Investigation of thermal cooling potential of Permeable Paving at an urban trial site in London, UK

The built environment is being forced to adapt to rising global temperatures and severe weather events such as more intense storms, longer heatwaves etc. The proliferation of impermeable surfaces has over time led to many urban design problems, such as storm surges overwhelming sewers. Increasing urban temperatures are also caused by the built environment, the Urban Heat Island (UHI) effect. These impacts can be tackled through better infrastructure. Permeable paving offers an alternative to many impermeable surfaces, providing a robust surface with the advantage of drainage. Its ability to mitigate heat, however, remains poorly understood.

To address this, a detailed performance evaluation of two permeable paving pads, one a control and the other actively (mains supply) and passively (rainwater retention) watered, was undertaken. The 16 m² permeable paving pads were installed at Imperial College London’s White City campus (London, UK) and monitored over 4 months (July to October 2021). The pads were bounded by a raised impermeable barrier and consisted of a block layer with foundations of grit underneath. Both pads were placed on a slope enabling them to be drained, a weir prevented flooding and a tap allowed for complete drainage. The pads were instrumented with internal heat and water content sensors, as well as surface thermal sensing, and a dedicated weather station. Several artificial wetting events were conducted during the summer of 2021 alongside controlled laboratory work. A significant cooling effect was found (average of 1, and up to 5 of cooling), which was around half that computed for well-watered green space. It was found that the evaporation rate of the wetted pad was dependent on the degree of saturation, with the greatest heat loss efficiency occurring when the grit layer was partially saturated. A variety of secondary observations were also made, including issues around water fouling, and porous bricks. Whilst permeable paving can assist with flood alleviation, is it hoped, through minor design modifications, that it can also help tackle extreme urban heat impacts.