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A Permeable Future

14 October 2009

The use of concrete block permeable paving is set to grow rapidly with new draft legislation making SUDS (Sustainable Drainage Systems) mandatory on all new developments within two years. Colin Nessfield explores the implications for FMs

Over two thirds of the 57,000 homes affected by the 2007 summer floods were flooded not by swollen rivers, but by surface water runoff or overloaded drainage systems. The Government’s Foresight Report estimates that currently 80,000 properties are at very high risk from surface water flooding causing, on average, £270m of damage every year. The continuing growth in urbanisation and ambitious government driven housing programmes, combined with more extreme weather events linked to climate change, will only exacerbate the problem.
Clearly, a sustainable approach to all surface drainage is needed to deal with existing overloaded systems and to accommodate future growth. It is now well recognized that Sustainable Drainage Systems (SUDS) offers the solution, SUDS manages surface water by attenuation and filtration with the aim of replicating, as closely as possible, the natural drainage from a site before development. The three pillars of SUDS are to:
 minimise water runoff quantity
 improve water quality
 provide amenity and biodiversity.
The Draft Flood and Water Management Bill, issued for consultation on 21 April, will apply to new surface water drainage from buildings and roads in England and Wales. It takes on board many of the proposals of the Pitt Review into the 2007 flooding events and the Government’s Water Strategy for England. From 2011, SUDS will need to be included in new developments “where practicable”. In particular, the draft Bill states that: “In new developments, permeable paving, swales or French drains should take the place of traditional impermeable roads and pavements draining to sewers.”
Current planning policies, Building Regulations and the Code for Sustainable Homes (CSH) already positively encourage SUDS but their use is by no means widespread yet. The new Bill goes much further, addressing ownership, maintenance and adoption issues which have limited take-up of SUDS so far. To achieve its aims, the Bill will replace current arrangements for approval, construction and maintenance of conventional piped drainage with similar procedures applying to SUDS techniques. A new set of National Standards will be developed by 2011, in conjunction with stakeholders such as trade associations, dealing with the various SUDS techniques.
Without doubt, concrete block permeable paving (CBPP) is the most versatile of these techniques but it does differ from the others, such as swales and other soft landscaping features. In particular, detailed guidance is already in place for CBPP from Interpave, based on decades of experience both here and abroad, which should form the basis for Standards. There is therefore no reason to prevent concrete block permeable paving being used now, in place of impermeable surfaces, gulleys and pipes, to meet current planning requirements.
By 2011, an approval system will also be set up, in line with the Standards, and a new SUDS Approving Body (SAB) established. Unitary and County authorities will be required to act as SABs and it is currently proposed that they should maintain and adopt all new SUDS schemes, except single properties. The draft Bill consultation explores various options to ensure that privately managed SUDS will be maintained properly. Developers will have to demonstrate compliance with Standards and approval will be required before development commences.
As part of the new legislation, the right to connect to public sewers will be conditional on meeting the Standards. In effect, infiltration of rainwater on site will be preferred but, where ground conditions preclude this, SUDS to reduce, attenuate and treat runoff to sewers will be required.
Clearly, CBPP is set to grow substantially in popularity under the new legislation. It has the same visual characteristics as other precast concrete paving and is available in a growing range of styles, colours and textures to meet today’s design requirements. It differs with enlarged joints created by larger than conventional spacer nibs on the sides of each unit. These joints are subsequently filled with a joint filling material specific to each product, which is an angular aggregate, not sand. This arrangement ensures that water will continue to pass through the joints over the long-term to a permeable sub-base below.
CBPP reduces the peak rate, total volume and frequency of rainwater runoff and is very effective at removing water-borne pollution. It is one of the most space efficient SUDS components available and does not require any additional land-take. It can handle runoff from roof drainage, as well as adjacent impermeable surfaces. Combinations of permeable and conventional precast concrete paving will therefore become a central feature of external areas in the future. CBPP deals with surface water close to where rainfall hits the ground, and conventional kerbs and channels are not necessary. It meets current accessibility requirements and also eliminates ‘ponding’, reducing the risk of ice forming on the surface and avoiding rain splashing from standing water.
There are three different CBPP systems in use, all incorporating important attenuation and pollution removal capabilities.
 System A simply allows full infiltration to ground with good permeability
 System B includes collection of any surplus from infiltration into outlet pipes
 System C effectively forms a storage tank for collection of all the water into outlet pipes with no infiltration.
CSH and BREEAM recognise improvement of water quality and protection of any receiving waters and acknowledge the recycling of rainwater which can be collected within CBPP with resulting reductions in the amount of mains potable water used for external uses. There is also potential for use of this stored water for WCs and washing machines instead of potable water, and CSH offers up to five credits for savings here. Trials are also being carried out with a heat recovery system using stored water from a concrete block permeable pavement installed at the Building Research Establishment.
Finally, the beneficial impacts that CBPP can have on wildlife can contribute to CSH and BREEAM credits available for ecological value, for example by avoiding the lethal traps which open gulleys present to wildlife. As CBPP allows the same pattern of run-off transfer to the ground as natural vegetation, it allows water to reach tree and shrub roots, despite providing a hard surface above. In fact, some tree protection systems incorporate permeable paving.
Most SUDS techniques are ‘soft’ landscaping features, still regarded as unconventional drainage devices with particular maintenance issues and some uncertainty over long-term performance. However, CBPP which uses established engineering technology and has predictable performance proven over decades in the UK and abroad.
The infiltration rate of CBPP will decrease due to the build-up of detritus in the jointing material, then stabilise with age. American and German experience recommends that the design infiltration rate through the surface should be 10 percent of the initial rate, to take into account the effect of clogging over a 20-year design life without maintenance. Even after allowing for clogging, studies have shown that the long-term infiltration capability of permeable pavements will normally substantially exceed UK hydrological requirements.
Recommended maintenance is minimal. Some manufacturers do recommend sweeping twice a year as a precaution against clogging, but this should be no greater than for conventional paving and experience suggests that this is rarely carried out on many sites where CBPP is still working. Of course, the maintenance required for conventional piped drainage is eliminated. With these conventional piped systems, regular cleaning of gulleys, oil separators and other equipment is often omitted, causing localised flooding during extreme weather events.
Colin Nessfield is Secretary of Interpave, the Precast Concrete Paving and Kerb Association and a product association of British Precast.

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