Urban Trees Slow Down Stormwater Flow
Kate Thompson, Dalhousie University
2021-01-19
Water is never at rest. In the city, people rely on flows of fresh water to sustain and cleanse, and to remove waste. A gentle, steady rain replenishes our parks and gardens. But the sudden rush of water that can’t be absorbed into the ground following a hard rain – stormwater – has harmful consequences. These range from mere annoyance (wet feet) to costly damage (wet basements) and more serious outcomes (contamination and injury), when urban drainage infrastructure is overwhelmed and flooding results.
Managing stormwater flow is a major challenge for urban engineers and planners. It is costly, too –Halifax Water spends around 10.6 million dollars managing stormwater each year (Halifax Water, 2021). With climate change, and the accompanying, more intense, and more frequent storms, the challenges will expand.
Impacts on people are only part of the overall stormwater problem. Stormwater scours surfaces such as roads, efficiently removing pollutants such as petrochemicals, salt, fertilizers, heavy metals, and sediment. Pollutants are sent directly into natural water bodies, damaging them as habitats. The force of the rushing water may degrade stream banks and channels. Almost all the water that might have been absorbed into the ground to sustain natural areas and replenish groundwater becomes a costly hazard.
It’s easy to blame flooding on the weather or on inadequate infrastructure. However, the immense volume of stormwater flow in urban areas is primarily generated from ‘hardened’ or impervious surfaces – roads, roofs, parking lots, and driveways – which cover a large proportion of most cities. Urban grey infrastructure – drains, pipes, ditches, and culverts –carry off stormwater into natural waterbodies as quickly as possible, only adding to the problem.
To supplement pipes and ditches, natural features in the urban landscape have been recognized and employed for their role in slowing down stormwater. These features range from small hollows in the ground that interrupt water flow, to wetlands that capture stormwater and cleanse and release it slowly.
Higher up, the urban tree canopy quietly and elegantly aids in slowing down stormwater flow by intercepting rainwater and storing it as a film on leaves and bark. You experience this valuable service when you take shelter under a tree when the first raindrops start to fall. The water eventually drips from the leaves or needles, runs down tree trunks, or evaporates. Fascinating studies are revealing that a tree’s ability to hold water on its leaves is related to microscopic characteristics of the leaf surface (Wang et al., 2015). The overall capacity of the urban forest to intercept rain and thus slow stormwater flows varies with factors such as the tree age, leaf size and type, canopy area and density, and climate (Berland et al., 2017).
Having a background in environmental planning, I was aware of the role of trees in intercepting rainfall. But during my background reading for this article, I was surprised to learn how effective the forest canopy can be in performing this function. In Vancouver, which has abundant rainfall throughout the year, a graduate student at the University of British Columbia found that deciduous broadleaf trees intercepted, on average, about two-thirds of the summer rainfall, and almost half of winter precipitation. Evergreen needle-leaved trees performed even more impressively, intercepting four-fifths of summer rain, and almost three-quarters of winter precipitation (Asadian, 2010).
The tree canopy also contributes to slowing stormwater flows by preventing rain from hitting soil and damaging its ability to absorb water. The canopy is not the only element in the urban forest to play a role in stormwater management, however. The organic litter beneath trees also protects soil from erosion due to rushing water and slows down its flow. Tree roots loosen the soil to enhance absorption of rain. In the summer, trees help to ‘de-water’ the soil, as their roots draw water into their trunk and branches, eventually releasing it into the air through their leaves. De-watered soil is more able to absorb water from the next rainfall, preventing it from becoming stormwater.
Where trees are planted is important. Street trees have the most effect on reducing stormwater volumes and slowing down its flow. In one Chicago study, a 30-year-old ash street tree was estimated to reduce stormwater runoff by 1250 L each year (Forman, 2014). That figure might seem small, but multiply it by the number of street trees in a city and you realize that the impact is significant.
As the articles in this blog series illustrate, urban trees are exquisitely suited to performing many valuable services in the city. A tree’s unheralded ability to slow down stormwater and help us to avoid costly engineered interventions is just one more compelling reason to appreciate the trees in our urban forests and work to increase their health and extent.
Photo #1 – Rainfall in this Halifax residential neighbourhood generates relatively low amounts of stormwater because of the generous canopy of street trees.
Photo #2 – Rainfall in this industrial/commercial neighbourhood in Toronto generates huge amounts of dirty stormwater. Trees would greatly reduce the volume of that polluted water.
References
Asadian, Y. (2010). Rainfall interception in an urban environment [Master of Science thesis]. The University of British Columbia.
Berland, A., Shiflett, S. A., Shuster, W. D., Garmestani, A. S., Goddard, H. C., Herrmann, D. L., & Hopton, M. E. (2017). The role of trees in urban stormwater management. Landscape and Urban Planning, 162, 167–177. https://doi.org/10.1016/j.landurbplan.2017.02.017
Forman, R. T. T. (2014). Urban Ecology: Science of Cities. Cambridge Univ. Press.
Halifax Water (2021). Understanding Stormwater [website]. Retrieved from https://www.halifaxwater.ca/understanding-stormwater#accordion
Wang, H., Shi, H., & Wang, Y. (2015). The Wetting of Leaf Surfaces and Its Ecological Significances. In M. Aliofkhazraei (Ed.), Wetting and Wettability. InTech. https://doi.org/10.5772/61205