiatives.

E2Designlab have used the Scenario tool to model areas at all scales (a specific site up to whole of catchment or city). Using place-based data to represent current conditions, we can then compare future scenarios to quantify the impacts of development, and benefits of a range of typical water sensitive interventions. The tool provides both 3D visualisations of the scenarios plus graphical and data outputs including:

  • Land Surface Temperatures
  • Air Temperatures
  • Catchment runoff
  • Total Nitrogen Generated
  • Water Balance

E2Designlab was a core partner involved with the development and testing of the Scenario Modelling Tool and we have utilised it on a number of demonstration projects in Brisbane, Townsville, Melbourne, Adelaide, and Sydney. The tool results can be used to inform cost benefit analysis and support a business case which favour liveability and sustainable outcomes. The images below show some of the heat mapping which can be done.

Interested in cooling your city and building heatwave resilience? Follow the links below to learn more or contact E2Designlab at: [email protected]

Designing for a cool city – Guidelines for passively irrigated landscapes (CRC WSC, April 2020)

Trees for Cooler and Greener Streetscapes - Guidelines for Streetscape Planning and Design (DELWP, 2020)

Impacts of WSUD solutions on human thermal comfort – Green cities and microclimate (CRC WSC, Dec 2014

How-to guide for planning urban greening and enhanced stormwater management in Victoria (DELWP, July 2017)

Greening Ballarat, a green-blue city action plan (City of Ballarat, March 2016)