Green Infrastructure Design Guidance

Design Considerations

Plan green infrastructure in conjunction with regional systems, taking into account conditions such as the water table, topography, and local climate. Consider the following design criteria:

Groundwater Table. Maintain a minimum of 3 m, with 1 m under drains, from the ground surface to the top of the groundwater table for all green infrastructure.

Soil Permeability. A minimum infiltration rate of 1.5 cm/h is required for green infrastructure. If infiltration rate is less, use underground storage tanks to hold excess water.

Underdrainage. Create proper underdrainage as a combination of fine aggregate placed under drainage pipes to allow treated stormwater to leave the rain garden.

Design and Grade for Swales. Carefully design the size, longitudinal grade, and location of swales to ensure that localized flooding does not occur. The grade should be between 2% and 5%. If the grade of the swale is less than 2%, the base may become clogged. If the grade is greater than 5%, issues with erosion and damage to the vegetation are likely.

Vegetation. Use plants that are tolerant of extended dry periods and periodic inundation, typically native grasses, sedges, shrubs, and trees. The plants absorb soil nutrients, support biological growth, maintain soil porosity, and prevent surface clogging of the filter media.

Climate Considerations

High Rainfall. Locate inflow and outflow structures close to each other or design the system to feed the inlet from the back, and allow high flows to bypass the system completely. Avoid pavers with loose material as these are vulnerable to erosion. Passive irrigation can be used for all climate zones but is most effective where rainfall is regular.

It is important that high water flows do not erode the vegetation or swale surface. Ensure that the swale width is adequate for the catchment and the expected velocities of water.

Dry Climates. Ensure filter media retains soil moisture with suitable media type and depth to sustain vegetation. Using a saturated zone bioretention system is the most effective way of maintaining planting health for longer periods between rainfalls. Select droughttolerant vegetation.

Cold Climates. Apply salt, sand, or cinder to streets in moderation to reduce contamination of subsoil in snowy climates. Plowing should be done carefully, and abrasives such as sand or cinders should be avoided to preserve the integrity of the system.

Placement Considerations

Ensure adequate pedestrian access and emergency egress within the sidewalk.

Curbside Strips Within the Sidewalk. Distribute green infrastructure along the sidewalk as continuous or noncontinuous  strips, while maintaining a clear path for pedestrians. These strips can consist of various green elements such as tree pits, swales, rain gardens, and permeable paving.

Curb Extensions. Use curb extensions to place smaller areas of green infrastructure. Place rain gardens and tree pits at intersection gateways, on bus bulbs, or between on-street parking spaces.

Side or Central Median. Provide green infrastructure within the side or central medians, depending on the grading of the street and the underground conditions. Medians help manage water runoff from adjacent impermeable surfaces.

Species Selection

At the core of any green infrastructure strategy is the goal to build resilience into the system. As climate change and other environmental threats impact urban forests and green  infrastructure, their viability ultimately hinges on their durability and adaptability. Traditionally, many cities have concentrated on planting a handful of species, rendering them vulnerable to pests, disease, and extreme weather. Species selection and increased diversity is central to creating resilience.

Species selection should ensure tolerance for today’s climate and resilience for future change. Consider the following criteria when planting within an urban context:

• Drought tolerance
• Compaction tolerance
• Heat tolerance
• Wind tolerance
• Longevity
• Pollution tolerance
• Pest and disease susceptibility
• Potential as an allergen
• Sun and shade tolerance
• Predicted maintenance
• Mosquito breeding