For the purposes of this guideline, pervious pavement refers to any system providing hard or trafficable areas which also provides for downward percolation of stormwater runoff.
This includes the following systems:
- No-fines concrete or porous asphalt
- Permeable paver systems, where water percolates through gaps between pavers
- Porous pavers, where water percolates through the paver
- Stabilised loose material such as pebble or shell held in reinforced units or bound by resin.
There are also many products in the marketplace for reinforced turf that are appropriate for occasional traffic use. Many off-the-shelf products in the market provide their own specification, but the key functional requirements are the runoff percolation rate, required traffic loading rates, and the provision of washed aggregates of specific grades in the base course.
Pervious pavements can be designed as standalone water quantity treatment practices by providing below-ground storage for the water quality volume within base course layers. This reduces peak flows and total volumes (through some evapotranspiration), and improves water quality through filtration and sedimentation. Pervious pavement systems have shown excellent water quality enhancement for total suspended solids and metals. However, nutrients bypass these systems unless an anaerobic water storage layer is provided in the base course for denitrification processes to occcur.
A stabilised catchment before and after construction is essential to prevent premature clogging of pervious pavement. Post-construction movement of sediment from landscaped areas must also be avoided.
Pervious pavement can come in a variety of colours, shapes and textures and can often be useful to demarcate bus stops and pedestrian areas. Pervious paving can also be used for low trafficked areas including vehicle and equipment storage, and parking zones.
Challenges and solutions
The following table describes some of the common issues relating to pervious pavement. In all circumstances there is a potential design solution, which must be balanced against other objectives for the project.
Auckland soils may limit potential for infiltration
Under-drains may be required to drain the base course above low permeability soils.
A restricted outlet can provide storage and achieve the required residence time for stormwater.
Paver movement may occur on slopes and highly trafficked areas if not suitably constructed
Particular attention must be applied to turning areas, as well as road lengths where frequent accelerating and decelerating occurs. Pervious strips can be applied outside these areas to receive sheet flow from the balance of the site.
A herring-bone pattern should be applied so that pavers lock together downhill. Pavers should be laid from the bottom of the slope upwards.
Compression strength of porous concrete may be less than normal concrete or asphalt
Porous concrete has been shown to have very high infiltration rates, and this is only partially reduced if smaller aggregates are required for higher compression strength. Super plasticisers can also improve final compression strength.
Strips of porous concrete can be used in areas where there is unlikely to be turning by vehicles, thereby reducing potential effects to the concrete surface.
Reduced compression strength is mitigated to an extent by an increased aggregate layer in the subbase.
Saturation of subsoils may affect the integrity of adjacent structures
Pervious pavement should be designed for the site conditions, with potential to include greater depths of drainage layers, impermeable liners, and geogrids to isolate stormwater from adjacent structures.
Sediment from an erosion event or spill may fill the voids of pervious pavers
Pervious pavers may be cleaned by mechanical brushing combined with vacuum trucks of a very specific power rating. Movement of fines to bedding layers may require replacement in some areas, but the base course is likely to remain unaffected.
Stabilise the sub-catchment before pervious pavement is brought online.
Porous pavers and concrete can be waterblasted.
Oil and grease spills are very difficult to clean out and may require replacement of the surface, if it is a concern.
General loss and wash-out of aggregates may occur between pavers
Topping up may be required, but in heavily impacted areas, water based resins or permeable grout materials may be considered as an alternative.
Total costs are higher than standard hard surfaces due to increased base course and care required during instalment
The stormwater management function of pervious paving saves on the equivalent cost of an alternative practice and/or land area to accommodate it. Specialist products are becoming more commonplace and contractors are becoming more efficient in their installation, bringing costs down over the medium term.
Pervious pavement should be inspected every two years if other areas drain to the surface and every five years if the installation is standalone. Any issues identified during inspection should be rectified.
If maintenance is performed correctly and the right pavement is chosen for the site, pavement life should be similar to non-pervious surfaces. Surface designs adjust construction materials to ensure pavement servicability is equivalent to their non-pervious alternatives. For example, pervious concrete mixes can contain up to 30% more cement than traditional concrete to provide extra strength to offset the effect of removing fines from the mix.