Case study: Assessing the performance of bioretention basins in an urban drainage system

Traditional urban rainwater management methods send runoff from impervious surfaces directly to a system of pipes and out to rivers, without treatment. Their capacity design utilized historic weather data. However, with increase in development and a projected increase in precipitation, many systems are seeing this capacity breached. Therefore, a new thinking around rainwater management in the urban setting has immerged focusing on nature’s method of infiltration. Best Management Practices (BMPs) are micro-solutions that control rainwater at the source, prior to it going directly to the sewage system. This research project involves the study of one type of BMP solution: bio-retention basins. These basins are part of a pilot project developed on a busy boulevard in the city of Montreal. It is important to understand the actual performance of these solutions to validate their design. Components studied in this research project include the reduction of the quantity of runoff drained to the sewage systems as well as the improvement in the quality of water passing through the basins. Flowmeters installed at the entrance and exit of the basins collect water height and velocity measurements. Sampling buckets installed in the same manholes enable testing of the water quality. The data will be available to the public in order to encourage conversation about these types of projects. Preliminary results of the study include an increase of approximately 15 minutes in runoff concentration time as well as a considerable reduction in the height of runoff towards the sewage system. In terms of water quality, the bio-retention basins decrease the presence of lead and zinc however increase the concentration of phosphorous and nitrate both found in plant matter.

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