Porous pavement and rain garden in a parking area (Source:
Porous pavement allows stormwater and snow melt to pass through voids in the paved surface and infiltrate into the subbase. In open (unlined) systems, infiltration into the underlying soil may also be possible.
Porous pavements may be constructed of four (4) basic material types:
- Porous asphalt
- Porous concrete
- Interlocking paver blocks
- Plastic grid
Porous asphalt and concrete often look the same as their conventional counterparts but are mixed with a low proportion of fine aggregates, leaving void spaces that allow for infiltration. Interlocking paver blocks themselves are impervious, but gravel- or grass-filled voids in between the blocks allow stormwater to enter the subbase. Plastic grid systems provide a stable structure in which each cell in the grid contains grass or gravel.
Drainage in porous pavements may be one of three types:
- Full exfiltration
- Partial exfiltration
- No exfiltration or tanked systems
The amount of exfiltration depends on the permeability of the existing soil. Regardless of which approach is used, overflow devices are usually provided to prevent ponding. In full exfiltration systems, all stormwater is expected to exfiltrate into the underlying subsoil. Pipes at the top of the subbase provide overflow and secondary drainage in case the base becomes clogged or loses capacity over time. Partial exfiltration systems are designed so that some water exfiltrates into the underlying soil while the remainder is drained by the overflow devices. No exfiltration occurs when the subbase is lined with an impermeable membrane and water is removed at a controlled rate through the overflow device. Tanked systems are essentially underground detention systems and are used in cases where the underlying soil has low permeability and low strength, there is a high water table, or there are water quality limitations.
Stormwater Management Objectives
Potentially 70-80 percent of the annual rainfall can be returned to groundwater through the use of porous pavement if underlying soils have a permeability of between 0.5 and 3.0 inches per hour. In lined systems, stormwater will be detained in the subbase and slowly pass through the underdrains into the sewer.
As a design rule, if the subbase can provide a storage volume equal to the volume of increased runoff during a local two-year storm event (that is, the difference between the pre- and post-development runoff volumes), this will provide sufficient storage to mitigate the peak rate of runoff during larger storm events (25-year to 100-year). For small events, the peak discharge is attenuated by stormwater movement through the subbase.
Porous pavements intercept TSS and larger sediment particles in the pavement structure and the subbase; annual vacuuming is required to preserve permeability. Brattebo and Booth (2003) compared the quality of infiltrated water from two (2) paver block systems and two (2) grid systems with that of surface runoff from conventional asphalt. Copper concentrations in the infiltrated runoff were 83-89 percent lower than in the surface runoff from the conventional asphalt. Zinc concentrations were reduced 39-69 percent. Motor oil concentrations were reduced to below detection limits.
In open systems, pollutants that are not easily trapped or adsorbed, such as nitrates and chlorides, may continue to move through the soil profile and into groundwater. Further scientific data is necessary before porous pavement is constructed near drinking water supplies. Porous pavements simultaneously serve as hardscape and as stormwater infrastructure, and are therefore especially practicable where space constraints preclude the use of other water quality BMPs.
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