Hard paved surfaces such as roads, car parks, driveways, and courtyards cover a significant proportion of urban areas. Impervious paving increases peak and total storm water discharge increasing demand on existing stormwater infrastructure and increases downstream flooding. Rapid discharge of storm water deteriorates stream habitat through bank erosion, while runoff from impervious surfaces carry pollutants that reduce water quality. Moreover, impervious surfaces prevent groundwater recharge necessary for the development of healthy trees.
Water sensitive urban design (WSUD), aimed at sustainable solutions to water management and broader environmental protection is gaining global momentum in contemporary urban planning. One WSUD strategy is to reduce impervious surfaces. The use of porous pavement provides an innovative method for achieving several WSUD objectives.
Porous paving is widely employed in the UK and northern Europe, where its use was principally as a flood mitigation technique to minimise the high cost associated with locking up expensive real estate as retention ponds and soaks. Porous paving is increasing in popularity in the United States where its use is being driven in part by the Environmental Protection Agencies (EPA) control on storm water pollution management.
In Australia, the driest continent in the world, there has traditionally been reluctance from government and urban planners to adopt this technology, despite its water management advantages. The adoption of WSUD objectives in Australian urban planning in recent years has provided an opportunity to expand the use of and improve this technology.
Porous paving can be separated into the main product types listed in Table 1 below.
Table 1: Main porous paving product types
Unlike impervious paving, porous paving is designed to allow water and air to flow through the pavement section, and depending on design, may allow infiltration and gaseous exchange into the underlying soil. Infiltration is determined by the rate at which the entire pavement including the base course and sub-base section can absorb, retain and drain water.
In flow through systems where the water is discharged into the natural underlying soil, soil properties will determine the suitability of and influence the design of the paving. Adequate hydraulic conductivity and the ability of the soil to maintain structure and load bearing capacity when wet are integral to the design and therefore success of the paving.
Contrary to common perceptions the structural capacity of some porous pavers extends beyond light load, low traffic areas such as driveways pedestrian paths, courtyards and small car parks. Modular block pavers, and in particular open jointed pavers have proven application in residential streets, commercial traffic zones such as bus terminals and even sites that experience regular heavy industrial traffic including several major shipping port container yards.
Porous pavement is gaining attention for its pollution control properties, having filtration capabilities to remove suspended solids thus improving water quality. Mostly, these pollutants are held in the jointing material or trapped in filtering layers in the base course or sub-base. Over time infiltration rates can be reduced and allowances for this should be made in the design stage.
The functionality of porous paving can be further reduced as oil, grease, fine solids and organic matter become trapped, clogging the system and necessitating the need for cleaning maintenance. Site design considerations that can reduce clogging include the installation of sediment traps, filter strips or gutter systems that pretreat stormwater to remove gross pollutants and sediment. Where paving is likely to receive large quantities of sediment and debris that cannot be controlled, installation should be avoided.
Generally porous pavement systems that rely on achieving porosity through open joints and cells are easier to maintain than porous asphalt, concrete or modular porous paving, which have finer voids that tend to clog more easily. Restoring infiltration capacity of open jointed and celled systems can be achieved by removing and replacing the top layer or drainage material. Cleaning porous asphalt, concrete or modular porous paving requires regular vacuuming or high pressure cleaning to maintain an acceptable level of porosity. Therefore, site design that avoids or minimises the accumulation of sediments over the paving and cleaning and maintenance are integral elements of any permeable paving system.
The benefit to trees of porous paving lies in its ability to provide a healthy rooting habitat, contributing to tree longevity. Trees bring to the urban landscape enormous social, functional and environmental benefits. Trees provide an intrinsic amenity through their ability to moderate the urban heat island effect, improve air quality, reduce glare, attenuate noise, bring a sense of scale to and compliment architecture. Trees have the capacity to contribute to mental and psychological well-being, add economic value to property and encourage patronage to commercial districts. Consequently, the importance and public veneration of trees in the urban forest is increasing.
The level of benefit a tree provides is commensurate with longevity and ultimate size. Many urban tree planting sites occur on compacted soils within or surrounded by impervious paving that present trees with hostile growing environment where ground water recharge is limited and gaseous exchange poor. Subsequently, many urban trees develop diminutive stature, are easily drought stressed and susceptible to pest and disease, ultimately limiting their potential lifespan.
Conversely, porous paving that allows moisture infiltration and gaseous exchange to the underlying soil, provides an improved rooting environment similar to a natural soil surface. In combination with other “tree friendly” technologies such as load bearing rooting media or “structural; soils”, providing modified growing environments that includes the application of porous pavement systems will allow more successful urban landscapes to be developed; a landscape in which increased opportunities for tree planting are provided.
No single porous pavement system has universal application and careful selection of the appropriate pavement material is required to achieve the design intent, functionality and cost effectiveness for a project. The uptake of WSUD in Australia has created an opportunity for the increased use of porous paving that will add to current knowledge influencing future developments and improvements in this useful technology that could also facilitate the increased presence of healthy long lived trees in cities.
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- Dierkes, C., Lohmann, M., Becker, M& Raasch, U., (2005) Pollution retention of different pavements with reservoir structure at high hydraulic loads, in Proceedings from 10th International Conference on Urban Drainage, Copenhagen/Denmark, 21-26 August 2005
- DSE (2006) Victorian Planning Provisions Practice Note practice Note Using the integrated water management provisions of Clause 56 – Residential subdivision http://www.dse.vic.gov.au
- Ferguson, B., K., (2005) Integrated studies in water management and land development. Porous pavements. Taylor and Francis
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- Water Sensitive Urban Design in the Sydney Region, (2003) Practice note 6. Paving