Advantages of Permeable Pavement
Research has shown that permeable pavement can significantly reduce runoff volumes, thereby reducing the erosive power of stormwater entering creeks and inter tidal areas. This helps to protect backwater refuges, brings less sediment to spawning areas, prevents down cutting of streams and loss of bank stability.
Long term research on permeable pavers shows their effective removal of pollutants such as total suspended solids, total phosphorous, total nitrogen, chemical oxygen demand, zinc, motor oil, and copper. In the void spaces, naturally occurring micro-organisms break down hydrocarbons and metals adhere.
In areas with suitable soils, permeable pavements allow stormwater to enter the sub-soils, replicating the natural hydrological cycle by allowing for groundwater recharge and moderating the fluctuations of flows in watercourses.
Porous pavement can help lower high runoff water temperatures commonly associated with impervious surfaces. Stormwater pools on the surface of conventional pavement, where it is heat by the sun and the hot pavement surface. By rapidly infiltrating rainfall, porous pavement reduces the water’s exposure to sun and heat. Cool stream water is essential for the health of many aquatic organisms, including trout and salmon.
Using permeable pavement surfaces reduces the amount of effective impervious area (EIA), in an existing development. (EIA is the hard surface area directly connect to municipal drainage systems.) Reduction of EIA improves the performance of existing on-site cleansing, infiltration and storage facilities, which thus process less stormwater flow.
A reduction in EIA can help reduce the size of the on-site stormwater storage technique required in many municipalities, potentially freeing up land surface for other more valuable uses.
While there is little historical evidence, many concrete paver manufacturers claim their product will last 50 years or more. In comparison, asphalt parking lots last a far shorter time, especially in freeze/thaw climates. Frequent crack filling and overlaying, some restriping and at least one reconstruction, would be required within a 50-year span.
Conversely, the maintenance would be on a permeable concrete paver system is claimed, depending upon the source, to be from minimal to onerous. Maintenance consists of restriping and occasional cleaning of the aggregate within the pore area by vacuum truck. The latter needs to perform only on a case-by-case basis, depending on how the pavement is performing.
Limitations of Permeable Pavement
Permeable pavements are design to replace effective impervious areas, not to manage stormwater from other impervious surfaces on site. Use of this technique must be part of an overall on site management system for stormwater, and is not a replacement for other techniques.
Highly contaminated runoff can be generated by some land uses where pollutant concentrations exceed those typically found in stormwater. These “hot spots” include commercial nurseries, recycling facilities, fuelling stations, industrial storage, marinas, some outdoor loading facilities, public works yards, hazardous materials generators (if containers are exposed to rainfall), vehicle service and maintenance areas, and vehicle and equipment washing and steam cleaning facilities. Since porous pavement is an infiltration practice, it should not be apply at stormwater hot spots due to the potential for ground water contamination. All contaminate runoff should be prevent from entering municipal storm drain systems by using best management practices for the specific industry or activity.
Weight & Traffic Volumes
Reference sources differ on whether low or medium traffic volumes and weights are appropriate for porous pavements. For example, around truck loading docks and areas of high commercial traffic, porous pavement is sometimes cite as being inappropriate. However, given the variability of products available, the growing number of existing installations in North America and targeted research by both manufacturers and user agencies, the range of accepted applications seems to be expanding. Some concrete paver companies have developed products specifically for industrial applications. Working examples exist at fire halls, busy retail complex parking lots, and on public and private roads, including intersections in parts of North America with quite severe winter conditions.
Permeable pavements may not be appropriate when land surrounding or draining into the pavement exceeds a 20% slope, where pavement is down slope from buildings or where foundations have piped drainage at their footers. The key is to ensure that drainage from other parts of a site is intercept and dealt with separately rather than being direct onto permeable surfaces.
Cold climates may present special challenges. Road salt contains chlorides that could migrate through the porous pavement into ground water. Snow plow blades could catch block edges and damage surfaces. Infiltrating runoff may freeze below the pavement, causing frost heave, though design modifications can reduce this risk. These potential problems do not mean that porous pavement cannot be use in cold climates. Porous pavement designed to reduce frost heave has been use successfully in Norway. Furthermore, experience suggests that rapid drainage below porous surfaces increases the rate of snow melt above.
Some estimates put the cost of permeable paving at two to three times that of conventional asphalt paving. Using permeable paving, however, can reduce the cost of providing larger or more stormwater BMP’s on site and these savings should be factor into any cost analysis. In addition, the off-site environmental impact costs of not reducing on-site stormwater volumes and pollution have historically been ignored or assigned to other groups (local government parks, public works and environmental restoration budgets, fisheries losses, etc.) The City of Olympia in Washington State is studying the use of porous concrete quite closely and finding that new stormwater regulations are making it a viable alternative to stormwater ponds.
Grass pavers require supplemental watering in the first year to establish the vegetation, otherwise they may need to be re-seede. Regional climate also means that most grass applications will go dormant during the dry season. While brown vegetation is only a matter of aesthetics, it can influence public support for this type of permeable paving.
Olympia, Washington has found that porous concrete mix quality can be difficult to control, as it is sensitive to water and difficult to blend correctly. The city is still working on how, and how often, to clean porous concrete. Olympia expects to solve these problems as it gains more experience.
Performance Records & Measures for Permeable Pavement
Results of studies have shown that permeable pavement systems dramatically reduce surface runoff volumes and peak discharge. As well, standard water quality indicators are significantly reduce. Research also indicates that, as with any stormwater management technique or device, permeable paving performs well over time if properly installed and maintained.