Porous Pavement
Porous pavement is a permeable paving surface overlying a stone reservoir that captures and temporarily stores stormwater runoff before it infiltrates into the underlying soil. By replacing conventional impervious surfaces like asphalt or concrete, this practice allows rainfall and runoff to pass directly through the pavement structure, reducing surface runoff volume and treating pollutants. Common types include porous asphalt, pervious concrete, and permeable interlocking concrete pavers (PICP). Porous asphalt and concrete are manufactured without fine aggregates, creating interconnected void spaces that allow water to pass through. Paver systems consist of solid blocks separated by joints filled with permeable aggregate.
The primary application for porous pavement is in low-traffic areas such as parking lots, access roads, residential driveways, and sidewalks. It functions as a source control, managing precipitation where it falls and promoting groundwater recharge. The system is designed to manage runoff from the paved surface itself, but in some configurations, the underlying stone reservoir can be oversized to accept and infiltrate runoff from adjacent impervious areas, such as rooftops. While highly effective when properly sited and maintained, the practice has a history of high failure rates primarily due to surface clogging, making site selection and long-term maintenance critical for success.
Applicability
Porous pavement is a versatile tool but has specific site constraints that must be met to ensure long-term performance. Its suitability depends on soil characteristics, groundwater conditions, climate, and land use. For assistance in determining if this practice is appropriate for a given site, designers can consult a comprehensive BMP selector tool.
Drainage Area and Land Use
Porous pavement is best suited for low-traffic and low-speed areas. High traffic volumes can compact the pavement structure and increase the delivery of fine sediments and pollutants that lead to clogging. Ideal applications include overflow parking areas, employee parking lots, residential driveways, fire lanes, and pedestrian plazas. The contributing drainage area to an individual porous pavement system should generally not exceed five acres. If runoff from adjacent areas is directed to the system, it must first pass through a pretreatment practice to remove coarse sediment.
Soils and Topography
As an infiltration practice, porous pavement relies on the permeability of the underlying native soils. Soils should have a minimum infiltration rate of 0.52 inches per hour, as confirmed by on-site geotechnical testing. The soil should have a clay content below 20% and a combined silt and clay content below 40%. Porous pavement should not be installed on slopes greater than 6% to prevent surface erosion and ensure even distribution of water. It is also unsuitable for installation on fill soils unless they are properly amended and certified by a geotechnical engineer.
Groundwater and Setbacks
To protect groundwater quality and ensure proper drainage, the bottom of the stone reservoir must be at least four feet above the seasonally high water table and any bedrock layer. A horizontal separation distance of at least 100 feet is required between the porous pavement system and any drinking water supply wells. To prevent moisture issues, systems should be set back at least 25 feet down-gradient from building foundations.
Regional and Special Conditions
Porous pavement can be used in most climates, but cold regions present unique challenges. The use of sand for winter traction is prohibited, as it will quickly clog the pavement surface. If deicing salts are used, there is a potential for chloride contamination of groundwater. Snowplows may damage the surface of permeable pavers if not equipped with a raised blade. However, experience has shown that snow and ice can melt faster on porous surfaces due to rapid drainage. In cold climates, the stone reservoir base should extend below the frost line to mitigate the risk of frost heave.
In ultra-urban areas with high land values, porous pavement is an excellent option because it provides stormwater management without consuming additional space. For stormwater retrofits, it is most cost-effective when incorporated into planned parking lot resurfacing or reconstruction projects. Porous pavement is not suitable for stormwater hotspots—areas with land uses that generate highly contaminated runoff—due to the risk of direct groundwater contamination.
Design Criteria
The design of porous pavement systems must address feasibility, conveyance, pretreatment, and treatment storage to ensure functionality and longevity. These criteria are similar to those for other infiltration practices, such as an infiltration trench or an infiltration basin.
Pavement and Reservoir Sizing
The system consists of several layers: a porous surface course (2-4 inches), a fine gravel choker course (1-2 inches), and a stone aggregate reservoir. The depth of the stone reservoir is sized to store the required water quality volume (WQv), typically based on a storm event between 0.5 and 1.5 inches of rainfall. The volume of the reservoir is calculated using the void space of the aggregate, which is typically assumed to have a porosity of 0.32 to 0.40. The system must be designed to completely drain the WQv within 48 hours. The infiltration sizing calculator can assist in determining the required surface area and reservoir depth based on soil properties and design storm depth.
Conveyance and Overflow
Runoff enters the system directly through the porous surface. For larger storm events that exceed the infiltration capacity of the pavement or the storage capacity of the reservoir, a positive overflow must be provided. This is often achieved by installing storm drain inlets with grate elevations set slightly above the pavement surface, allowing for shallow ponding before overflow occurs. Alternatively, an “overflow edge”—a gravel trench along the perimeter of the pavement connected to the storm drain system—can manage excess flows.
Pretreatment
The pavement surface itself acts as the initial form of pretreatment by filtering coarse particles. However, this makes the surface highly susceptible to clogging. Therefore, minimizing sediment delivery to the pavement is the most critical design goal. The contributing drainage area must be fully stabilized with dense vegetation before the pavement is installed. If runoff from adjacent impervious areas is directed to the system, it must first pass through a dedicated pretreatment practice, such as a vegetated filter strip or a sedimentation chamber. A minimum pretreatment volume of 25% of the WQv is required for any external runoff sources.
Construction sequencing is paramount for porous pavement. The practice should be one of the last items installed on a site. The area must be protected from all construction traffic and sediment-laden runoff during the entire construction phase. Failure to do so is a leading cause of immediate or premature failure.
Geotextile and Liners
A non-woven geotextile fabric is placed along the sides of the prepared subgrade excavation to prevent soil migration into the stone reservoir. It is also placed over the top of the stone reservoir, beneath the pavement bedding course. A sand layer (approximately 6 inches deep) may be placed on the bottom of the uncompacted subgrade to improve drainage and provide a stable, flat base for the reservoir.
Pollutant Removal
Porous pavement provides effective pollutant removal through filtration, adsorption, and biological processes within the pavement structure and underlying soils. It is particularly effective at removing suspended solids and associated pollutants. While monitoring data is somewhat limited, available studies show high removal rates for many common stormwater pollutants. The data presented here should be considered in context with the broader information available in the national pollutant removal database.
| Pollutant | Pollutant Removal (%) |
|---|---|
| Total Suspended Solids (TSS) | 95 |
| Total Phosphorus (TP) | 65 |
| Total Nitrogen (TN) | 82 |
| Nitrate (NOx) | Not Available |
| Metals (Cadmium, Copper, Lead, Zinc) | 98 – 99 |
| Bacteria | Not Available |
| Source: Winer (2000). Data based on a limited number of studies. | |
Construction and Cost Considerations
The construction cost of porous pavement is typically higher than that of conventional pavement. Porous asphalt or pervious concrete can cost between $2.00 and $3.00 per square foot, compared to $0.50 to $1.00 per square foot for traditional asphalt. However, these upfront costs can be offset by significant savings from reduced or eliminated storm drain infrastructure, such as pipes, inlets, and curbs. Additionally, by providing stormwater management within the footprint of required parking, porous pavement can increase the amount of developable land on a site, providing a substantial economic benefit.
Proper construction techniques are critical. The underlying subgrade should not be compacted, and care must be taken to prevent sediment from washing into the system during construction. The stone aggregate must be clean, washed stone. Specialized contractors with experience in porous pavement installation are recommended.
Maintenance
Diligent, specialized maintenance is essential for the long-term function of porous pavement. The failure to perform routine maintenance is the primary reason for the practice’s high reported failure rate. A legally binding maintenance agreement should be established to ensure required tasks are performed. Key maintenance activities include routine inspection and cleaning of the pavement surface to prevent clogging.
| Activity | Schedule |
|---|---|
| Inspect pavement for debris, sediment, and ponding water. | Monthly |
| Ensure pavement dewaters completely between storms. | Monthly |
| Clean pavement surface with a vacuum sweeper (not a bristle or regenerative air sweeper). | 3 to 4 times per year, or as needed |
| Mow adjacent vegetated areas and remove clippings. Reseed bare spots. | As needed |
| Inspect surface for deterioration, spalling, or joint failure. | Annually |
| Post signs identifying porous pavement areas and restricting sanding and improper resurfacing. | One-time installation |
| Never seal or repave with non-porous materials. | N/A |
Limitations
The primary limitation of porous pavement is its susceptibility to clogging from fine sediment. A study in Prince George’s County, Maryland, found a failure rate as high as 75% within two years of installation, largely attributed to a lack of proper maintenance (Galli, 1992). This underscores the need for a rigorous maintenance plan and owner education.
Other limitations include:
- Not suitable for high-traffic or heavy vehicle load areas.
- Cannot be used where winter road sanding is practiced.
- Requires specific soil permeability and separation from the water table.
- Higher initial construction cost compared to conventional pavement.
- Potential for groundwater contamination if used at stormwater hotspots.
Frequently Asked Questions
What is porous pavement?
Porous pavement is a special type of paving surface that allows water to pass through it instead of running off. It is installed over a subsurface stone reservoir that temporarily holds the water before it soaks into the ground. This method helps manage stormwater at its source, reducing runoff, filtering pollutants, and recharging groundwater. Common types include porous asphalt, pervious concrete, and permeable interlocking pavers. It is typically used for low-traffic applications like parking lots, driveways, and walkways.
What is the most common reason porous pavement fails?
The most common cause of failure is surface clogging. Fine particles of sand, silt, and other debris accumulate in the pores of the pavement over time, sealing the surface and preventing water from infiltrating. This is almost always due to a lack of proper and timely maintenance, particularly routine vacuum sweeping. Construction site runoff, winter sanding, and runoff from unstabilized landscapes are major sources of sediment that can cause rapid clogging and system failure. Proper site design and a strict maintenance schedule are essential for long-term success.
Can porous pavement be used in cold climates with snow and ice?
Yes, but with important considerations. Sand cannot be used for traction as it will clog the pavement. Alternative deicing materials or limited salt application may be necessary, though this raises concerns about groundwater contamination. Snowplow blades should be fitted with shoes or a rubber edge to avoid catching on and damaging the edges of permeable pavers. On the positive side, some studies suggest that snow and ice melt faster on porous surfaces because meltwater can drain away quickly, reducing refreezing.
How is porous pavement maintained?
Maintenance is critical and primarily involves keeping the surface clean. The most important task is vacuum sweeping, which should be performed three to four times a year to remove accumulated sediment from the surface pores. Regenerative air or bristle sweepers should not be used as they can force debris deeper into the pavement. Other maintenance includes regular inspections for ponding, keeping the surrounding landscape stabilized, and immediately cleaning any spills. Signs should be posted to prevent accidental sealing or improper deicing practices.
Is porous pavement more expensive than traditional pavement?
The initial material and installation costs for porous pavement are typically higher than for conventional asphalt or concrete, often two to three times more per square foot. However, a life-cycle cost analysis may show it to be more economical. By integrating stormwater management directly into the paved surface, porous pavement can reduce or eliminate the need for expensive storm drain infrastructure like pipes, catch basins, and detention ponds. It can also maximize usable land on a development site, providing significant economic value.
What happens during a very large storm?
Porous pavement systems are designed to manage a specific volume of runoff, known as the water quality volume, which corresponds to smaller, more frequent storms. During a larger storm that exceeds the system’s storage capacity or infiltration rate, an overflow mechanism is activated. This is typically a raised storm drain inlet or an overflow channel at the edge of the pavement. These structures are set at an elevation that allows for some shallow ponding before safely conveying excess flow to the conventional storm drain system.
Can porous pavement be used for high-traffic roads?
Generally, no. Porous pavement is not recommended for high-traffic or high-speed roadways. The structural design of some porous systems may not withstand the repeated heavy loads of constant traffic, leading to degradation. More importantly, high-traffic areas generate more fine particulates from tire wear and vehicle emissions, which would accelerate surface clogging and lead to premature failure. The best applications are low-traffic areas like overflow parking, residential streets, alleys, and driveways where traffic loads and speeds are minimal.
What types of soil are suitable for porous pavement?
Porous pavement requires underlying soils that can adequately infiltrate water. Ideal soils are sandy loams or loamy sands with an infiltration rate of at least 0.52 inches per hour. Soils with high clay content (greater than 20%) or high silt/clay content (greater than 40%) are generally unsuitable. An on-site geotechnical investigation, including soil borings and infiltration tests, is required to confirm the suitability of the soil before design and construction. The system will not function correctly if the underlying soil cannot drain the stored water within the required 48-hour drawdown period.
Does porous pavement pose a risk to groundwater?
There is a potential risk if the practice is sited improperly. Because it is an infiltration system, it should never be used to treat runoff from stormwater hotspots—land uses like gas stations or vehicle maintenance areas that generate high concentrations of toxic pollutants. For other land uses, the filtration provided by the pavement layers and underlying soil effectively removes many pollutants. To further protect groundwater, design standards require a minimum vertical separation of four feet between the bottom of the stone reservoir and the seasonally high water table.
How long can porous pavement be expected to last?
The structural lifespan of porous pavement materials is similar to conventional pavement, typically 20 years or more. However, the hydraulic lifespan—its ability to infiltrate water—is entirely dependent on maintenance. With a rigorous and consistent maintenance program, including regular vacuum sweeping, a porous pavement system can function effectively for its entire structural life. Without proper maintenance, the surface can clog and fail hydraulically in as little as two to five years, effectively becoming an impervious surface.