Permeable paving is pretty simple – water drains freely and quickly through it and into the ground underneath. But why do councils (and engineers, and architects, and town planners, and municipal water authorities….) love permeable paving so much? We’ll have a look at this, and the environmental benefits in this blog post.
But first, where does stormwater actually go?
It’s not something we often think about. Stormwater systems basically discharge stormwater from our homes, roads and cities into waterways and out to sea – rather like the water cycle we learn about at school. But, because of all the hard paved surfaces and pollutants on them, water goes into the waterways much quicker causing erosion and flash flooding, and carries with it a large amount of pollutants. Municipal authorities (such as Melbourne Water) develop strategies to mitigate and offset the detrimental effects of urbanisation on waterways and health, and this is called Water Sensitive Urban Design (WSUD). This then feeds into planning laws and development environmental targets that are ultimately implemented by local councils.
(For more information on WSUD, visit Melbourne Water’s very informative website –here)
Permeable paving helps meet a variety of WSUD targets, and this is what this blog post is about.
Benefit #1 – water into the ground, not into the municipal stormwater system
This one is the most obvious benefit of permeable paving. By installing permeable paving on a fully permeable base such as permeable concrete, water can completely drain through the pavement and infiltrate the soil underneath. So instead of water running off the surface into the stormwater system, it’s much more akin to what happens when it rains on your lawn in the garden – water drains slowly into the ground, like nature intended!
So, the amount of water discharged from a property is reduced, meaning that the chance of flash flooding (when the stormwater system is overwhelmed) is reduced. but the soil on the property contains more water, meaning adjacent landscaped areas (plants, trees etc.) flourish.
Benefit #2 – reduced amounts of pollutants enter the stormwater system
A large source of pollutants in the urban water cycle is hard paved areas, such as driveways and roads. Dust, grime, fuel and oil build up during dry periods, and then are washed straight down the drain when it rains. But on a permeable pavement, instead they flow down into the pavement reducing the amount of pollutants that enter waterways.
What about when you’ve got subsurface drainage? Doesn’t that defeat the point of using permeable paving?
Not at all! We’ve written about subsurface drainage before, and on clay soils it’s pretty much essential. But you still get the environmental benefits – here’s how:
Flash flooding reduction – the cause of flash flooding is a large “first flush” into the stormwater system upon commencement of a heavy rain event that overwhelms the capacity of the municipal system. Permeable paving that connects to the stormwater system acts as a retention system, in that it holds back water during the first flush. Because the water has to flow down through the pavement, and then along the clay soil (after saturating it) to the stormwater drain point, it takes much longer to drain than a typical hard surface such as a concrete driveway.
Pollutant reduction – even when connected to the stormwater system through subsurface drainage, permeable paving retains the ability to filter out and reduce the amount of environmental pollutants. Because the stormwater has to drain through the pavement and along the soil underneath to the drainage point, a lot of the surface pollutants are retained or absorbed on the subsoil. Therefore, even if 100% of the rainfall enters the stormwater system, it still occurs slower and with less pollutants compared to a normal impermeable paved area.
Benefit #3 – the Urban Heat Island Effect
Another slightly less obvious benefit of permeable paving is in helping combat the urban heat island effect. Cities, due to their black asphalt roads and dark roofs, are hotter than rural areas (with trees, grass and not some many buildings and roads). Anyone who’s ever worn a black t-shirt on a hot summer’s day will tell you why!
A major factor in mitigating the heat island effect is the planting and maintaining of urban treescapes – such as City of Melbourne’s Urban Forest project. But trees need water to their roots to grow, which is why local councils insist on permeable paving within tree protection zones. (And why local planning laws require the planting of new canopy trees in new developments).
Transpiration, another part of the stormwater cycle that is often overlooked, also helps reduce urban heating. Transpiration is the evaporation of water from plants through foliage, with 80% of the cooling effect from canopy trees a result of nature’s own evaporative cooling system! But again, why irrigate urban foliage when you can install a pavement that does it for you?
How do you design permeable pavements to comply with council requirements?
That’s the hard part! Any new subdivisions must comply with Clause 56.07-4 of the Victorian Planning Provisions, which sets stormwater management objectives. Certain councils go further than these objectives with their own specific planning overlays. Compliance with Clause 56 is often assessed with computer modelling using STORM or MUSIC.
A key factor in compliance is the amount of permeable space in a development. Building footprints are impermeable, so creating the maximum amount of permeable space allows the maximum building footprint. And permeable paving counts as permeable space under the planning provisions.
But how do you ensure compliance with local councils? There is no specific Australian Standard for permeable paving so permeability and performance rests on the suppliers and installers of the pavement. So in order to achieve compliance, make sure you specify and use an established product installed correctly.
Here at New Dawn, we have been doing permeable paving for over half a decade. We have installed and achieved compliance in all council areas in Melbourne. We issue certificates of compliance for all new installations, and guarantee council satisfaction. We pride ourselves on not being a paving manufacturer trying to sell everyone pavers, but a professional permeable contractor offering a specialist service done right.
We also offer complimentary in-office design advice and assistance with regards to permeable paving. So whether you’re building or designing, we’re here to help! To get in touch today, call (03) 9543 3013 or email [email protected].