Onsite stormwater detention, also known as OSD, is a common requirement of new townhouse or unit developments that forms up part of your site’s drainage engineering strategy. In this article we are going to explore why such systems are required, and some of the common alternatives for OSD systems.
When it rains, stormwater is created, however, not all stormwater is created equally. The stormwater that lands on grass is mostly absorbed by the ground, whereas stormwater that lands on a hard artificial surface such as a concrete driveway or a tiled roof will stay above ground and become runoff.
To understand the growing need for OSD systems, we need to understand how our suburban landscape and associated infrastructure came to be…
A Historical Perspective
Once upon a time, back when metropolitan suburbs were first subdivided, the planners of the day envisaged that the golden age of big backyards would never end. Alas, this was not to be so.
In the late 1990s the rise of higher density construction and larger luxury houses started to have an observable impact on existing infrastructure. In particular, the proportion of grassed areas on property allotments were shrinking, and they were shrinking fast.
For a typical lot with a single house, it used to be that around 30% of the site was considered impermeable ‘hard surface’, whereas in recent times it hasn’t been uncommon to see this percentage in the range of 60-70%.
The result of this progressive increase in total impermeable ‘hard surface’ area has been higher levels of stormwater runoff. Meanwhile, the existing Council infrastructure (the pits and major pipes beneath our roads), having not been designed for these higher runoff levels, are being overloaded more frequently resulting in some areas of the wider drainage network experiencing flooding when it rains.
Onsite Stormwater Detention
The approach that most Council’s with established infrastructure have adopted is the introduction of OSD requirements for new townhouse and unit developments. They are also not uncommon for developments where substantially larger homes are also having the same effect of drastically increased impermeable surfaces.
The underlying principle behind any OSD system is that the runoff flows from the site are restricted to the same level and intensity as when the site had more grassed areas.
To achieve this, every OSD system will have some mechanism for storage (i.e. large underground pipes or above ground tanks) and some mechanism for limiting the flow being discharged into the Council network.
While cast insitu concrete pits with baffle walls and orifices are still very common, there have been a recent rise in proprietary alternatives that are also available.
You can find more information on proprietary systems through these links:
SVC Products
Kingspan Water Tanks
Novaplas Group
As a drainage design incorporating an OSD system has an impact on the Council’s wider infrastructure network, it is common for these plans to require Council approval prior to construction. Varying from Council to Council are the conditions of their existing infrastructure networks, their preferences and understanding of various design methods, the acceptance of different proprietary products, and their requirements for final sign off.
Additionally, it is worth noting that while OSD systems don’t typically have any mechanical components, they still require occasional maintenance. A good drainage designer will ensure that there is overflow redundancy built in, but you should expect that in the long term there will be the need to service discharge pits or proprietary flow control devices.
For a more technical understanding of how OSD requirements are developing in the context of urban development, we recommend this article from Water Science and Engineering.
A Final Word
While the principle behind OSD systems is quite straightforward, the design of a particular site’s OSD system needs to be well considered. There are a number of factors both on your site, as well as your site relative to the wider drainage network, that influence a design.
We recommend that you ensure that your chosen drainage engineer is able to navigate these nuances, in particular when advocating on your behalf to Council during the approval process, to achieve the best result for your project.
If you are looking for structural and drainage design engineers for your next unit, townhouse or multi-dwelling development, please get in contact with us so we can provide you with a comprehensive fee proposal for our services. We look forward to partnering with you on your next project.
