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PI 4: Addressing urban heat

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Purpose

This indicator helps to understand the value of green infrastructure such as tree canopy, vegetation and waterways in reducing the impact of extreme heat on people’s health and improving local amenity.

Goal

The goal is to increase the contribution that the urban tree canopy and water in the landscape can make to the quality of public places, streets and open spaces which improves amenity.

Measures

  • Number of hot days (at or above 35ºC) (updated)
  • Urban heat impacts (new)
  • Proportion of permeable surface cover (new)
  • Investment in tree planting (new)

Hot days and heatwaves are a significant hazard in Greater Sydney, impacting people’s health and local amenity. Green infrastructure such as tree canopy can mitigate this by reflecting radiant heat, providing shade and cooling the microclimate through transpiration. Programs to increase urban tree canopy cover include the NSW Government’s 5 Million Trees Program and council-led tree planting programs39.

Our evidence base has expanded to include insights on permeable surfaces, recognising how retaining more water in the landscape can also help mitigate the urban heat island effect.

Further research also highlighted how Sydney Water’s investments in reinstating more natural conditions in highly modified waterways not only helps retain more water in the landscape but can also support increased tree canopy cover and a range of liveability and sustainability objectives.

Measures

Hot days and heatwaves

In 2019-20 the number of hot days (at or above 35 degrees Celsius) was above the long-term average across all districts (Figure 20). For example, Penrith (in Western City District) recorded 35 hot days, well above the long-term average of 22.7 days. Similarly, Parramatta (in Central City District) recorded 16 hot days (the long-term average is 11.3 days). In the Eastern City District, nine hot days were recorded at Observatory Hill (3.2 average). During the summer of 2019-20, there were fewer hot days in the Western City, Central City and South districts than the previous year (2018-19), and more hot days in the Eastern City and North districts (see Figure 21).

Heatwaves occur when temperatures remain high for that location for three or more consecutive days40. For example, there were six consecutive days over 35 degrees in Penrith from 26 to 31 December 2019, with a peak temperature of 46.3 degrees on 31 December 2019. By contrast, Observatory Hill in the Eastern City District did not record any heatwaves in 2019-20.

 

Figure 20: Number of days at or over 35°C (July 2009-June 2020)41

Figure 21: Number of days at or over 35°C (July 2019-June 2020)42

Urban heat

Mapping of the urban heat island effect in last year’s Pulse showed that the urban areas of the Western City, Central City and Eastern City districts are all exposed to urban heat. These are the most heavily populated districts of Greater Sydney.

A significant proportion of the South District, north of the Georges River, is also exposed to urban heat while the North District is much less exposed. Data on exposure to heat is available on the Greater Sydney Dashboard.

Permeable surfaces allow water to penetrate the soil, helping to retain more water in the landscape. They can help support the growth of trees and green ground cover, which can mitigate the urban heat island effect. Permeable paving allows water to drain and evaporate through urban surfaces, helping to mitigate the urban heat island effect.

Evaporative cooling from permeable paved surfaces may decrease the surface temperature by up to 20ºC compared to other impermeable hard surfaces43. Studies of different paving materials show while permeable hard surfaces such as brick pavers retain more heat than natural surfaces such as grass and sand, they can be up to 30 degrees cooler than artificial surfaces such as synthetic grass and recycled rubber, which are commonly used in children’s playgrounds (Figure 22).

Figure 22: Heat impacts on children's playground surfaces44

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Permeable surfaces also reduce the amount and speed of stormwater, which helps reduce the flow of pollutants entering waterways and the risks of flash flooding. In this way, permeability can be a useful indicator for measuring urban heat, the health of catchments and resilience to flood risks.

The Eastern City District has the lowest proportion of permeable surface, reflecting its longer history of dense urban development and renewal. Fifty-two per cent of the developed urban area of the District is permeable. By comparison, the developed urban areas of all other Districts have between 65 per cent and 68 per cent permeable surface area (Figure 23). Even though the Eastern City District has fewer hot days than other parts of Greater Sydney, the lower levels of permeable surface means that the urban heat island effect can be significant.

 

Figure 23: Proportion of permeable surfaces within developed urban area by district45

Land uses within each catchment can impact on water quality in waterways, as well as exposure to urban heat. There are more than 15 catchments areas (also known as hydrological catchments or drainage basins) in Greater Sydney where networks of streams flow to a common point.

Measuring the proportion of each catchment’s permeability provides a useful insight into the area’s ability to retain water in the landscape, which will help mitigate the urban heat island effect. A high proportion of permeable surface increases infiltration and reduces the volume and speed of stormwater flows and is therefore a proxy indicator of overall waterway health.

The Cooks River is Greater Sydney’s least permeable catchment, with 52 per cent of the catchment’s total area, or over 5,800 hectares, impermeable (Figure 24).

Greater Sydney catchments with more than one-third impermeable surface area include the Sydney Harbour, Parramatta River, Curl Curl Lagoon and Dee Why Lagoon catchments. This reflects the high degree of urbanisation and the lower levels of remnant native vegetation in these catchments.

Considering levels of permeability can also highlight challenges with mitigating urban heat and managing water quality and waterway health in other highly urbanised catchments.

Other catchments, such as the Pittwater and Port Hacking catchments, have a much lower proportion of impermeable surface area, reflecting the lower levels of urbanisation and the presence of large national parks in these areas.

There are many other factors, in addition to the proportion of impermeable surface, that can impact water quality and waterway health. The NSW Government has prepared a risk-based framework to assist in managing the impacts of land-use planning decisions on waterways46.

 

Figure 24: Catchments and proportion of impermeable surface area47

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Urban tree canopy cover

The proportion of urban tree canopy cover varies greatly. Data from 2016 shows that the urban area of the North District has 40 per cent urban tree canopy cover, compared to the overall average of 21 per cent of the urban area of Greater Sydney. More information is available on the Greater Sydney Dashboard.

In 2018–19, the NSW Government’s Five Million Trees Program provided over $5.3 million to 20 councils in Greater Sydney to support tree planting programs. Figure 25 shows the grants are supporting programs in areas with poor tree canopy. Blacktown in the Central City District is running an extensive $1 million program for planting trees in streets and parks and Liverpool (Western City District) is investing in urban forest, parklands and street tree planting.

 

Figure 25: Five Million Trees Grant Recipients 2018-1948

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Findings and future focus

All districts other than the North District have lower than benchmark tree canopy cover, which exacerbates urban heat. The significant difference in the average daily maximum temperature between Western City and Eastern City districts emphasises the need to prioritise an increase in tree canopy cover in the Western City District. While Eastern City District has a much smaller proportion of permeable landscape, the urban heat island effect can be significant.

Increasing tree canopy in the urban area will improve amenity and address urban heat. This will progress the evolution to a metropolis of three sustainable cities. Projections from the NSW and ACT Regional Climate Model (NARCliM) show that air temperatures in Greater Sydney are expected to increase as a result of climate change and increasing urbanisation49.

Maintaining or increasing the amount of vegetation landscape cover and permeable surfaces can help maintain more comfortable temperatures. Protecting and integrating waterways near where people live can both moderate the urban heat island effect and support urban tree canopy. It can also help support local habitat, healthy waterways, reduced flood risk and the delivery of Green Grid connections.