Making urban landscapes more absorbent to stormwater

Presentation Overview:

Draining roof run-off to pervious landscaped areas like the lawns and garden beds around our properties is standard practice in most communities.  Yet few studies have been done to quantify the runoff reduction benefit of simple roof downspout disconnection, nor to understand how planting soil depth and organic matter content affects how absorbent urban landscapes are to stormwater. This webinar presents findings from a three-year field monitoring study comparing the run-off characteristics of simulated lawn test plots receiving roof drainage, located at the Kortright Centre in Vaughan, Ontario.  Outflow from each of the four 20 square metre test plots was measured during both natural and simulated rain events and used to calculate run-off coefficients for each simulated lawn.  Hydrologic models (SWMM5) of each roof and simulated lawn area were developed and calibrated with the field monitoring data and used to predict average annual run-off reduction performance.  Scenario modelling and analysis was performed to examine how varying planting soil depth and impervious (roof) to pervious (lawn) area ratio affects run-off reduction performance. Results are compared to recent research literature.
Based on study findings and other recent research, it is recommended that landscaping best practices and municipal urban design standards in Ontario support restoration of 20 to 30 centimetres of planting soil containing a minimum of 5% organic matter by dry weight to all landscaped areas.  A coefficient suitable for modelling the run-off characteristics of landscapes constructed with these minimum standards is provided to allow stormwater management system designers to incorporate this best practice into their calculations.

Learning Objectives:

  • Recognize how soil management best practices during landscape construction help to create healthier, more absorbent and easier to maintain lawns and gardens.
  • Summarize findings comparing run-off coefficients from lawns constructed according to standard and recommended best practices, and effects of varying planting soil depth and impervious to pervious area ratio on runoff reduction performance.
  • Identify recommended minimum standards for restoring healthy soil in municipal urban design standards and landscape construction specifications and ways incorporate the run-off characteristics of this best practice into the design of stormwater management systems.


Dean Young, Toronto and Region Conservation Authority (TRCA)

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