Designing effective stormwater management policies : the role of the urban forest and impervious cover in Vancouver, B.C. Lefrançois, Camille B.
Cities across the world deal with issues of water quantity and quality due to stormwater runoff, and many of those problems are expected to increase with climate change. To address those problems, the use of urban forest to manage stormwater quantity and quality issues is becoming more common. However, the way trees and plants contribute to stormwater management varies significantly based on the local climate and topography. In this project, I investigate the role of the urban forest for stormwater management in a small sewershed in Vancouver, BC. The area is located in Hastings-Sunrise within the single-family residential zone and contains approximately 200 homes. I use the i-Tree Hydro hydrology and water quality model to quantify the effect of surface cover changes on the sewershed. I first test the effect of changes in tree canopy in comparison with changes in impervious cover. Then, I investigate the effect over time of a business as usual case where the properties build laneway houses and street trees mature, and of a possible future where the country lanes program is implemented, as well as a downspout disconnection policy. My results indicate that changes in impervious cover have an impact eleven times higher than changes in canopy cover on surface runoff. My scenarios show that the biggest factor affecting stormwater runoff quantity and quality is directly connected impervious areas. It is illustrated with the downspout disconnection scenarios, which generate a 65% reduction in surface runoff. However, the substantial decrease also results in an increase in pervious flow (saturated soils). Increases in tree canopy have a significant impact on the reduction of pervious flow. Overall, I found that the most effective change for stormwater management policies in Vancouver would be to reduce directly connected impervious areas. Policies recommending a reduction of these surfaces should also enhance the urban forest and consider engineered green infrastructure solutions to allow for sufficient infiltration and evapotranspiration. Tools such as i-Tree Hydro require a significant time investment, but can offer key informations for planners and decision-makers to inform decisions on the contribution of the urban forest and other surface covers to stormwater management.
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