Design considerations
for cellular porous paving
The Treatment Train
The treatment train approach is fundamental to the creation of integrated SUDS designs compliant with best practice guidance.
Porous 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 porous pavements microbes develop naturally in response to ongoing contamination and this treatment capability is further enhanced by filtration and aeration within the construction.
Porous pavements also limit the concentration of pollutants by immediate localised interception and long term research has demonstrated that the following are effectively removed from stormwater run-off:
- Heavy metals
- Hydrocarbons
- Phosphorus
- Nitrogen
There are 3 primary forms of construction to meet specific requirements:
- Infiltration – it is recognised that total infiltration is generally the preferred design option and should be utilised as source control wherever possible.
- 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.
- Attenuation – this assumes no infiltration and utilises an impermeable geomembrane to achieve temporary storage prior to controlled discharge or re-use.
In addition, the following design issues should be considered:
Rainwater re-use – porous pavements can be utilised for harvesting rainwater with the sub-base creating a storage tank prior to re-use for a range of non-potable applications including toilet flushing and irrigation.
Sloping sites – porous pavements can be successfully installed on sloping sites – dependent upon the gradients it may be necessary to construct dams/baffles within the sub-base layer to control the flow of water.
SUDS and brownfield sites – a common misconception is that SUDS cannot be used on contaminated sites. In fact, appropriately designed porous paving can be successfully incorporated into any brownfield development and offers advantages over conventional drainage solutions.
SUDS retro-fit to existing development – SUDS can also be used very effectively to overcome drainage problems on existing development and the use of porous paving can provide a cost effective solution without increasing land-take.
Physical
- Soil type – Can infiltration be used, CBR values, etc
- Former land use – is contamination present and will a lined system be required?
Structural
- Properties of proposed materials (interaction with specification)
- Traffic loads
- Construction loads
Hydraulic
- Hydrology of catchment – is there a flood risk?
- Rainfall data for the site
- Discharge criteria if draining to a sewer or ditch
- Frequency of surface ponding that is acceptable
- Void ratio and permeability of the proposed sub base material
Environmental
- Levels of contamination below the site (from site investigation)
- Details of receiving water course – quality designation and any environmental quality standards (EQS) that are applicable
- Groundwater vulnerability and classification for site – From environmental regulators (EA / SEPA).
Design specific parameters
- Type of development – will site use cause polluted runoff?
- Potential areas of porous pavement – car parking, etc
- Use during construction – need to keep the area clean and mud free
Health and safety
The treatment train approach is fundamental to the creation of integrated SUDS designs
Porous pavements also limit the concentration of pollutants