SuDS and permeable paving
By Bob Bray
If we are to have successful SuDS schemes in our towns and cities, they must incorporate well-designed permeable paving.
Sustainable drainage systems, or SuDS, are coming of age. SuDS structures, once seen as stand-alone drainage features, are now being integrated into the landscape as part of everyday urban design. This evolution in thinking requires both a change in the way urban design is approached and an understanding of how SuDS are integrated into the urban fabric.
Firstly SuDS must be seen as part of the urban landscape rather than merely as a drainage system alone. Secondly there are some techniques, like permeable surfaces, and components, like flow controls, that have a special role in SuDS. The shift from conventional drainage to SuDS can be expressed as moving from a preoccupation with getting rid of water as quickly as possible to creating ‘a controlled flow of clean water’ within developments. Rainfall then becomes an opportunity to enhance city life rather than a problem that will only increase with climate change.
Nature inspires the new approach to managing rainfall as SuDS mimic nature. In nature, rainfall is collected across the whole surface of the landscape. There are natural losses of water in a temperate landscape due to evaporation, transpiration and soakage into the soil before water either flows across the surface as runoff or soaks into the ground as infiltration.
On impermeable ground, water collects in vegetated soils and depressions in the landscape. These have inspired the swales, basins, ponds and wetlands of SuDS. The porous soils and rocks, like sandstones and limestone, have in a similar way been the templates for infiltration basins and permeable pavements. In SuDS, the slow flow through these complex landscapes is mirrored using flow-control devices.
The capability for permeable paving to remove pollutants and attenuate water flow during rainfall is well known. But this principle is transformed by considering distinct storage compartments, or sub-catchments, of permeable paving using flow controls with an orifice on the outlet. This enables water storage to be deployed around a site, with demonstrable flow rates for the SuDS design approval process.
The following schemes demonstrate how permeable paving and shallow flow controls can integrate SuDS into urban spaces and provide the ‘controlled flow of clean water’ necessary for amenity and biodiversity.
Australia Road in White City, London
The Australia Road urban renewal project began as a SuDS initiative by Hammersmith and Fulham Council to introduce permeable paving into the streetscape. Consultation with local people and a realization of the potential for the site led to an integrated design that linked two disconnected spaces and created a social arena celebrating rainfall.
Concrete-block permeable paving was used to break the existing formal road alignment and introduce a ‘piazza’ within Bridget Joyce Square. However, the scheme also connected the Randolph Beresford Early Years Centre to an adjacent play area and generated a social hub for parents and children.
The SuDS landscape celebrates roof water collection with sculptural gutters, downpipes and twisted steel halyards bringing water into planted rain gardens. Some roof water, together with car park runoff, flows along sett channels and through stainless steel letterboxes into planted basins. The concrete block permeable paving overlay replaces a tarmac road surface. It is laid on a 2-6mm grit bedding layer over a geo-composite conveyance sheet that transports water to the basins very near the surface.
Meandering through the whole space is the ‘wiggly wall’ – a reminder of a low wall used as a balance beam by residents in their youth. Finally, two ‘Controflow’ flow control chambers protect the combined sewer, allowing water to flow from the site at 1 litre/second through 20mm orifices. Thus, rainfall remains within the SuDS landscape until storms have passed and the sewer can deal with water again.
Parkside Civic Centre re-development, Bromsgrove
The Edwardian, Grade II listed Parkside School building, with surrounding landscape, has been re-developed into a civic centre and library for the town. Its location on sandy soil that is generally free draining suggested a fully infiltrating SuDS strategy, although affected by several site factors. The frontage of the school currently drains to ground, soakaways or the combined sewer. This situation was retained on the grounds of conservation issues and cost.
Proposed 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 a Controflow chamber restricting flow, 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 but 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, flags with spacers, grass surfaces and free-draining plant beds. The tarmac access road is laid on open graded crushed stone linked to the rest of the courtyard surfaces. The central grass lawn is slightly lower than its surroundings and can function 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 1 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 225mm stainless steel spout that pours water down a granite sett cascade into a wetland rill. Some water is diverted to a tank and re-circulated down the cascade by a solar pump when the sun shines. Permeable surfaces that are all fully accessible, together with a series of flow control chambers to ensure full infiltration potential, define the SuDS solution.The infiltration rate for the site, together with the storage provided within the pavement profiles, very nearly meet the 1 in 100 year return period including a 30% allowance for climate change.
Springhill Cohousing – Stroud
This community has created one of the most interesting housing developments in England in
recent times. The 50 unit/hectare development is on a sloping site near the centre of Stroud. Prospective homeowners were able to consider the SuDS proposals from the start during consultation.
Rainfall is celebrated by keeping water at or near the surface. Every time it rains, water is visible as it flows from downpipes into open rills, down a 4-metre cascade on a retaining wall or filling a raised pool outside the community house. Rainfall on the access road and car park at the top of the development is collected and cleaned in a lined, concrete block permeable pavement. A flow-control chamber releases water at ‘greenfield rate of runoff’ which, together with some un-attenuated roof water, cascades down the tile-hung retaining wall into a swale on its way to the raised community pool. This makes up the upper sub-catchment.
Runoff from the pedestrian street and adjacent roofs is collected in a rill that flows in front of the lower terrace of houses. Each household has personalized its rill by filling it with cobbles, stones or planting. Day-to-day flows are directed into storage below the street surface, with bigger volumes overflowing into a play space that is also an informal detention basin. The series of box storage features is linked by flow controls in a sequence down the street to maximize storage potential.
Designing SuDS in the urban realm
Space has to work hard in the city. Parks, gardens and recreational green space between buildings should all be multifunctional, with the management of rainfall one of the key uses for this valuable social asset. The use of ‘rain gardens’, informal detention basins and swale connectivity are familiar methods of exploiting these green spaces.
The more universal hard surfaced landscape common to the built environment has to play its part in managing rainfall sustainably too. Green and blue roofs can exploit the buildings themselves but at ground level permeable surfaces provide the most promising opportunities for SuDS in the future.
These landscape surfaces must collect rainfall at or near the surface, clean and store volumes of water and then convey it to watercourses, the storm sewer or the combined sewer at modest flow rates. The use of simple, passively functioning and easily maintained flow control structures completes the picture of features and components that enable SuDS to serve our urban landscape.