Permeable Pavements
The solution.
The Un Earth Summit at Rio de Janeiro in 1992 introduced the central theme of Sustainable Development and all Governments were asked to adopt the basic tenet of Local Agenda 21 of which Sustainable Drainage is an integral part. The UK Government has endorsed a number of initiatives to increase awareness of the issues, these being taken up by the Environment Agency in England and Wales and its Scottish counterpart. To date no definitive legislation dictates that Sustainable Drainage techniques must be used but a number of technical guides such as those issued by CIRIA offer guidance and “best practice techniques” to fulfill the requirements of the building regulations in England and Wales and methods of complying with discharge consents set by planning. Indeed, the planners are in many instances the driving force in demanding change and compliance with Sustainable Drainage techniques. Research has been ongoing from the early 1990’s into the area of “Sustainable Urban Drainage Systems” (SUDS) to reduce the impact of development on the natural water environment. A series of techniques have been adopted which have uses in various sectors of our industry. These include a variety of Infiltration and Attenuation systems, one of the most widely used for urban areas being permeable paving systems.
Permeable paving systems are block paved surfaces which have wide joints filled with fine aggregate, laid on a free draining base which allows for water to be intercepted where it falls, and so are true “source control systems”. These systems can be designed to provide for infiltration or attenuation or a combination of both, depending upon site specific conditions. The passage of water through the base course filters the water, testing at Coventry University has shown that these pavements can become biological reactors, degrading the hydrocarbons by some 97% and retaining the heavy metals in the sediment at the base of the system.
The design of these systems is entirely site specific, and manufacturers of Concrete block Permeable paving are now, in the main, offering to provide a fully indemnified Engineers design for each proposed site. This provision should be availed of as it offers the client fully indemnified design at no cost.
Uses.
Concrete Block Permeable pavements are suitable for a wide variety of residential, commercial and industrial applications. They are particularly suited to carparks and residential roads, but are also capable of substantially heavier uses, when designed to do so.
As housing layouts and densities required by the latest planning guidelines could result in a 50-60% impermeable site area, CBBp offers a real opportunity to reduce this. Permeable pavements are especially cost effective in introducing parking in existing developments, where there is insufficient space for detention ponds. It also optimizes land use by combining parking with water handling within a single construction element. Importantly, permeable pavements can also be designed to accept additional drainage from adjacent impermeable areas such as hardstandings, roofs, site roads etc.
Benefits.
Conventional Concrete block paving has many benefits over other surfacing finishes, combining aesthetic appeal with a slip and skid resistant surface which is highly durable, load bearing and provides long life with minimal maintenance. Couple this to its ease of reinstatement, and it being fully recyclable, and it is easy seen why The Building Research Establishment credits Block paving with an “A” rating for least environmental impact compared with a “C” rating for Asphalt. Concrete Block permeable pavements take these impressive criteria a step further with sustainable drainage offering what the industry should see as being the preferred solution to site drainage.
As with all SuDS techniques, CBBp meets the criteria set down covering Quantity, Quality, Amenity and Economy
Quantity
- Reduces run-off
- Eliminates need to set aside land for retention systems
- Reduces downstream flows in storm conditions
- Increases groundwater recharge
- Increases dry weather flows in streams
Quality
- Water quality improved by filtration
- Retention of pollutants
- Breakdown of hydrocarbons
- Method of preventing storm water entering contaminated sub-grades
- Provision of water to planting areas close to paved areas
Amenity
- No standing water in extreme rainfall events
- No hazard to wildlife
- Aesthetic appeal
Economy
- Total capture of rainwater within site
- Eliminates need for on site drainage
- Comparable or lower cost to conventional drainage and surfacing systems
- Minimal maintenance and lower whole life cost
Conclusion.
The benefits of Concrete Block permeable pavements not only embrace stormwater management and pollution control but are also seen to have economic advantages by minimizing the costs of surface drainage works while reducing the demands on stormwater systems and optimizing land use. Importantly, as sites for development are becoming more and more scarce and more difficult to develop, permeable paving allows flexibility of design and good movement accommodation needed on the more difficult sites.
Clearly these systems will help in many ways with storm water discharge issues, on sites where they are incorporated. The challenge is to develop what is a “Model T Ford” system into a Ferrari which will deal with storm water discharge in an “effective” manner, and contribute positively to what will necessarily need to become a construction industry which has sustainability as its core principle.
Initial concerns about clogging and long term maintenance have largely been allayed by tests in Europe, North America and Australia. This has been the burning issue with local authorities, who contend that maintenance will be required regularly during the pavements lifetime. The tests indicate that CBBp’s can achieve service lives in excess of 25 years without maintenance.
Worldwide, the usage of CBBp is increasing steadily and is attracting the attention of not only Landscape Architects, but Municipal and Civil Engineers. It provides a powerful tool for environmentalists and designers seeking to achieve a sustainable future.
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