The power of one
Cross-disciplinary research shows how each individual tree
contributes to urban environment’s sustainability
It is common knowledge that cutting down a large area of trees has a significant environmental impact, but many may find it surprising that cutting down just one city tree has a measurable effect on local airflow, air quality and temperature regulation.
Researchers at the University of British Columbia have developed a computational tool for simulating the lower atmospheric wind field and predicting the role of the built urban environment and tree coverage on airflow. It’s a significant advance in the field of urban systems modellingand insight into the value of trees. Findings also have the potential to inform policy decisions and improve city planning.
The cross-disciplinary team of researchers wanted to look at trees’ impact on airflow and energy consumption and to answer two main questions, says Marc Parlange, dean of the faculty of applied science at UBC and professor of civil engineering. “What happens in winter when the leaves fall off deciduous trees? And what would happen if you removed the trees entirely?”
To answer these questions, the researchers built a computer model of a Vancouver neighbourhood using data collected by Nicholas Coops, associate dean of research and innovation in forestry and Canada Research Chair in Remote Sensing. Using aircraft-mounted LiDAR – a remote sensing laser technology – Dr. Coops surveyed every building, tree and bush in Vancouver’s leafy Sunset neighbourhood.
With this extremely accurate representation of the neighbourhood, the team then used computer models to simulate the impact of various scenarios at the level of individual streets and houses.
“Historically, architects made use of energy models for a single building, and atmospheric scientists like myself would use separate, coarse weather forecasting models to study weather over entire regions,” says Andreas Christen, associate professor in the Department of Geography and the atmospheric science program. “There are limitations to both. Our approach, on the other hand, allows us to integrate the two scales, modelling weather at a level of detail not available before, and allowing us to look at the effect of mean wind and turbulence on buildings at a resolution of half a metre.”
What they learned is that each individual tree in an urban environment plays a role in improving quality of life and reducing energy costs.
“One of the most significant results emerged from looking at the impact of trees on reducing wind speed in a city,” says Marco Giometto, a post-doctoral fellow in civil engineering.
“Removing a tree can increase wind speed by a factor of two. If you imagine someone walking down the street, there is a big difference between a wind speed of 15 kilometres per hour and one of 30. This also affects the pressure loading on buildings – the increased airflow that infiltrates through small gaps and openings. It’s estimated that pressure loading is responsible for up to 30 per cent of a building’s energy consumption.”
The model also points to the differences that emerge in the winter and summer with changes in foliage. Although foliage plays an important role in moderating airflow, even trees barren of leaves in the winter contribute to more comfortable urban environments.
“Tree branches mitigate airflow,” says Dr. Parlange. “There’s no need to just plant evergreen trees to get beneficial effects – deciduous trees also reduce pressure loading on buildings throughout the year.”
The predictions generated by the computer model have been validated by comparing the scenarios against measured data. This data has been collected over the past eight years from a sophisticated climate tower in Vancouver that can measure turbulence fluctuations 20 times per second.
“Our tool affirms that advances in engineering and numerical modelling can be used in policy development and decision-making,” says Dr. Parlange. “It allows us to see the impact of both large and small changes to the treed landscape on quality of life and the energy costs associated with heating and cooling buildings.”
He adds that this type of cross-disciplinary research effort is needed to solve increasingly complex issues in urban infrastructure and planning, noting that UBC’s Master of Engineering Leadership in Urban Systems provides mid-career professionals from engineering, architecture and planning the opportunity to develop the technical and management skills to lead in this in-demand area.
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