Rivers help reduce the Urban Heat Island Effect

In a previous post, Climate Change: Implications for Cities, The Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change claims that city temperatures could rise by up to 4 degrees by 2100 with peak seasonal temperatures even higher. More hot days will exacerbate the urban heat island effect resulting in greater heat-health related problems and air pollution.  The development of green infrastructure (GI) heat management strategies including green zones, wind corridors, green roofs and water features need to be factored into urban planning.

The Urban Heat Island effect describes the relatively higher temperatures found in urban areas compared with rural surroundings, and is the result of several different factors. Building materials that absorb heat, the loss of moisture in the air due to reduced vegetation and paving over soil, as well as sources of heat, such as traffic, can all contribute to the Urban Heat Island.

Barnes wetland

Wildfowl and Wetlands Trust London Wetlands Centre.

“The role of green infrastructure in addressing the challenges of
the 21st century cannot be underestimated. It is a natural service providing
infrastructure that is often more cost effective, more
resilient and more capable of meeting social, environmental and
economic objectives than ‘grey’ infrastructure.”
—GREEN INFRASTRUCTURE : AN INTEGRATED APPROACH TO LAND USE,
LANDSCAPE INSTITUTE POSITION STATEMENT T (2013)

Researchers recorded temperature and humidity between April and August at 12 sites located at different distances from a small river running through the city of Sheffield, UK. Sites were located in areas of different ‘urban form’; either in an open square, an open street, a closed street or completely enclosed by buildings. To quantify the effect of the river on temperatures, these measurements were compared with another site, which was distant from the river but similar in all other properties except that of altitude, which was accounted for in the analysis.

The study demonstrated that the river did have a significant cooling effect, especially at higher ambient air temperatures. It led to an average reduction of 1°C during temperatures higher than 20°C. Cooling only occurred during the daytime and ranged from 0.25 to 1.82°C. The effect was also greater in May (between 1.01-1.82°C), rather than June (between 0.25-0.98°C), which was thought to be a result of higher water temperatures in the summer months.

The cooling effect did not extend beyond 30 metres from the river and was negligible at 40 metres. However, at shorter distances, the amount of cooling was significantly affected by urban form. Streets which were open to the river, combined with river banks with more vegetation, led to more effective cooling, which was sustained over a greater distance. For example, in an open street cooling was 1.2°C greater than a closed street.

The researchers conclude that rivers do have cooling effects and that future policies to uncover underground rivers could be of value in urban environments where high temperatures can have a negative effect on health and wellbeing. However, they stress that urban form surrounding the river corridor is more important than the simple presence or absence of a river and that cooling effects can be greatly enhanced by careful consideration of urban design.

CASE STUDY: CHEONGGYECHEON RIVER, SEOUL

Korea1

 CHEONGGYECHEON RIVER, SEOUL: BEFORE AND AFTER

Mayoral candidate Lee Myung-bak focused his mayoral election campaign on restoring the Cheonggyecheon River in Seoul. The river was erased by a three-lane stretch of elevated highway creating one of the most congested and polluted areas of the city. Following his election the area was completely transformed with the return of the river bringing with it 3 miles of fresh running water with open space and tree-lined pedestrian walkways. The park now attracts 64,000 visitors a day. Whilst the beauty of the park is a major draw, this landscape lowers ambient temperatures to 3°C lower than city average, land prices have soared and biodiversity increased by 639% along this new blue-green corridor. Summing up this transformation in his book “What Has Nature Ever Done for Us?”, the environmentalist Tony Juniper suggests it reflects a growing appreciation of the essential role that GI and nature play in city design.

korea2

 CHEONGGYECHEON RIVER, SEOUL: BEFORE AND AFTER

Source:

Hathway, E. A. and Sharples, S. (2012). ‘The interaction of rivers and urban form in mitigating the urban heat island effect: a UK case study.’ Building and Environment. 58: 14-22. DOI 10.1016/j.buildenv.2012.06.013.

Saraiva M.G, Ramos I.L, Vaz L, Bernardo F, Condessa B.  ‘Towards sustainability in rehabilitating urban river landscapes. Crossing ecology with social concerns’, 4th ECRR Conference on River Restoration

Armour T, Hargrave J and Luebkeman, C and  (2014)  ‘Cities Alive: Rethinking Green Infrastructure’.  Arup Foresight Report

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