Our cities are growing and at the same time becoming more affected by heat stress due to climate change. At her PhD defence, Laura Kleerekoper of the faculty of Architecture and the Built Environment presented her research on the application of urban adaptation measures in Dutch cities aiming for a comfortable city climate and optimal water management.
She studied options to adapt to changes that are already observable at the urban level. Our general climate is getting more extreme with more dry periods and heat waves, together with more heavy rainfall and violent storms. These extreme weather events are more severe in cities where the predominant stony paved areas trap and store heat and do not allow water to infiltrate into the ground. “The health of the elderly and children are most at risk from heat stress,” said Kleerekoper.
Her research involved computer model simulations at the building and city block scales. “I varied the positions of trees and the materials to study how they interact and their effects on the temperature,” she said. Kleerekoper also applied a human thermal comfort indicator capturing not just air temperature, but also radiant temperature, air speed and water vapour pressure. She found that radiation and wind are important for local thermal effects, while air temperature influences effects at larger scales. In the Netherlands, designing using wind direction is tricky because the prevailing north easterlies can be both hot and cold and the opposing south westerlies are strongest.
Investigating different Dutch urban typologies she found historical urban centres tend to be very stony and are hard to green, while modern underground infrastructures like tunnels and car parks also tend to exclude planting trees. In garden villages she identified the key role of green areas in private gardens which should be preserved to promote water infiltration. In these neighbourhoods local authorities should plant street trees and encourage owners not to pave their gardens.
Luckily designers have tools that can influence a city’s climate such as vegetation in parks, street trees, and green roofs and façades. Plants reduce temperature by providing shading and cooling by evapotranspiration. Water bodies reduce temperatures by evaporation and acting as a buffer slowly absorbing heat. The shape, colour and materials of urban forms affect how heat is trapped or air can flow. Ventilation can be maximised using the wind to cool buildings and urban areas. During heat waves wind speeds are slow but designs can create pressure differences making hot air rise from hot spots, attracting cooler air.
Laura Kleerekoper, Thesis: Urban Climate Design. Improving thermal comfort in Dutch neighbourhoods. PhD supervisor: Dr. A. van den Dobbelsteen (Faculty of Architecture and the Built Environment). Defence date: July 7, 2016
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