Innovative Stormwater Management Practices
General Descriptions
Source: CVC & TRCA Low Impact Development Stormwater Management Planning and Design Guide (2010) unless otherwise stated.
  1. Bioretention
  2. Enhanced Swales
  3. Green Roofs
  4. Perforated Pipe Systems
  5. Permeable Pavements
  6. Rainwater Harvesting
  7. Soakaways, Infiltration Trenches and Chambers
  8. Vegetated Filter Strips
  9. Other


Bioretention

Bioretention areas are excavated cells with shallow surface depressions that can be used to capture, treat and infiltrate roof, road and parking lot runoff from small drainage areas. They contain a gravel layer along the bottom and a prepared soil bed above, that filters and treats runoff and provides a growing medium for specially selected vegetation planted on the surface. During storms, runoff enters the shallow depression, filters through the prepared soil bed and root zone and is temporarily stored in the gravel layer. The filtered runoff can either infiltrate into the underlying native soil or be collected in a perforated pipe underdrain that flows to the storm sewer system. They also feature a layer of mulch, cobble and/or river-run stone on the surface. Depending on native soil infiltration rate and physical constraints, the system may be designed without an underdrain for full infiltration, with an underdrain for partial infiltration, or with an impermeable liner and underdrain for filtration only (i.e., a biofilter). They remove contaminants from runoff through filtration in the soil and uptake by plant roots and can help to reduce runoff volume through evapotranspiration and infiltration. They can also provide wildlife habitat and aesthetic benefits. Bioretention can be adapted to fit into many different development contexts and provide a convenient area for snow storage and treatment. They can also be referred to as rain gardens, biofilters and bioswales.

York University - Bioretention Cell
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Enhanced Swales
Vegetated open channels designed to convey treat and attenuate roof, road and parking lot runoff from small drainage areas. Enhanced swales are typically applied in series with other practices. Vegetation in the swale slows the water to allow sedimentation, filtration through the soil and root zone, and infiltration into the underlying native soil. Specific designs can vary but all improve upon traditional drainage ditches. Designs incorporate modified geometry and check dams that make enhanced swales both a treatment and conveyance practice. Dry swales incorporate a prepared soil bed, gravel storage layer and perforated pipe underdrain, much like a linear bioretention cell. Where development density, topography and depth to water table permit, enhanced swales are a preferred alternative to both curb and gutter and storm sewers as a stormwater conveyance system. When incorporated into a site design, they can reduce impervious cover, accent the natural landscape and provide aesthetic benefits. They can also be referred to as enhanced grass channels, vegetated swales and bioswales.
York University - Dry Swale
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Green Roofs
Green roofs, also known as “living roofs” or “rooftop gardens”, consist of a thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof. Green roofs are touted for their benefits to cities, as they improve energy efficiency, reduce urban heat island effects, and create greenspace for passive recreation or aesthetic enjoyment. They are also attractive for their water quality, water balance, and peak flow control benefits. The green roof acts like a lawn or meadow by storing rainwater in the growing medium and ponding areas. Excess rainfall enters underdrains and overflow points and is conveyed in the building drainage system. After the storm, a large portion of the stored water is evapotranspired by the plants, evaporates or slowly drains away. There are two types of green roofs: intensive and extensive. Intensive green roofs contain greater than 15 cm depth of growing medium, can be planted with deeply rooted plants and are designed to handle pedestrian traffic. Extensive green roofs consist of a thinner growing medium layer (15 cm depth or less) with herbaceous vegetative cover.
Brookside Public School - Green Roof
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Perforated Pipe Systems
Perforated pipe systems can be thought of as long infiltration trenches or linear soakaways that are designed for both conveyance and infiltration of runoff from roofs and low to medium traffic roads or parking lots with adequate pre-treatment. They are composed of stormwater conveyance pipes that are perforated along their length and installed in gently sloping gravel filled trenches lined with geotextile fabric. They allow water to infiltrate into the underlying native soil while it is being conveyed from source areas or other stormwater best management practice to an end-of-pipe facility or receiving waterbody. Perforated pipe systems can be used in place of conventional storm sewer pipes, where topography, water table depth, and runoff quality conditions are suitable. They can also be referred to as pervious pipe, clean water collector, percolation drainage or exfiltration systems.
Nepean Exfiltration System Diagram
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Permeable Pavements
Permeable pavements, an alternative to traditional impervious pavement, allow stormwater to drain through them and into a stone reservoir where it is stored, allowing it to infiltrate into the underlying native soil or be temporarily detained. They can be used for low traffic roads, parking lots, driveways, pedestrian plazas and walkways. Permeable pavement is ideal for sites with limited space for other stormwater best management practices. Examples of permeable pavement types include:

- Permeable interlocking concrete pavers (i.e., block pavers);
- Plastic or concrete grid systems (i.e., grid pavers);
- Pervious concrete; and
- Porous asphalt.

Openings in permeable interlocking concrete pavers and plastic or concrete grid pavers are typically filled with gravel, sand or topsoil and grass. Depending on the native soils and physical constraints, the system may be designed with no underdrain for full infiltration, with an underdrain for partial infiltration, or with an impermeable liner and underdrain for a detention and filtration only practice. They can also be referred to as porous pavement or pervious pavement.

Permeable Pavement at Kortright Center for Conservation
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Rainwater Harvesting
Rainwater harvesting is the process of intercepting, conveying and storing rainfall for future use. The rain that falls upon a catchment surface, such as a roof, is collected and conveyed into a storage tank. Storage tanks range in size from rain barrels for residential applications (typically 190 to 400 litres in size), to large cisterns for industrial, commercial and institutional applications. A typical pre-fabricated cistern can range from 750 to 40,000 litres in size. Cisterns intended for use year-round must be installed underground, below the local maximum frost penetration depth, or be located in a temperature controlled environment (i.e., indoors). With minimal pretreatment (e.g., gravity filtration or first-flush diversion), the captured rainwater can be used for outdoor non-potable water uses such as irrigation and pressure washing, or in the building to flush toilets or urinals. It is estimated that these applications alone can reduce household municipal water consumption by up to 55%. The capture and use of rainwater can, in turn, significantly reduce stormwater runoff volume and pollutant load. By providing a reliable and renewable source of water to end users, rainwater harvesting systems can also help reduce demand on municipal treated water supplies. This helps to delay expansion of treatment and distribution systems, conserve energy used for pumping and treating water and lower consumer water bills.
Rainwater Collection System
(Image source: U. of Guelph)
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Soakaways, Infiltration Trenches and Chambers

Soakaways, infiltration trenches and chambers are subsurface (i.e., underground) stormwater best management practices designed to filter, store and infiltrate runoff. Soakaways are rectangular or circular excavations lined with geotextile fabric and filled with gravel or other void forming material. They typically service individual lots and receive only roof and walkway runoff but can also be designed to receive overflows from rainwater harvesting systems. Soakaways can also be referred to as infiltration galleries, dry wells or soakaway pits. Infiltration trenches are rectangular trenches lined with geotextile fabric and filled with gravel or other void forming material. Like soakaways, they typically service an individual lot and receive only roof and walkway runoff. This design variation is well suited to sites where available space for infiltration is limited to narrow strips of land between buildings or properties, or along road rights-of-way. They can also be referred to as infiltration galleries or linear soakaways. Infiltration chambers are another design variation on soakaways. They include a range of proprietary manufactured modular structures installed underground, typically under parking or landscaped areas that create large void spaces for temporary storage of stormwater, allowing it to infiltrate into the underlying native soil. Structures typically have open bottoms, perforated side walls and optional underlying gravel layers for additional storage. They can be installed individually or in series in trench or bed configurations. They can infiltrate roof, walkway, parking lot and road runoff with adequate pre-treatment. Due to the large volume of underground void space they create in comparison to a soakaway of the same dimensions, and the modular nature of their design, they are well suited to sites where available space for other stormwater best management practices is limited, or where it is desirable for the facility to have little or no surface footprint (e.g., high density development contexts). They can also be referred to as infiltration tanks.

Parking Lot Infiltration Chamber
(Image source: Schollen and Company)
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Vegetated Filter Strips
Vegetated filter strips are gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They slow runoff velocity and filter out suspended sediment and associated pollutants, and provide some infiltration into underlying soils. Originally conceived as an agricultural runoff treatment practice, filter strips have evolved into an urban stormwater best management practice. Vegetation may be comprised of a variety of trees, shrubs and native plants to provide wildlife habitat and aesthetic value as well as water quality benefits. With proper design and maintenance, filter strips can provide relatively high pollutant removal benefits. Maintaining sheet flow into the filter strip through the use of a level spreading device (e.g., pea gravel diaphragm) is essential. Vegetated filter strips are typically used to pre-treat runoff being directed into another stormwater best management practice. They also provide a convenient area for snow storage and treatment during winter. They are also referred to as buffer strips and grassed filter strips.
Vegetated Filter Strip Design
(Image source: Landmark Design Group)
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Other

Other innovative stormwater management practices may include underground detention tanks, infiltration basins, or other practices that supplement conventional end-of-pipe detention ponds and oil and grit separators that are in common use today. Underground detention tanks may be the only type of stormwater best management practice feasible in highly constrained ultra urban development contexts, where land is highly valued and not typically available for surface practices. Tanks are typically constructed of concrete or plastic and are installed below the local maximum frost penetration depth. Storm sewer flow captured by these facilities is temporarily stored, allowing sedimentation to occur. Following a storm event captured runoff may be pumped to a water pollution control plant for further treatment if it contains overflows from combined sewers, or to a receiving waterbody. Infiltration basins are large excavated depressions designed to temporarily pond and infiltrate roof, road and parking lot runoff from medium to large sized drainage areas. As an end-of-pipe stormwater infiltration practice, they are only suitable in areas with highly permeable soils. Like detention ponds, they require dedication of a significant area of land, which limits their feasibility in ultra urban development contexts.
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