The simple concept of the permeable paving system is to allow water to drain through the pavement, preventing ponding and reducing the risk of flooding and pollution during periods of heavy rainfall.
The problems associated with the failings of conventional storm-water drainage systems have become painfully evident with the increasingly regular occurrence of major flood events. This has been further exacerbated by on-going development and climate change which is estimated to increase the volume of annual rainfall by 20% to 40% within the next 50 years.
As a consequence, the recent European Water Framework Directive now requires that we must manage our water resources in a more sustainable way and provide an enhanced level of protection to water quality.
This requirement has necessitated a fundamental change in the overall approach to drainage design with the primary aim of replicating natural processes.
This can be achieved by the use of sustainable drainage systems (SUDS) incorporating source control techniques which endeavour to mimic the natural movement of storm-water from a development, reducing flood risk, enhancing water quality and providing an improved environment.
The benefits of using SUDS in terms of minimizing flood risk are now well recognized by local authorities,county councils and national government, which promote and endorse the use of SUDS on all new development.In addition correctly designed permeable pavements can provide petrol / oil interception in accordance with PPG3 (Pollution Prevention Guidelines) without the need for expensive conventional separators.
With increasing pressure on developers to place high density housing on sites, this will inevitably lead to more hard surfaces. Research has shown that permeable pavements are one of the most space efficient Suds techniques, and yet further research shows their cost effectiveness. The most cost-effective solution – the key findings of a recently published independent report by Interpave has confirmed that permeable pavements can offer significant cost savings from both initial construction and whole life (over 40 years) costs. Innovatively designed SUDS solutions by easydriveways can offer many benefits including reducing the risk of flooding, improving water quality, enhancing wildlife habitats, providing natural groundwater recharge and minimising pollution risk.
The overall benefits of using block paving are well known and understood, what is less clear is the additional benefits that permeable paving provides.
Much in-depth research has been carried out, in particular a recent report carried out by Scott Wilson, (the costs of paving, www.paving.org). This has shown that in comparison to other recognised surfacing materials, permeable pavements proved the most cost effective paving solution for the types of project investigated from Supermarket carparks to Airfield pavements. The cost comparison in the Scott Wilson document, used the System C no infiltration option for comparison, where infiltration or partial infiltration can be achieved costs can be further reduced.
What must always be remembered are the significant environmental advantages permeable pavements have over asphalt and concrete surfaced pavements, both in the improvement in water quantity control, water quality improvement and the ease of recycling of materials which permeable pavements provide. Concrete block pavements in general, have been described as being the most “green” surfacing solution as they have least environmental impact during their manufacture and their disposal, permeable pavements take that one step further.
Cost Benefits of Permeable Paving
and Sustainability Benefits of Drainage Systems
Natural Water Treatment – Water quality is improved by the naturally occurring process which takes place within permeable pavements:
- Facilitates Rainwater re-use
- Shallow installation minimises excavation
- Enhances biodiversity and amenity values
- Shallow installation minimises excavation
- Enhances biodiversity and amenity values
- Shallow non man-entry solution compliant with Health & Safety and CDM regulations
Health & Safety Benefits
There is a misconception that standing water in permeable pavements may be a health hazard. This is not the case as these pavements drain down fairly quickly after rainfall occurs and therefore there will be no significant volume of water within these pavements for most of the time. As they are very shallow structures the issue of accessibility does not arise and indeed these are very safe structure when compared with alternative storm drainage solutions.
When using permeable pavements, separate oil separation techniques are not required, and indeed research has shown that these systems are more effective at removing a wide range of pollutants that oil interceptors are. If additional treatment is required, then the treatment train approach should be used.
A further misconception is that freezing of the water stored within the sub-base may cause damage to the pavement structure. In fact these pavements will drain down before any freezing can occur, and monitoring of projects done in much more extreme climates than our own has shown that non have failed due to frost heave. In fact, it was found that frost penetration was shallower within permeable pavements than in conventional pavements.
easydriveways are fully committed to source control which is internationally accepted as the preferred option in any sustainable urban drainage scheme forming a fundamental part of the treatment train approach to ensure that both run-off quantity and quality are addressed.
Infiltration should be the primary method used wherever possible, however if this is not achievable or appropriate then partial infiltration or attenuation solutions can be developed utilising the Acheson & Glover range of Sustainable Paving Systems.
The key performance criteria for all SUDS techniques including sustainable paving are:
- meeting design requirements for drainage
- improving the local environment
- removing pollutions
An independent report recently published by Interpave confirms that permeable pavements can offer significant cost savings over traditional pavement and drainage design from both initial construction and whole life (over 40 years) costs.
One of the best techniques that can be used to develop a sustainable drainage system (SuDS) effectively is to use source control. This is defined as controlling run-off from a surface at or near its source. This, where possible, follows the natural pattern of drainage, by controlling run-off where it is generated and returning it to its natural environment as soon as practicable.
Using permeable pavements, one of the principal methods of achieving source control, manages both quality and quantity of run-off at the earliest possible stage. The basic idea is that within the voided sub-base of the pavement, water entering the pavement through the joints, is stored, recycled, infiltrated where possible, or conveyed to the outfall at a controlled rate.
Where possible this should be the first approach taken to the design process, and become the ethos around which the design is developed.
Whilst it is clearly understood that permeable pavements are highly efficient control mechanisms for water quantity less is known about the water quality enhancement, which takes place as the water passes through the large volume of stone in the sub-base.
In extreme situations eg, where the water may be carrying a high nutrient load, a series of SuDS systems, in train, may be used to clean the water further.(polishing) The choice of, and the way in which the systems interact, will depend upon site circumstances and this site specific design is a common approach with SuDS design.
Permeable pavements have been installed and monitored for more than 20 years to demonstrate their performance in terms of water treatment. It is now well proven that within permeable pavements, microbes develop naturally in response to ongoing contamination and this treatment capability is further enhanced by filtration and aeration within the construction.
Permeable pavements also limit the concentration of pollutants by immediate localized interception and long term research has demonstrated that the following are effectively removed from storm-water run-off:
- Heavy metals
Hence permeable pavements can negate the need for costly traditional petrol/oil interceptors and can be used as a primary treatment facility in accordance with the Greater Dublin Sustainable Drainage Strategy (Volume 3 Chapter 6) often without the need for further polishing of the water.
The mechanisms by which permeable pavements intercept pollutants and provide water quality enhancement, are widely understood to be:
- Filtration/sedimentation and retention of solids
- Adsorption(pollutants attach or bond to elements within pavement)
- Bio degradation of organics such as petrol
- Tanking of systems preventing contamination of groundwater
As permeable pavements provide mechanisms that encourage these processes to occur, and designs ensure that flows are controlled to allow these processes to occur, permeable pavements are very effective in providing water quality enhancement. Information collated by CIRIA from worldwide research shows that between 50 and 95% of water borne pollutants are removed by permeable paved systems. This can be further enhanced by the treatment train approach.
Surely, when we stop to consider the existing pipe methods of stormwater drainage, where surface water and its contaminants are simply conveyed to the nearest watercourse, these systems offer a vast and much more environmentally friendly solution.
How it works
The basic systems of permeable pavement design fall into three categories depending upon site specific requirements:
It is recognised that total infiltration is generally the preferred design option and should be utilized as source control wherever possible. In this solution rainfall passes through the joints, through the pavement and infiltrates into the sub-grade, helping with ground water recharge. In many sites due to site conditions, this simply is not a workable option.
B) Partial Infiltration
This option is being increasingly used on low permeability sites where a proportion of the run-off is drained through infiltration and the remainder is discharged into an appropriate outfall.
For more information please contact one of our paving specialists on 0845-121-7175
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