Green Infrastructure for Stormwater and Runoff Management

CentrEau and IWA Young Water Professionals Canada invited Quebec’s water management specialists to their first Aper’eaux of 2021. The event was held simultaneously in three cities: Montréal, Québec City and Sherbrooke.

The guest speaker in Sherbrooke was Christian Vézina, Eng., strategic and organizational leader at Avizo Experts-Conseils. Attendees at Refuge des Brasseurs had the opportunity to talk with this water management specialist, who has nearly 30 years of experience.

We invite you to listen to the recording of Mr. Vézina’s presentation (in French). The slides are available on SlideShare.

Stormwater management facilities

Tonight’s presentation will focus on stormwater and runoff management. How can we manage it at the property level? Directly on a lot, or with somewhat larger facilities for public applications, such as highways or residential developments?

With this presentation, we want to give an overview of the different types of infrastructure that exist. You will see that there are two main families of stormwater management facilities: grey infrastructure and green infrastructure.

The purpose of a stormwater management facility is to collect, convey and store stormwater and runoff to prevent sewer system overflows.

When a residential development is built, all the surfaces that allow rainwater to infiltrate into the ground are replaced by pavement, sidewalks and buildings. This greatly reduces infiltration. That water is then carried to sewer systems, which often were not designed to handle these stormwater inputs.

Tonight, I will mainly be presenting green infrastructure.

Table of contents: green infrastructure

• Grey infrastructure
• Green infrastructure
  • Infiltration trench
  • Rain garden
  • Constructed wetland
  • Vegetated swale
  • Wet pond
  • Dry pond
  • Permeable pavement
  • Green roof
• The importance of maintenance programs
• Summary table

Stormwater management in urban areas

Quebec’s Ministère de l’Environnement and municipalities require this water to be retained, either directly on site or in retention facilities that allow it to be released at a controlled rate.

These facilities ensure that the sewer system does not become surcharged. There are no overflows, and erosion in watercourses is also minimized. The real goal of stormwater management facilities is to delay the flow of water downstream.

Take an urban application: I live on the other side of the mountain, in a new development with income properties. This new development is within the urban perimeter, but to avoid large surges of water going to the City of Sherbrooke’s stormwater system, retention ponds were built. These ponds store the water and gradually release it to the sewer system.

Stormwater management at the property level

When we want to manage stormwater at the property level, or for small residential developments without a sewer system, the water is discharged to the environment: into a river, a small stream or a ditch. In that case, the goal is not only to control the quantity of water released into the environment, but also its quality. There are performance objectives to meet for suspended solids, to retain them as much as possible on the site where the water is generated.

As I mentioned, there are two main families of stormwater management facilities: grey infrastructure and green infrastructure.

Grey stormwater infrastructure

Grey infrastructure includes all facilities that use concrete and high-density polyethylene (HDPE) pipes. These are underground facilities, like the one shown here, whose purpose is simply to intercept runoff, store it and release it to the municipal sewer system at a controlled rate. That is essentially what grey infrastructure is for.

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Green stormwater infrastructure

Green infrastructure relies on vegetation, on wildlife to some extent, and mainly on bacterial activity, to achieve better performance in removing sediment and phosphorus, as well as hydrocarbons that can be washed off roadways.

Among the facilities I am presenting tonight, the infiltration trench is used mostly on private and residential properties, or on small public sites such as schools and community centres. The same goes for rain gardens, which are found mainly on residential properties. Then there are constructed wetlands and vegetated swales, which are seen more on public sites, wet ponds and dry ponds, permeable pavements and, finally, green roofs, which essentially intercept water from rooftops.

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1. Infiltration trench

The first facility is the infiltration trench. Its design is very simple: it is a trench placed at the bottom of a slope and filled with granular material. Sometimes there is also a perforated drain inside. The goal is to intercept runoff and capture it in the trench to promote infiltration.

When a perforated drain is installed, the excess that cannot infiltrate into the soil because of its limited absorption capacity is conveyed to another type of facility, such as a retention pond or a rain garden. Either way, this first facility intercepts, infiltrates and sends the excess on to another facility.

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2. Rain garden

The second facility is the rain garden. It is found mainly on residential properties, where it intercepts water from gutters. Gutter water is directed to the rain garden, which is planted with vegetation selected to withstand both fully submerged conditions and much drier conditions when there is no precipitation.

Plant selection is very important. Once again, the goal is to promote infiltration into the soil. There is also some evapotranspiration through the plants in the garden. Rain gardens are also used for small public applications, such as libraries or schools.

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3. Constructed wetland

The third facility is the constructed wetland, which has two roles: intercepting and controlling the volume of water, and treating it.

It is expected to do more extensive work in capturing suspended solids and reducing phosphorus. One example that comes to mind is on the outskirts of Lac Saint-Augustin in Québec City. Much of the water from Highway 40 and from all the development near the lake is intercepted and directed to a wetland, where sediment and phosphorus are captured.

We are currently working on a mandate with a Montréal firm to evaluate the treatment performance of this wetland. We are responsible for sampling and analysis, and the other consultant in Montréal will assess the treatment performance.

The key point to remember is that the constructed wetland has a dual role: controlling the quantity of water discharged and controlling the quality of that water.

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4. Vegetated swale

Next are vegetated swales. A vegetated swale is simply a ditch, like the ones you see in the countryside, but much wider. The larger surface area allows water to infiltrate, and the slower flow velocity allows suspended solids to settle more effectively.

We are seeing more and more of them in municipalities, for example in Boucherville, where vegetated swales were built in one area to infiltrate as much water as possible through these ditches. The excess goes to a wet pond, where further settling takes place. As you can see, these are very wide ditches.

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5. Wet pond

Then there are wet ponds, or ponds with permanent storage. This is what is in the residential development on the other side of Mont Bellevue. These are large ponds that receive all the runoff intercepted by the area’s storm sewer system.

In these two ponds, the water is kept at a certain elevation, and aquatic plants have been planted. At the outlet, there is a large manhole with an orifice plate to control the flow.

The water level in the pond fluctuates, but some water always remains inside.

The main role of a wet pond is quantity control: it reduces the amount of water released to the municipal sewer system.

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6. Dry pond

A dry pond works in a similar way, except that the water level drops completely. In some municipalities, I have even seen the bottom of the pond landscaped as a park, with benches and playground equipment.

As soon as there is heavy precipitation, the water level rises, then drops back within 24 to 48 hours after the event. Its main role is flow control and the settling of suspended solids.

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7. Permeable pavement and grass pavers

We are starting to see more and more permeable pavers. In fact, my colleague Normand went to see one last week in the Municipality of Plessisville. For one of its public buildings, part of the parking lot was paved with permeable pavers. Water running off the parking lot infiltrates through the cells, which promotes infiltration.

Sometimes there is granular material underneath. The excess is intercepted by a flow control mechanism and, once again, released to the municipal sewer system.

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8. Green roof

The last one is the green roof, which we are seeing more and more often. This one is at Université Laval, on the De Koninck pavilion, where part of the roof has been planted with vegetation.

Roof water supplies the garden built on the rooftop. Of course, you have to make sure the roof and building structure can support this type of facility, given the weight of the growing medium and the water. A structural analysis is therefore needed to confirm that this type of facility can be built on a roof.

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The importance of maintenance programs

As with any type of water management infrastructure, whether for drinking water, wastewater or stormwater, maintenance is critical. Vegetation must be removed to prevent excessive growth, and sediment that accumulates over time must be cleared out.

A minimum amount of maintenance is required. At present, since this is a fairly new discipline, there are no maintenance programs in place to properly manage the upkeep of these facilities.

Things are starting to take shape, though: some companies are beginning to offer maintenance services for these facilities to help maintain their treatment performance.

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Summary table of green stormwater infrastructure

This gives you a very simple, very general overview of green stormwater infrastructure.

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