Going Green with Stormwater Management

After photo of the Bovaird stormwater management facility in Brampton, Ont. (Photo courtesy of GHD)

Over the past decade, municipalities across Canada have experienced pressures of population growth, aging infrastructure, degraded ecosystem health, and climate change, all while needing to foster economic growth and social equity. To meet these challenges, many are turning to green infrastructure.

Between 2014 and 2016, the Region of Peel conducted performance monitoring of its Bovaird stormwater management facility in Brampton, Ont., determining that a retrofit was needed to address a reduction in performance specific to drawdown and water quality.

Through the use of sustainable stormwater design practices, the region was able to complete the project within the urban setting footprint while still meeting key project objectives. Following the completion of the project, monitoring indicated an average peak flow reduction of 98 per cent and volume reduction of 86 per cent from the facility inlet to the outlet. A net cooling effect was also observed, with an average temperature reduction of 2.6°C from the facility inlet to outlet.

Before photo of the Bovaird stormwater management facility in Brampton, Ont. (Photo courtesy of GHD)

Although stormwater originates from precipitation in the form of rainfall, snow, and hail, it ultimately collects on hard surfaces and eventually makes its way into streams, rivers and lakes.

It will take one of two routes as it travels through stormwater management systems, traditionally comprised of grey infrastructure features such as catch basins, maintenance holes, conveyance channels, underground storm sewers, and stormwater management ponds before discharging to the natural environment through an outfall. Otherwise, stormwater discharges directly into surrounding waterbodies via surface runoff.

COLLECTING AS IT GOES

As it moves along impervious surfaces, such as streets, sidewalks, roofs, and driveways, stormwater picks up almost everything in its path. This can include dirt, garbage and other pollutants, such as oil and grease, fertilizer, and pesticides. There can also be a change in temperature as it flows.

Although stormwater management ponds, and specifically wet ponds, are designed to trap pollutants and provide temporary storage, they are also known to have a warming effect on stormwater due to their large permanent pool surface areas. The downside to this is that the warm water eventually makes its way into a downstream receiver, which may be a naturally cool or cold-water habitat for aquatic communities. Cold and cool-water species, such as Brook Trout (Salvelinus fontinalis) and Redside Dace (Clinostomus elongatus), can be vulnerable to warmer water temperatures.

To mitigate the impact of stormwater runoff, stormwater system designers usually turn to known best management practices, such as bottom-draw outlets in stormwater management ponds, cooling trenches, controlled outlets that release water at night versus during the heat of the day, or landscaping to provide shading of a wet pond or within the upstream catchment area.

PROTECTING AT-RISK SPECIES

The Bovaird stormwater management facility was originally constructed to manage an upstream drainage area of 3.74 hectares from the surrounding Bovaird Drive right-of-way. The downstream receiver, however, is Fletcher’s Creek, a known habitat for Redside Dace, which is a federally and provincially identified species at risk that prefers cool streams with good water quality. Monitoring indicated that a facility retrofit was necessary. Engineering firm GHD evaluated retrofit options based on objectives that included meeting specified flood control parameters, providing enhanced water quality with at least 80 per cent removal of total suspended solids, and providing thermal mitigation to protect the Redside Dace in Fletcher’s Creek.

The retrofit incorporates multiple design features using a treatment train approach for the stormwater before entering the facility, which now operates as a dry, infiltration facility as opposed to maintaining a wet facility design. Pre-treatment features include an Oil Grit Separator (OGS) followed by a filter unit, both of which were selected to improve the longevity of the dry pond infiltration system. The system is predicted to have a conservative lifespan of approximately 25 years.

A HYBRID APPROACH

The main basin now consists of a hybrid infiltration system, which includes two stages of treatment at the surface. The first is a hardwood mulched area located at the inlet that acts to slow the effluent down as it enters the treatment area while also promoting the further settling of sediments. The mulched area is then underlined with a layer of biofilter media — a second stage of treatment to further promote infiltration and settling of sediments.

A berm was also incorporated across the infiltration basin as a means of flow dispersion, which allows for water to accumulate within the area of the inlet where it develops hydraulic head to force the water through the biofilter media layer.

If the water within the inlet area rises above the berm, the stormwater overflows into the larger bioretention area. Below the mulched and biofilter areas is an infiltration basin that consists of a clear stone layer wrapped in a geotextile. The clear stone layer provides approximately 700 cubic metres of stormwater storage. Once stormwater makes its way through the clear stone layer, it enters the surrounding native soils to infiltrate as it naturally would. Any water that does not infiltrate is collected through perforated pipes installed within the stone layer, which acts as a subdrain that discharges the water to the downstream watercourse via an outlet control manhole. An overflow outlet was also installed along the perimeter of the facility should a need arise from larger storm events.

The storage capacity provided within the infiltration basin exceeds the required volume by five times. This was intentionally done to ensure all storms are forced through the surface filter media and then into the infiltration basin, where all incoming flows encounter the clear stone layer.

Based on the performance of the retrofit, the system received an Ontario Public Works Association (OPWA) Project of the Year Award.

Sarah Andrew is an associate and senior water resources engineer for the Ontario Integrated Water Management team at GHD. Samantha Paquette is a project manager for the Infrastructure Programming and Studies team at the Region of Peel.