One of the most common and simple ways to direct stormwater into Silva Cells is to install pervious pavers above the system so that rain can drain through the pervious pavers directly into the Silva Cells. (For other ways to get water in, check out our stormwater schematics.)
One of the most common questions we are asked about using this solution is whether the pavers will become clogged over time.
A 2009 literature review by the Toronto and Region Conservation found that although clogging was a problem with some of the earliest permeable paver installations, more recent permeable pavement installations maintain much higher infiltration rates with age due to improved siting, design, and maintenance:
“…clogging has been a serious issue in some of the early permeable pavement installations (Lindsey et al, 1992). Many of the early permeable pavement installations were installed with sand as a bedding layer. Further, garden and grassed areas around the perimeter often drained onto the pavement, rather than away from it. These conditions tend to increase potential for clogging. More recent installations use washed stone in the pavement openings and bedding layer because it resists breaking down into smaller particles with age and the pore spaces are large enough to transmit fine particulate matter into the base course layers, thereby reducing potential for surface sealing (Toronto and Region Conservation 2009).”
A study that compared 8 year old experimental permeable paver plots constructed with a bedding layer of clear washed stone vs. permeable pavers over a bedding layer of clear washed stone mixed with sand found that the plots over clear washed stone bedding layer not only had significantly higher infiltration rates, they also responded much better to maintenance to restore the infiltration capacity (Toronto and Region Conservation 2009).
So, what can be done to minimize clogging?
- Site pervious pavement where it will not collect runoff from disturbed soil areas or pervious surfaces.
- Prevent sediment from entering the permeable pavement during construction.
- Use clear washed stone per manufacturer’s recommendations for the bedding layer and joint fill material; do not include sand in the bedding layer or the joint fill material.
- Regular maintenance: vacuum sweeping at least once or twice or more per year using street cleaning equipment without brooms and water spray action are the most effective to loosen and remove sediment from the openings. Frequency will depend on site use and sources of sediments to the pavement.
Putting It All In Perspective
Studies have found that permeable pavement generally has a very high infiltration rate when new. With time, the infiltration rate decreases and eventually levels off.
But how much can clogged permeable pavement infiltrate? While infiltration rates of permeable pavement can decrease by orders of magnitude if not sited right, designed right, or maintained, even “clogged” permeable pavement can still provide considerable stormwater management. A study that compared permeable interlocking concrete (PICP) sites in close proximity to sites free of loose fines, for example, found that while the sites free of loose fines had an average infiltration rate of 900 inches (2,286 cm) per hour, “clogged” sites close to loose fine particles still had an average infiltration rate of 1.6 inches (4 cm) per hour (Bean et al 2004), which is still high enough to work well with Silva Cells and provide significant stormwater treatment.
Based on many studies of long term infiltration rates, the Interlocking Concrete Pavement Institute (ICPI), recommends that “Since there are infiltration differences between initial and long term performance, construction, plus inevitable clogging, a conservative design rate of 3 in./hr (2.1 x10-5 m/sec or 210 L/sec/hectare) can be used as the basis for the design surface infiltration rate for a 20 year life” (Smith, 2006).
References for more information:
Bean, E.Z., W.F. Hunt, D.A. Bidelspach, and J.T.Smith. 2004. Study on the Surface Infiltration Rate of Permeable Pavements. Prepared for the Interlocking Concrete Pavement Institute.
Hunt, W.F. and K.A. Collins. 2007. Urban Waterways: Permeable Pavement: Research Update and Design Implications. North Carolina Cooperative Extension Service.
Smith, D.R. 2006. Permeable Interlocking Concrete Pavements: Selection, Design, Construction, Maintenance, 3rd Edition. Interlocking Concrete Pavement Institute.
Toronto and Region Conservation (TRCA). 2009. Review of the Science and Practice of Stormwater Infiltration in Cold Climates. 2009. Downloaded December 2010 from http://www.sustainabletechnologies.ca/Portals/_Rainbow/Documents/SW_Infiltration%20Review_0809.pdf