Difference between revisions of "Permeable pavements"
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To minimize risk of groundwater contamination the following management approaches are recommended (Pitt et al., 1999; TRCA, 2009b): | 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); | + | - 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, | + | - 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. | + | - 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). | *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). |
Revision as of 20:07, 6 September 2017
Overview[edit]
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:[edit]
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.
- 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.