Permeable pavements

Overview

Permeable pavements, an alternative to traditional impervious pavement, allow stormwater to drain through them and into a stone reservoir where it is infiltrated into the underlying native soil or 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 surface stormwater BMPs. The following are some of permeable pavement types:

• Permeable interlocking concrete pavers
• Plastic or concrete grid systems
• Previous concrete; and
• Porous asphalt.

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 no infiltration or detention and filtration only practice (Figure ). Permeable paving allows for filtration, storage, or infiltration of runoff, and can reduce or eliminate surface stormwater flows compared to traditional impervious paving surfaces like concrete and asphalt.

Planning Considerations:

Common Concerns

Common concerns about permeable paving include the following:

• Risk of Groundwater Contamination: Most pollutants in urban runoff are well retained by infiltration practices and soils and therefore, have a low to moderate potential for groundwater contamination (Pitt et al., 1999). Chloride and sodium from de-icing salts applied to roads and parking areas during winter are not well attenuated in soil and can easily travel to shallow groundwater. Infiltration of deicing salt constituents is also known to increase the mobility of certain heavy metals in soil (e.g., lead, copper and cadmium), thereby raising the potential for elevated concentrations in underlying groundwater (Amrhein et al., 1992; Bauske and Goetz, 1993). However, very few studies that have sampled groundwater below infiltration facilities or roadside ditches receiving de-icing salt laden runoff have found concentrations of heavy metals that exceed drinking water standards (e.g., Howard and Beck, 1993; Granato et al., 1995).

To minimize risk of groundwater contamination the following management approaches are recommended (Pitt et al., 1999; TRCA, 2009b):

- Stormwater infiltration practices should not receive runoff from high traffic areas where large amounts of de-icing salts are applied (e.g., busy highways), nor from pollution hot spots (e.g., source areas where land uses or activities have the potential to generate highly contaminated runoff such as vehicle fuelling, servicing or demolition areas, outdoor storage or handling areas for hazardous materials and some heavy industry sites);

- Prioritize infiltration of runoff from source areas that are comparatively less contaminated such as roofs, low traffic roads and parking areas; and,

- Apply sedimentation pretreatment practices (e.g., oil and grit separators) before infiltration of road or parking area runoff.

• Risk of Soil Contamination: Available evidence from monitoring studies indicates that small distributed stormwater infiltration practices do not contaminate underlying soils, even after more than 10 years of operation (TRCA, 2008).
• Winter Operation: For cold climates, well-designed mixes can meet strength, permeability, and freeze-thaw resistance requirements. In addition, experience suggests that snow melts faster on a porous surface because of rapid drainage below the snow surface. Also, a well draining surface will reduce the occurrence of black ice or frozen puddles (Cahill Associates, 1993; Roseen, 2007). Systems installed in the Greater Toronto Area have generally not suffered from heaving or slumping (TRCA, 2008b). Permeable pavement is typically designed to drain within 48 hours. If freezing should occur before the pavement structure has drained, then the large void spaces in the open graded aggregate base creates a capillary barrier to freeze-thaw. Permeable pavers have the added benefit of having enough flexibility to handle minor heaving without being damaged. Permeable pavement can be plowed, although raising the blade height 25 mm may be helpful to avoid catching pavers or scraping the rough surface of the porous pavement. Sand should not be applied for winter traction on permeable pavement as this can quickly clog the system.

Design

• On Private Property: If permeable pavement systems are installed on private lots, property owners or managers will need to be educated on their routine maintenance needs, understand the long-term maintenance plan, and may be subject to a legally binding maintenance agreement. An incentive program such as a storm sewer user fee based on the area of impervious cover on a property that is directly connected to a storm sewer (i.e., does not first drain to a pervious area or LID practice) could be used to encourage property owners or managers to maintain existing practices.
• Clogging: Susceptibility to clogging is the main concern for permeable paving systems. The bedding layer and joint filler should consist of 2.5 mm clear stone or gravel rather than sand. Key strategies to prevent clogging are to ensure that adjacent pervious areas have adequate vegetation cover and a winter maintenance plan that does not include sanding. For concrete and asphalt designs, regular maintenance that includes vacuum-assisted street sweeping is necessary. Isolated areas of clogging can be remedied by drilling small holes in the pavement or by replacing the media between permeable pavers.
• Road Salt: Care needs to be taken when applying road salt to permeable pavement surfaces since dissolved constituents from the road salt will migrate through the bedding and into the groundwater system. A well-draining surface will reduce the occurrence of black ice or frozen puddles and requires less salt than is applied to impervious pavement (Roseen, 2007).
• Structural Stability: Adherence to design guidelines for pavement design and base courses will ensure structural stability. In most cases, the depth of aggregate material required for the stormwater storage reservoir will exceed the depth necessary for structural stability. Reinforcing grids can be installed in the bedding for applications that will be subject to very heavy loads.
• Heavy Vehicle Traffic: Permeable pavement is not typically used in locations subject to heavy loads. Some permeable pavers are designed for heavy loads and have been used in commercial port loading and storage areas.

Physical Suitability and Constraints

In general, permeable pavement systems can be used almost anywhere a traditionally paved system might have been installed. However, these systems have the same site constraints of any infiltration practice and should meet the following criteria:

• Wellhead Protection: Permeable pavement should not be used for road or parking surfaces within two (2) year time-of-travel wellhead protection areas.
• Winter Operations: 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 salts applied sparingly (i.e., not as a preventative measure).
• Site Topography: 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 (Smith, 2006). Pervious surfaces should not drain onto the pavement
• Water Table: The base of permeable pavement stone reservoir should be at least one (1) metre above the seasonally high water table or bedrock elevation.
• Soils: Systems located in low permeability soils with an infiltration rate of less than 15 mm/hr (i.e., hydraulic conductivity of less than 1x10 -6 cm/s), require incorporation of a perforated pipe underdrain. Native soil infiltration rate at the proposed location and depth should be confirmed through measurement of hydraulic conductivity under field saturated conditions using methods described in Appendix C.
• Drainage Area and Runoff Volume: In general, the impervious area treated should not exceed 1.2 times the area of permeable pavement which receives the runoff (GVRD, 2005). The storage layer under the permeable pavement must be sized to accommodate runoff from the pavement itself and any impermeable areas draining to it.
• Pollution Hot Spot Runoff: To protect groundwater from possible contamination, source areas where land uses or human activities have the potential to generate highly contaminated runoff (e.g., vehicle fueling, servicing and demolition areas, outdoor storage and handling areas for hazardous materials and some heavy industry sites) should not be treated by permeable pavement.
• Setbacks from Buildings: 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) metres down-gradient from building foundations is recommended.
• Proximity to Underground Utilities: 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.

Design

Aplications

Permeable pavements are designed to provide treatment for the rain that falls directly onto their surface, but can also be designed to receive runoff from adjacent conventional paving and building roof downspouts. They are particularly useful in high density areas with limited space for other stormwater BMPs. Treatment of runoff from pervious areas is discouraged due to clogging potential. Permeable pavement may be applied on residential lots, school grounds, parks, shopping centres, and around commercial, institutional or municipal buildings (Figure 4.7.2). Permeable pavement practices should not be applied in pollution hot spots such as vehicle fuelling, service or demolition areas, outdoor storage and handling areas for hazardous materials and some heavy industry sites.

Design Guidance

Geometry and Site Layout

Permeable pavement systems can be used for entire parking lot areas or driveways or can be designed to receive runoff from adjacent impervious paved surfaces. For example, the parking spaces of a parking lot can be permeable pavers while the drive lanes are impervious asphalt or vice versa depending on the drainage pattern. In general, the impervious area should not exceed 1.2 times the area of the permeable pavement which receives the runoff. A hybrid permeable pavement/soakaway design can feature connection of a roof downspout directly to the stone reservoir of the permeable pavement system, which is sized to store runoff from both the pavement surface and the roof drainage area.

Pretreatment

In most permeable pavement designs, the surface acts as pretreatment to the stone reservoir below. Periodic vacuum sweeping and preventative measures like not storing snow or other materials on the pavement are critical to prevent clogging (see Maintenance Section). Another pretreatment element is a pea gravel choking layer above the coarse gravel storage reservoir.

Conveyance and Overflow

All pavement designs require an overflow outlet connected to a storm sewer with capacity to convey larger storms. One option is to set storm drain inlets slightly above the surface elevation of the pavement, which allows for temporary shallow ponding above the surface. If the surface is overloaded or clogged, then flows that are too large to be treated by the system can be bypassed through the storm drain inlets. Another design option intended as a backup water removal mechanism is an overflow edge (Figure 4.7.5). An overflow edge is a gravel trench along the downgradient edge of the pavement surface that drains to the stone reservoir below. If the pavement surface is overloaded or clogs, stormwater will flow over the surface and into the overflow edge and underlying stone reservoir, where infiltration and treatment can still occur. On smaller sites, overflow can simply sheet flow onto the traditional paving and drain into the storm sewer system.

    Pavements designed for full infiltration, where native soil infiltration rate is 15 mm/hr or greater, do not require incorporation of a perforated pipe underdrain. Pavements designed for partial infiltration, where native soil infiltration rate is less than 15 mm/hr (i.e., hydraulic conductivity less than 1x10-6 cm/s) should incorporate a perforated pipe underdrain placed near the top of the granular stone reservoir. Partial infiltration designs can also include a flow restrictor assembly on the underdrain to optimize infiltration with desired drawdown time between storm events (Figure ).

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

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

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.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

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.

The temporary storage of stormwater to control discharge rates, and allow for sedimentation.

The technique of removing pollutants from runoff as it infiltrates through the 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.

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.

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.

The water below the surface, and typically below the groundwater table.

Toronto and Region Conservation Authority

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

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 broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

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.

Land owned by private individuals or companies.

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.

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.

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.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

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.

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

A parameter that describes the capability of a medium to transmit water.

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

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.

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.

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.

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.

The period between the maximum water level and the minimum level (dry weather or antecedent level).