As you may know, development in Sydney and the Central Coast is booming and has been for a while. As a civil and environmental engineer, I’ve had the opportunity to see a large number of designs and proposed solutions when it comes to treating stormwater runoff on residential and industrial subdivisions.
Currently, all subdivisions in the north and south-western growth centres of Sydney require some form of temporary water quality treatment. These systems are to be in place until Council is able to construct the planned regional stormwater management infrastructure. At this point, the temporary measures can be removed and those additional lots released.
To meet these requirements, engineers often opt for a detention basin combined with a bioretention zone at the base. These basins take up at least one lot on smaller developments (depending on how thorough you are in your design method) and 10 or more lots on larger developments. Lowering a development’s return and increasing costs in the process.
Bioretention basins are designed to capture stormwater runoff and treat water through a depressed landscaped area with an engineered filter media. Stormwater filters through this engineered media and is allowed to permeate through the ground where existing soils have a high enough permeability and a low groundwater table. However, in north-western Sydney, it is mandated that an impervious liner is placed at the base and slotted sub-soil drainage lines take the treated stormwater away.
Figure 1 A typical section through a bioretention system
These systems are designed to mimic the pollutant removal mechanisms that operate in natural ecosystems.
While this sounds simple enough in principle, in practice, you get convoluted designs, patchwork solutions, and oftentimes, non-existent maintenance regimes. I’ve come across everything from a simple letter report explaining why temporarily treating stormwater runoff is not required for a certain development to full-fledged, bioretention basins with all the bells and whistles that you would expect to see on a permanent system.
Recently, I came across a stamped set of DA plans that had a bioretention basin with a surcharge pit as the outlet and no low-level outlet to drain the filtered stormwater. Unintentionally creating a pond for future houses that will back onto the basin.
Another design went through so many changes and iterations as the development progressed through Council reviews and approvals, that the constructed basin will have no use whatsoever as stormwater would never enter it.
Figure 2 A bad example of a bioretention basin.
Now you may say this is an issue with the designers and contractors implementing these systems. Or maybe it’s the fact that Council’s themselves don’t know what is required or what constitutes a logical solution. Your design could be perfect, yet each time it goes to the certifying authority, it’s a 50/50 call whether or not it gets approved.
While you might expect the odd detail to slip through the cracks here and there, it shouldn’t be the case that two very similar developments can be approved with huge differences in proposed temporary infrastructure.
So, what’s the solution to this mess? I believe temporary water quality treatment measures are a waste of time, money and resources. Often times creating more problems to downstream watercourses through increased sediment loads in their first year of operation.
Don’t get me wrong, I think bioretention systems are great and I am a huge proponent of them being integrated as part of a whole of water-cycle management approach. They not only have an environmental benefit but also provide public amenity and can be attractive focal points of a community. However, this takes a willing developer and invested public servants.
Figure 3 Rain garden under construction (Image credit: Mithun)
If funding was made available to Councils and they were able to build the infrastructure that was planned BEFORE houses started to be erected then we wouldn’t be in this mess. But what can we do about it now?
I believe stringent policing and fines for the implementation of erosion and sediment control measures during construction along with a policy to retain stormwater and reduce potable water consumption would be a much more effective strategy.
So next time you’re part of a team tasked with developing a broad-scale stormwater management strategy, don’t go for the end of line solution. Integrate stormwater management as part of each public street and each development wherever possible.