SuDS and techniques, such as concrete block permeable paving, are essential tools in the fight against flooding and pollution – particularly with overloaded sewers, urbanisation and climate change. They are also a firm requirement around the UK and the National Planning Policy Framework (NPPF) requires SuDS on new developments of 10 or more dwellings in England.
In addition, NPPF prioritisation of SuDS in areas of flood risk and requirements that developments should not make flood risk worse elsewhere apply to developments of any scale. Localised planning policies are also appearing, spelling out what local authorities expect from sustainable drainage on the ground.
By its very nature, concrete block permeable paving is uniquely placed to help meet the multifunctional requirements for SuDS on developments. Of course, hard surfaces are necessary anyway – whether for traffic, parking, cycling, walking or play. But permeable paving also provides an inherent drainage system, addressing both flooding and pollution issues by attenuating and cleaning water run-off at source.
Concrete block permeable paving can simply infiltrate to the ground where conditions allow or, more commonly, collect water for transmission to other SuDS features along the ‘management train’ or to conventional drainage and watercourses. And, after more than two decades of use, it has proved to be a predictable, reliable and low-cost SuDS technique. Its capability to attenuate water flow during rainfall for gradual discharge is well-known.
This principle is transformed by considering distinct storage ‘sub-catchments’ of permeable paving – importantly, using simple orifice plate flow control chambers on outlets. This enables water storage to be strategically deployed around a site, with the flow controls demonstrating straightforward compliance to local authorities as part of the SuDS design approval process.
Dedicated water storage on valuable land and associated excavation and construction costs are avoided, and this technique can help satisfy SuDS requirements on high-density urban schemes without expensive storage structures. It is also useful for controlling flows within the pavement construction, maximising storage on sloping sites and increasing treatment times to optimise removal of pollutants. And it can also be applied to infiltrating systems, exemplified by the comprehensive SuDS scheme at the Parkside Civic Centre re-development in Bromsgrove.
The listed Parkside School building and surrounding landscape has been re-developed into a civic centre and library for the town. Its location on generally free-draining sandy soil suggested fully infiltrating SuDS, although affected by several site factors. Accessible and useable permeable paving and landscaping, together with a series of flow control chambers to ensure full infiltration potential, define the SuDS solution designed by Robert Bray Associates.
Parking to the north of the access road is on contaminated ground and so required a liner beneath the permeable pavement. Water is therefore collected, cleaned and stored in the pavement, with each compartment having an orifice plate flow control chamber with internal overflows in case of exceedance rainfall conditions. The flow from the car park passes down the western boundary in solid pipes next to buildings, then through perforated pipes and stone trenches where infiltration can be achieved.
The main courtyard is designed as an extensive, wall-to-wall infiltration blanket using concrete block permeable paving and flags with spacers, grass surfaces and free-draining plant beds. The tarmac access road is laid over open-graded crushed stone for water storage, linked to the rest of the courtyard surfaces.
The central grass lawn is slightly lower than its surroundings, acting as a detention basin in very heavy rain. A perforated pipe at the lowest part of the site provides a flow route, via a control chamber, to the western boundary as calculations suggest water may not soak away quickly enough beyond the one in 30-year return period. There is a final pipe link to the storm sewer that may also receive water from the library entrance.
Roof water from the western elevation is collected by downpipes and conveyed to a stainless steel spout that pours water down a granite sett cascade into a wetland rill. Some water is diverted to a tank and recirculated down the cascade by a solar pump when the sun shines. The infiltration rate for the site, together with the storage provided within the pavement profiles, almost meets the one in 100-year return period including a 30% allowance for climate change.
Of course, paving will help to define the design and character of any development. The growing choice of concrete block permeable paving products available from Interpave manufacturers – with numerous shapes, styles, finishes and colours – allows real design freedom. At the same time, they can provide completely level, well-drained, firm and slip-resistant surfaces that are accessible to all, without the need for cross-falls, channels, gulleys or other interruptions. Rainwater ‘ponding’ is eliminated, reducing the risk of ice forming on the surface and preventing splashing from standing water.