A road reconstruction project presented a great opportunity to improve stormwater management through implementation of low-impact development (LID) in the Cooksville Creek watershed of Mississauga, Ont.
The road’s right-of-way in front of the Adult Education Centre at Elm Drive—constructed within seven months and completed in June 2011—now hosts a demonstration green street project comprised of LID practices. These practices consist of permeable pavement lay-bys within the right-of-way directing excess stormwater flow into adjacent bioretention planters spanning school and city-owned property. Monitoring results show a marked improvement in water quality and volume control. Runoff volume from a two-year return storm has been reduced by 78 per cent.
Communications and partnerships remain critical to all aspects of successful LID practices, and the unique attributes of this public site with the school property being adjacent to the road right-of-way resulted in a unique partnership among CVC, the city, and the Peel District School Board (PDSB).
The facilitation of an integrated design entailed negotiation of terms, roles, and responsibilities among partners. Terms agreed on during negotiations included granting Mississauga use of PDSB property for stormwater management purposes as well as construction and maintenance of the LID features.
It is what is underneath that counts
Stormwater runoff from the roadway flows into the permeable pavement and infiltrates through aggregate layers to the perforated pipe subdrain into bioretention cells or directly into the subsoil. The permeable pavement section (i.e. lay-by and sidewalk) comprises an area of 670 m2 (7212 sf) of green street and consists of interlocking concrete pavers with 6-mm (0.25-in.) narrow spacings allowing stormwater through them. The joint spaces are filled with a crushed angular chip stone rather than the sand typical for non-permeable paver applications.
Underneath the interlocking pavers are three layers of different-sized aggregates to treat stormwater:
- 50-mm (2-in.) bedding layer of 6-mm diameter clear stone;
- 250-mm (10-in.) base layer of 19-mm (0.75-in.) clear stone; and
- 400-mm (16-in.) subbase layer of 50-mm diameter clear stone.
A drainage fabric geotextile prevents soil from migrating into the base by separating the subsoil from the subbase. Clear stone, free of fine particles, avoids clogging subsoil or geotextile fabric and allows for temporary storage of stormwater in the pavement structure. From the aggregate layers, any stormwater that does not soak into the subsoil enters the 150-mm (6-in.) diameter perforated subdrain located under the lay-by and connects to the bioretetion planters.
A series of bioretention planters
Stormwater runoff is detained and treated in a series of bioretention planters connected to each other by a perforated pipe. Runoff is piped to the planters after flowing through permeable pavement and catch basins.
The seven bioretention planters consist of varying aggregate layers, engineered soil, plants, and a perforated pipe. Each one is about 32 m2 (345 sf) and excavated to a depth of 2 m (6.5 ft). The excavation is lined with geotextile drainage fabric backfilled with 1 m (3.25 ft) of 19-mm (0.75-in.) diameter angular washed limestone also known as high-performance bedding. A 250-mm (10-in.) diameter high-density polyethylene (HDPE) peforated pipe runs the length of the series of planters within the high-performance bedding.
The retaining walls for the bioretention planters and a 150-mm (6-in.) thick course concrete sand layer sit above the high-performance bedding. This is followed by a 450-mm (18-in.) bioretention filter media mix primarily comprised of sand, some fines, and three to five per cent of organic matter. The planters are planted with species that can serve the stormwater function and also provide colour and a year-round esthetic.
Preliminary monitoring of rainfall events during 2011—ranging from 1.6 to 45.6 mm (1/16 to 1.8 in.)—showed events smaller than 13 mm (0.5 in.) have had little to no discharge to the storm sewer. This indicates runoff was either infiltrated or lost through evapotranspiration. Larger events (>13 mm) also showed a delay in the time to peak in outflows from the planters. Through its Stormwater Management Monitoring Program, CVC began monitoring at Elm Drive in September 2011.*
* For additional information on the Elm Drive green street project, visit www.creditvalleyca.ca/lid-elm-drive.
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