Hard drainage pavements, also known as porous pavements, are nature-based infrastructure whichprovides opportunities for increased percolation of rain and surface water through a paved surface. Theyare constructed of smaller areas of impermeable surface compared to more traditional paving, which isinterspersed with greased or areas of sand to allow water to dissipate through the surface more quickly.Hard drainage pavements can also trap suspended solids and thus filter out pollutants associated withstormwater. The central goal of hard drainage pavements is to control stormwater, reduce runoff andsurface water stagnation and improve water quality in substrate layers via additional filtration. They aresuitable for pedestrian and cycling activity, as well as car parks and other areas of standing water.

Hard drainage pavements can be constructed from a variety of materials whose main property is theadditional filtration of rainfall/stormwater from a surface area to a substrate. The most common formsof hard drainage pavement are:

• Porous concrete
• Porous asphalt
• Permeable interlocking concrete pavers 
• Polymer-based grass pavers, grids and geocells

Their construction includes the creation of a series of layers that provide opportunities for rainfall/waterto permeate through the surface to the substrate. This includes the porous top layer (porousconcrete/asphalt or interlocking pavers) which are placed over a rock/stone reservoir or filter layer,which provides space for water to leach through the different layers. Depending on the location ofintervention there may also be a fabric membrane installed as the bottom layer. The thickness of thereservoir/filter membrane varies depending on the climatic conditions, with areas of heavily rainfallrequiring a deeper layer to mitigate the additional flow and time needed to dissipate.