October 7


Swinburne Method of OnSite Detention (OSD) tank sizing

By orgeng

October 7, 2020


We have been very busy these last few weeks integrating the Swinburne Method of OSD calculation into the Australian version InSite Water (This upgrade has not been implemented into the SA tool).

For those non-engineers out there, the Swinburne Method is a popular way of sizing stormwater detention tanks.  Unfortunately the only software that calculates the Swinburne Method, OSD4W was withdrawn from sale.  We are stepping into the gap to allow you to continue using this methodology, originally developed by Bruce Bowditch and Dr. Donald Phillips at the Swinburne University of Technology.

For the civil engineers out there – enjoy! For the non-engineers – please leave the FLOW settings as ‘defaults’ unless otherwise instructed by your Council / consulting Engineer. The default settings are suitable for most sites.

You can still print reports from previous projects, and there is the option to keep using our previous OSD methodology (Boyd’s equation).

Details of Recent Upgrades:
​Recent changes include:
– Integration of SQIDEP approved devices​
– Optimisation of the UI to reduce visual clutter
– ​Upgrade of the server – the site now runs much more smoothly 
– Integration the Swinburne Method into reporting
– Upgrade of our PSD algorithms – the Swinburne Method calculations now run much faster.
– Allowing ‘uncontrolled areas’ to be part of OSD calculations.
– Removal or the ‘calculate’ button. The new more powerful server meant that this was not necessary as the 20 year rain tank water balance and irrigation simulation runs almost instantaneously without lag. All calculations run automatically whenever any relevant input is changed.

FAQ: Why do I not get exactly the same number as the OSD software?

Some notes for engineers and hydrologists with experience with the Swinburne Method: 

InSite now is very close to other Swinburne Method software (usually +/-10%) however there are a few reasons the numbers are not exactly the same.

  • We are using the 2016 IFDs from the BoM instead of the 1987 IFDs. This will give slightly different (but more valid) answers.
  • The BoM no longer publishes ARIs (Average Return Interval) for 5 and 10 year storms. Instead they publish: 20%AEP (Annual Exceedance Probability) which is roughly a 1 in 4.5 ARI storm; and a 10%AEP which is roughly a 1 in 9.5 year ARI. We have used 20%AEP and 10%AEP in our calculations where previously OSD4 would have used 5 and 10 year ARIs. Further details on the new probability terminology can be found in Book 1; Chapter 2; Section 2.2 Terminology of ARR2019 http://arr.ga.gov.au/arr-guideline.
  • We calculate the pervious area runoff coefficient for each site rather than using the default of 0.13.  This is using the standard formula of 0.1+(0.7-0.1) x (i – 25)/(70 – 25) as per the Rational Method (where i is the 1 hour 20%AEP IFD intensity for the site). This is important, as this changes a lot as you head north in Australia.

Please purchase a license for further support or attend training for more information.


For more information on the role of OSD in flood prevention.

Book 9: Runoff in Urban Areas: Coombes, P., and Roso, S. (Editors), 2019 Runoff in Urban Areas, Book 9 in Australian Rainfall and Runoff – A Guide to Flood Estimation, Commonwealth of Australia, ©Commonwealth of Australia (Geoscience Australia), 2019. http://www.arr-software.org/pdfs/ARR_190514_Book9.pdf

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