Louise Arbour and Northbrae Public School (London) – Swale and Rain Garden

A swale was installed on a school yard next to the parking lot between Northbrae Public School and Louise Arbour French Immersion Public School as a Stormwater Management Low Impact Development demonstration site in London, Ontario. The UTRCA received funding from the Great Lakes Commission’s Great Lakes Emerging Champions Mini-Grant program and Fred A. and Barbara M. Erb Foundation with design time contributed by MTE Consultants for LID installations in London, Ontario.

The LID feature was installed to slow down the runoff from the impervious parking lot and filter pollutants before the runoff enters the municipal stormwater sewer system, and eventually flows into the Thames River, which is approximately 500 metres away. The swale was designed to collect runoff from approximately 25 mm of rainfall from the impervious parking lot (an area of approximately 2800 m2) and convey it towards an existing ditch inlet catch basin. Check dams in the swale slow runoff, and grass creates resistance to clean the runoff before it reaches the rain garden around the existing catch basin. The runoff flows into the existing system through the catch basin.

Immediately following the student rain garden planting events, thunderstorms occurred in the area. Over 30 mm of rain fell within a short duration during the first event and another, similar event occurred days later.

The LID was not designed for a peak flow of this size in such a short time period. Due to the existing orifice tube in the ditch inlet catch basin, the flow caused ponding in the swale and the adjacent parking lot for a short period of time on these two occasions. Because of the intensity of the rainfall and the ponding in the swale, many of the woodchips began to float and ended up in the receiving sewer system.

While the LID improved issues of parking lot flooding, the swale did not completely eliminate the problem as it was unable to handle such extreme events. Therefore, the UTRCA is currently working with a consultant to review and investigate any LID ideas for additional stormwater runoff storage to alleviate the parking lot flooding during extreme events including further plantings. A new survey and site plan have been created and design work is underway. The UTRCA has consulted with the Credit Valley Conservation Authority and continues to share this information.

The intention, once this new plan is prepared, is to implement any suggestions and continue with the plan to place interpretive signage at the site. Teachers from both Northbrae and Louise Arbour schools are already using the swale to illustrate stormwater management green infrastructure. The UTRCA will evaluate the LID’s performance by monitoring water depth, flows and quality during and after storm events.