In North Carolina, engineers built a permeable parking lot with interlocking concrete blocks; during an 88 mm storm, it produced no measured runoff

When heavy rain strikes ordinary concrete or asphalt, it typically has nowhere to go. During a major storm, water might overflow the nearby drainage system, rush across the surface, and accumulate in drains. A different strategy was explored by engineers in North Carolina, using permeable interlocking concrete pavers that let rain pass through the surface and into the stone layers beneath. During monitored rainfall events as large as 88 millimetres, the Swansboro pavement produced no measured surface runoff, according to a 2007 study in the Journal of Irrigation and Drainage Engineering.The result comes from a larger North Carolina State University study of permeable pavement across North Carolina’s Coastal Plain. The researchers weren’t just interested in whether water disappeared; they evaluated rainfall, runoff, infiltration and water quality to see where the water went. The Swansboro site was especially notable since an 88 mm rainfall storm generated no runoff from the permeable pavement.The operation of the paver systemThe Swansboro site was a 740-square-metre public overflow parking lot rather than a traffic lane. It was made of permeable interlocking concrete pavers with a thickness of 76 millimetres over stone layers, and the system was installed directly on top of sandy soil. The North Carolina State University research describes a 740-square-metre installation with 76-millimetre permeable interlocking concrete pavers over 75 millimetres of No. 72 pea gravel and 200 millimetres of No. 57 washed gravel. The washed gravel served as the stormwater storage layer, and the system was installed directly over sandy soil without a liner.The spaces between the blocks were important. Unlike the solid, sealed asphalt, the joints allowed rain to seep into the pavement rather than just sit on the surface. The coarse stone beneath the pavers then left open room for water to pool for a time before draining into the sandy soil below. Because the Swansboro system was unlined, it was meant to allow groundwater to replenish, rather than force all the stored water into a pipe.The 88-mm rain testResearchers observed the Swansboro PICP, or permeable interlocking concrete pavement, for around 10 months. The site received over 1000 mm of rainfall during this time, including five events with rainfall depths exceeding 50 mm. The biggest event was 88 millimetres of rainfall. The 2007 study in the Journal of Irrigation and Drainage Engineering found that no runoff was generated from the Swansboro location for rainfall events up to that depth.Using the SCS Curve Number method, the researchers calculated an equivalent curve number for individual rainfall events. For the site's largest monitored event, 88 mm of rain, the calculated equivalent curve number was 37. To put it plainly, the pavement was more like an extremely absorbent surface, rather than a normal paved lot, during that occurrence. The sandy soil of the site and the enormous storage layer facilitated the result. Those circumstances are important because permeable pavement relies on the underlying conditions. The speed at which stored water moved through the system depended on the soil and drainage conditions beneath the blocks.How it contrasted to typical pavementThe North Carolina investigation didn’t just look at the Swansboro pavers. Related testing monitored four permeable pavement systems, consisting of two types of permeable interlocking concrete pavement, porous concrete and concrete grid pavers, and conventional asphalt. A hydrologic study of permeable pavement and asphalt in eastern North Carolina in 2008 showed that all permeable systems considerably reduced surface runoff volumes and peak flow rates relative to asphalt.When the flow of stormwater is slowed, it lessens the abrupt pulse of water entering the downstream drainage systems. It may also provide additional chances for sediment and pollutants to filter out before water reaches groundwater or surface-water systems. The researchers also observed discrepancies among the permeable designs. The amount of water remaining in the system and the speed at which it left were affected by the open surface area, the stone beneath the pavement and the drainage arrangement.The maintenance problemPermeable pavement has an obvious weak point: the openings that let water in can also collect sediment. A 2007 field study by North Carolina researchers examined surface infiltration rates at 40 permeable pavement sites. It found that locations affected by fine sediment had much lower infiltration rates than cleaner sites. The researchers also found that simulated maintenance could substantially improve infiltration on concrete grid pavers. The result highlights an important practical point: permeable pavement is not maintenance-free simply because rain can pass through it.When soil and fine debris clog the joints or apertures, the surface can lose some of the hydraulic capacity that made it effective. So, to keep the system working properly, regular inspection and proper cleaning are part of the process.The real meaning of the 88-mm stormThe Swansboro result is impressive in that no runoff was produced from the monitored pavement. 88 mm of rain is a significant event. But the conclusion is not generalisable to all permeable pavement sites. The parking lot consisted of sandy native soil, with a well-designed stone storage layer and reasonably effective infiltration capacity. Likewise, a pavement built on clay or a compacted subgrade may behave very differently.So the whole North Carolina research gives a more meaningful lesson than the headline alone. Permeable interlocking concrete pavers have the potential to greatly minimise runoff if the surface, stone layers, soil and maintenance techniques are suitable for the site. At Swansboro, such a combination allowed the pavement system to absorb an 88 mm rainfall event without producing measured surface runoff. The research showed that permeable concrete blocks can form part of an effective stormwater system when the pavement, storage layer, underlying soil and maintenance regime are suited to the site.You use AI every day. Now get your AI Quotient. Take the AIQ test.