Performance of green stormwater infrastructures (GSI)

In a context where urban infrastructures are aging, the urban population is constantly growing, and the climate is changing, cities must adapt and need access to performance indicators for the adaptation measures at their disposal. Sewer systems (for both wastewater and rainwater) are extremely vulnerable to climate change, and it is important to reduce the volume of water that enters these systems and is eventually discharged into our watercourses through better control of runoff at the source.

Green stormwater infrastructures (GSI), such as bioretention cells, filter marshes and green roofs, serve to retain stormwater runoff and contribute to the treatment of rainwater. GSI also help reduce heat islands through revegetation and water retention and increase biodiversity in urban areas. Furthermore, they provide additional secondary aesthetic and social benefits.

Quantify the performance of GSI in current and future climate (peak flow, water volume, infiltration and water quality parameters) to improve the design and implementation of future GSI.

• Take part in the characterization of the site prior to bioretention cell installation, the design phase, and the development of implementation scenarios ;

• Test different configurations and choices of plants adapted to the specific issues facing the city and to current and future climate features ;

• Collect field data on GSI performance in current climate at the infrastructure and drainage basin level (peak flow, water volume, infiltration and water quality parameters) ;

• Evaluate the performance of GSI in future climate by modelling the sector under study using field data on water quantity and quality ; Improve the design and implementation process for future GSI.

Fifty-four bioretention areas with complete infiltration were built along a residential street in Trois-Rivières, Quebec, Canada. Four plant species (Iris versicolor, Cornus sericea ‘Arctic fire,’ Sesleria autumnalis and Juncus effusus) were monitored in eight bioretention areas (BR-1 to BR-8) containing either commercial or local substrate. The plantings were completed in late July 2018 and the monitoring of the plants was done in summer 2021. Hydrological, water quality and groundwater monitoring was done in BR-4 in 2019, 2020 and 2021.

Figure 1. Description of the components of the bioretention areas and their positioning along Saint-Maurice Street.

Experimental hydrological and water quality monitoring

At the local level, bioretention areas can help reduce peak flows observed in municipal storm sewers. In this project, the bioretention areas were able to completely manage rainwater even during intense rainfall events. During field visits, no accumulation of water was observed in the bioretention areas. This better-than-expected performance, which can be attributed to the design of the bioretention areas, in particular their extra large size (25% of the drained area rather than the recommended 5-10%), to the fact that some of the runoff water did not enter the sidewalk catch basins, and to the collection of groundwater, caused some monitoring problems.

Although a general improvement in the quality of runoff water was observed at the outlet for most contaminants, the results show that there was a significant release of nitrogen, as well as significant concentrations at the outlet, mainly during the spring thaw. Metal removal performance also seems to be correlated with the season. A decrease in removal was observed for some during the spring thaw, in particular manganese.

The factors behind the release of metals are still poorly understood and future research is needed to elucidate them and establish recommendations to limit this impact. The major project on Saint-Maurice Street is one of the first bioretention projects of this scale built in a winter climate, and it nevertheless shows that these systems offer appealing prospects for rainwater management in Quebec.

Substrate, mulch and plants

Plant growth and plant health indicators were measured. Leaf nutrient and trace metal concentrations were also analyzed. These indicators are all functional traits that can potentially be used to guide the choice of plants in bioretention areas. The substrates used allowed the plants to grow well during the three years of monitoring despite drought and contamination with de-icing salts in some sections of the bioretention areas.

The conductivity of the substrate was greater in the parts near the sidewalks and streets than near the runoff water inlet. This may be due to the spraying of salt during road maintenance and to the de-icing of sidewalks by the municipality and individuals. These constraints should be considered in the selection of species and their location during future projects.

Phytosanitary problems had more of an impact on the plants than runoff water or the type of substrate. This aspect must be taken into account when selecting plants. Regular monitoring of plants afterwards is desirable. Mulch must be chosen carefully to avoid an excessive supply of nutrients while maintaining the expected benefits.

The addition of shrub or tree species or a plant composition with a high leaf area index to the bioretention areas would increase the contribution of plant perspiration to volume reduction.

Lastly, it is also important to recall that, in the context of this project, the bioretention areas studied had particularities due to their large size, the low flow of runoff and the height of the groundwater at certain times of the year. The growth environment of the vegetation was therefore atypical compared to bioretention areas designed according to standards. This situation could hinder the application of the results to other bioretention areas.

To achieve their goals, the catch basins delivering water to the bioretention areas must be designed to fully collect runoff from the pavement.

Additional research on a larger number of species, including shrub and tree species, is needed to confirm the indicators (functional traits) on which to base plant selection. Meticulous plant observations every month during the growing season are recommended to quickly detect diseases and insects and to intervene accordingly. Longer-term monitoring should be considered to assess the performance and maintenance of mature systems. The recommendations arising from this pilot project could be disseminated by means of a summary page.

• INRS-ETE

• Université Laval

• Université de Melbourne, Australie