Structural practices summary

LID Design Guide Detail

	Available space	Site topography	Available head	Soils	Pollution hot spot runoff	Setbacks from buildings	Proximity to underground utilities	Drainage area and runoff volume	Wellhead protection	Water table	Overhead wires	Flow path length across impervious catchment	Winter operations
Roof downspout disconnection	Simple downspout disconnection requires a minimum flow path length across the pervious area (at least 5 metres) and suitable soil conditions. If the flow path length is less than 5 m and soils are hydrologic soil group (HSG) C or D, roof downspouts should be directed to another LID practice such as a rainwater harvesting system, soakaway, swale, bioretention area or perforated pipe system.	Disconnected downspouts should discharge to a gradual slope that conveys runoff away from the building. The slope should be between 1% and 5%. Grading should discourage flow from reconnecting with adjacent impervious surfaces.		If the infiltration rate of soils in the pervious area is less than 15 mm/hr, they should be tilled to a depth of 300 mm and amended with compost to achieve an organic content in the range of 8 to 15% by weight or 30 to 40 % by volume.	Downspout disconnection can be used where land uses or activities at ground-level have the potential to generate highly contaminated runoff as long as the roof runoff is kept separate from runoff from ground level impervious surfaces.			For simple downspout disconnection the roof drainage area should not be greater than 100 m2.
Soakaways, Infiltration Trenches and Chambers		Facilities cannot be located on natural slopes greater than 15 %.		Soakaways, infiltration trenches and chambers can be constructed over any soil type, but hydrologic soil group A or B soils are best for achieving water balance and channel erosion control objectives. If possible, facilities should be located in portions of the site with the highest native soil infiltration rates.	To protect groundwater from possible contamination, source areas where land uses or human activities have the potential to generate highly contaminated runoff should not be treated by soakaways, infiltration trenches or chambers.	Facilities should be setback a minimum of 4 m from building foundations.	Local utility design guidance should be consulted to define the horizontal and vertical offsets. Generally, requirements for underground utilities passing near the practice will be no different than for utilities in other pervious areas	They can be designed with an impervious drainage area to treatment facility area ratio of between 5:1 and 20:1. A maximum ratio of 10:1 is recommended for facilities receiving road or parking lot runoff	Facilities receiving road or parking lot runoff should not be located within 2 year time-of-travel wellhead protection areas	The bottom of the facility should be vertically separated by 1 m from the seasonally high water table or top of bedrock elevation
Bioretention	Designers should reserve open areas of about 10 to 20 % of the size of the contributing drainage area. These are areas that would be typically set aside for landscaping. More space is required for designs with soft and shallow side slopes than those with hard, vertical edges.	Bioretention is best applied when contributing slopes are between 1 to 5%. Ideally, the proposed treatment area will be located in a natural depression to minimize excavation	If an underdrain is used, then 1 to 1.5 m elevation difference is needed between the inflow point and the downstream storm drain invert. This is generally not a constraint due to the standard depth of storm drains. For bioretention without an underdrain, the design will only require enough elevation difference to move large event flows through the overflow or bypass without generating a backflow or flooding problem.	Bioretention can be located over any soil type, but hydrologic soil group A and B soils are best for achieving water balance benefits. Facilities should be located in portions of the site with the highest native soil infiltration rates. Where infiltration rates are less than 15 mm/hr an underdrain is required.	To protect groundwater from possible contamination, source areas where land uses or human activities have the potential to generate highly contaminated runoff should not be treated by bioretention facilities designed for full or partial infiltration. Facilities designed with an impermeable liner (i.e.stormwater planters) can be used to treat runoff from pollution hot spots	If an impermeable liner is used, no setback is needed. If not, a four 4 m setback from buildings should be applied	Designers should consult local utility design guidance for the horizontal and vertical clearances required between storm drains, ditches, and surface water bodies. It is feasible for on-site utilities to cross linear bioretention; however, this may require design of special protection for the utility. For road right-of-way applications, care should be taken to provide utility specific horizontal and vertical offsets	Bioretention cells work best for smaller drainage areas, as flow distribution over the filter bed is easier to achieve. Typical drainage areas are between 100 m2 to 0.5 Ha. The maximum recommended drainage area to one bioretention facility is approximately 0.8 Ha (Davis et al., 2009). Ideally, bioretention should be used as a source control for small drainage areas and not as an end of pipe control. Typical ratios of impervious drainage area to bioretention cell area range from 5:1 to 15:1.	Facilities receiving road or parking lot runoff should not be located within 2 year time-of-travel wellhead protection areas.	Bioretention should be separated from the seasonally high water table by a minimum of 1 m to ensure groundwater does not intersect the filter bed.	Designers should also check whether maximum future tree canopy height in the bioretention area will not interfere with existing overhead phone and power lines.
Vegetated Filter Strips	The flow path length across the vegetated filter strip should be at least 5 m to provide substantial water quality benefits (Barrett et al., 2004). Vegetated filter strips incorporated as pretreatment to another water quality best management practice may be designed with shorter flow path lengths.	Filter strips are best used to treat runoff from ground-level impervious surfaces that generate sheet flow (e.g., roads and parking areas). The recommended filter strip slope is between 1 % to 5 %		Filter strips are a suitable practice on all soil types. If soils are highly compacted, or of such low fertility that vegetation cannot become established, they should be tilled to a depth of 300 mm and amended with compost to achieve an organic content of 8 to 15 % by weight or 30 to 40 % by volume	To protect groundwater from possible contamination, source areas where land uses or human activities have the potential to generate highly contaminated runoff should not be treated by vegetated filter strips.						Filter strips should only be used where depth to the seasonally high water table is at least 1 m below the surface.	A limiting design factor is that the maximum flow path length across the impermeable surface should be less than 25 m.
Permeable pavement		The slope of the permeable pavement surface should be at least one percent and no greater than five percent. The impervious land surrounding and draining into the pavement should not exceed 20 % slope.		Systems located in low permeability soils with an infiltration rate of less than 15 mm/hr, require incorporation of a perforated pipe underdrain.	To protect groundwater from possible contamination, source areas where land uses or human activities have the potential to generate highly contaminated runoff should not be treated by permeable pavement	Permeable pavement should be located downslope from building foundations. If the pavement does not receive runoff from other surfaces, no setback is required from building foundations. Otherwise, a minimum setback of four 4 m down-gradient from building foundations is recommended.	Local utility design guidance should be consulted to define the horizontal and vertical offsets. Generally, requirements for underground utilities passing under or near permeable pavement will be no different than for utilities in other pervious areas. However, permeable pavement has a deeper base than conventional pavement which may impact shallow utilities.	In general, the impervious area treated should not exceed 1.2 times the area of permeable pavement which receives the runoff.	Permeable pavement should not be used for road or parking surfaces within 2 year time-of-travel wellhead protection areas.	The base of permeable pavement stone reservoir should be at least 1 m above the seasonally high water table or bedrock elevation.			Sand or other granular materials should not be applied as anti-skid agents during winter operation because they can quickly clog the system. Winter maintenance practices should be limited to plowing, with de-icing salt applied sparingly.
Enhanced swales	Grass swales usually consume about 5 - 15 % of their contributing drainage area. A width of at least 2 m is needed.	Site topography constrains the application of grass swales. Longitudinal slopes between 0.5 - 6% are allowable. This prevents ponding while providing residence time and preventing erosion. On slopes steeper than 3%, check dams should be used.		Grass swales can be applied on sites with any type of soils	To protect groundwater from possible contamination, source areas where land uses or human activities have the potential to generate highly contaminated runoff should not be treated by grass swales.	Enhanced grass swales should be located a minimum of 4 m from building foundations to prevent water damage.	Utilities running parallel to the grass swale should be offset from the centre line of the swale. Underground utilities below the bottom of the swale are not a problem.	The conveyance capacity should match the drainage area. Sheet flow to the grass swale is preferable. If drainage areas are greater than 2 hectares, high discharge through the swale may not allow for filtering and infiltration, and may create erosive conditions. Typical ratios of impervious drainage area to swale area range from 5:1 to 10:1.		Designers should ensure that the bottom of the swale is separated from the seasonally high water table or top of bedrock elevation by at least 1 m.
Bioswales	Dry swale footprints are approximately 5 to 15 % of their contributing drainage area. When applied to residential areas, the swale segments between driveways should be at least 5 m in length.	Dry swales should be designed with longitudinal slopes generally ranging from 0.5 to 4%, and no greater than 6 % (PDEP, 2006). On slopes steeper than 3%, check dams should be used.		Dry swales can be located over any soil type, but hydrologic soil group A and B soils are best for achieving water balance benefits. Facilities should be located in portions of the site with the highest native soil infiltration rates. Where infiltration rates are less than 15 mm/hr, an underdrain is required	To protect groundwater from possible contamination, source areas where land uses or human activities have the potential to generate highly contaminated runoff should not be treated dry swales designed for full or partial infiltration. Facilities designed with an impermeable liner can be used to treat runoff from pollution hot spots	Dry swales should be setback four 4 m from building foundations. When located within 3 metres of building foundations, an impermeable liner and perforated pipe underdrain system should be used.	Dry swales typically treat drainage areas of less than two hectares. If dry swales are used to treat larger areas, the velocity through the swale becomes too great to treat runoff or prevent erosion. Typical ratios of impervious drainage area to dry swale area range from 5:1 to 15:1.	Facilities receiving road or parking lot runoff should not be located within 2 year time-of-travel wellhead protection areas.	Designers should ensure that the bottom of the swale is separated from the seasonally high water table or top of bedrock elevation by at least 1 m to prevent groundwater contamination.
Perforated pipe systems	Perforated pipe systems should be located below shoulders of roadways, pervious boulevards or grass swales where they can be readily excavated for servicing. An adequate subsurface area outside of the 4 m setback from building foundations and suitable distance from other underground utilities must be available.	Systems cannot be located on natural slopes greater than 15 %. The gravel bed should be designed with gentle slopes between 0.5 - 1 %		Underlying native soil conditions do not constrain the use of perforated pipe systems but greatly influence their runoff reduction performance	To protect groundwater from possible contamination, source areas where land uses or human activities have the potential to generate highly contaminated runoff should not be treated by perforated pipe systems.	Facilities should be setback a minimum of four 4 m from building foundations.	Local utility design guidance should be consulted to define the horizontal and vertical offsets. Generally, requirements for underground utilities passing near the practice will be no different than for utilities in other pervious areas	Systems typically receive foundation drain water and runoff from roofs, walkways, roads and parking lots from multiple lots. They are typically designed with an impervious drainage area to treatment facility area ratio of between 5:1 to 10:1	Facilities receiving road or parking lot runoff should not be located within 2 year time-of-travel wellhead protection areas.	Designers should ensure that the bottom of the gravel bed is separated from the seasonally high water table or top of bedrock elevation by at least 1 m to prevent groundwater contamination.
Green roofs								Green roofs are designed to capture precipitation falling directly onto the roof surface. They are not designed to receive runoff diverted from other source areas.
Rainwater harvesting	Space limitations are rarely a concern with rainwater harvesting if considered during building design and site layout.	Site topography influences the placement of storage tanks and the design of the rainwater conveyance and overflow systems.	The needed head depends on intended use of the water. For residential landscaping uses, the rain barrel or cistern should be sited upgradient of the landscaping areas or on a raised stand. Gravity-fed operations may also be used for indoor residential uses, such as laundry, that do not require high water pressure. For larger-scale landscaping operations, locating a cistern on the roof or uppermost floor may be the most cost efficient way to provide water pressure.	Cisterns should be placed on or in native, rather than fill, soils. If placement on fill slopes is necessary, a geotechnical analysis is needed. Underground tanks and the pipes conveying rainwater to and from them, including overflow systems, should either be located below the local frost penetration depth (MTO, 2005), or insulated to prevent freezing during winter	Rainwater harvesting systems can be an effective stormwater BMP for roof runoff at sites where land uses or activities at ground level have the potential to generate highly contaminated runoff	Rainwater harvesting system overflow devices should be designed to avoid causing ponding or soil saturation within three 3 m of building foundations	The presence of underground utilities (e.g., water supply pipes, sanitary sewers, natural gas pipes, cable conduits, etc.), may constrain the location of underground rainwater storage tanks.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Areas where certain land uses or activities have the potential to generate highly contaminated runoff (e.g., vehicle fuelling, service or demolition areas, outdoor storage and handling areas for hazardous materials and some heavy industry sites).

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

The minimum linear distance of water flow across a surface.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

The practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

An underground water storage reservoir into which stormwater is directed and allowed to percolate into the underlying native subsoil.

An excavated area lined with geotextile filter cloth and filled with clean granular stone or other void forming material, that receives runoff and allow it to infiltrate into the native soil; can also be referred to as infiltration galleries, French drains, dry wells or soakaway pits.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

Includes the protection of soil from dislocation by water, wind or other agents.

The natural ground material characteristic of or existing by virtue of geographic origin.

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

A perforated pipe used to assist the draining of soils.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

A practice or structural measure that is usually implemented at the beginning of a drainage system or at the lot level, to reduce the volume of runoff and minimize the concentration of pollution in overland flow from private property and prevent pollutants from entering Stormwater runoff or other environmental media, as described by the Ministry of Environment.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

a gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

Any form of rain or snow.

The practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets.

Tank used to store rainwater (typically roof runoff) for later use.

Tank used to store rainwater (typically roof runoff) for later use.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.