Porous for thought

During the Great Floods of Autumn 2000 politicians on all sides were quick to point the finger of blame at builders for concreting over the countryside and thereby increasing run-off speeds. Whatever the merits of this argument, it has helped to bring about a reassessment of the techniques used for creating hard standing areas and, in particular, focussed a lot of interest on porous paving systems and sustainable drainage.

The idea of porous paving is to control the way rainfall interacts with the land it falls on, to flatten out the peaks which the drainage system finds so hard to cope with. Instead of rain cascading off the surface into the overloaded drains, the water is directed down into the voids of the sub-base, which act like a reservoir. From there, the run-off slowly trickles away either into the groundwater or into surface water drains. It's a technique widely practised in Germany but little observed in the UK until the Scottish Environmental Protection Agency (SEPA) became interested in the mid-1990s and began requesting that schemes included features to manage run-off. The buzz phrase here is sustainable urban drainage schemes, SUDS for short.

Block paving has proved the most readily adaptable surface to date and a number of manufacturers have taken differing approaches. RMC produces UniEcoloc, a porous paving system based around an L-shaped block that fits together leaving a 25 mm by 25 mm hole at the centre. Filled with gravel this channels water into the sub-base.

Another alternative is Formpave, based on a patent belonging to Professor Chris Pratt of Coventry University who had been studying the subject. Professor Pratt's invention is a detention sub-base which, it is claimed, stores and cleans the water.

Formpave's Stormwater Source Control System doesn't require any holes in the pavement and has channels cut into the edges of the individual blocks.

Wavin Plastics, best known for their Osma pipes and guttering, has recently entered the market with a stormwater storage product, which can be used in conjunction with porous paving.

Manufactured from recycled polypropylene, it comprises a series of infiltration modules, looking not unlike oversized milk crates, which slot together to give the required void.

It has been used on the innovative Bryce Road, Dudley scheme built by Black Country Housing Association in conjunction with Formpave infilterable blocks.

All of the systems depend on the construction of the sub-base which acts as either a filter or tank system.

Infiltration works by allowing the water held in the sub-base to trickle down slowly through a geotextile layer into the groundwater.

Tanking replaces this geotextile layer with an impermeable membrane - in effect creating a pond - and then piping the water away to a drain or, increasingly, to a rainwater harvesting system. Infiltration is the simplest and most cost effective option but it's not always suitable.

Design standards assume that over time drainage capability will be reduced to around 10% of the newly constructed value - although evidence seems to suggest that run-off rates tend to stabilise at much higher levels after a few years.

Formpave recommends that areas of porous paving are cleaned twice a year with a mechanical suction brush and that at much longer intervals, probably not less than 25 years, the blocks will have to be lifted and relaid in a fresh laying course because of a build up in silting and possibly toxins above the top geotextile layer. Formpave designs to a worst case scenario which assumes a 90% loss of surface permeability - that's still eighteen times what is required to absorb 50mm of rain in an hour.