For any property developer, an On-Site Detention (OSD) tank is a “grudge purchase.” It occupies valuable real estate, requires expensive excavation, and generates zero revenue. Yet, it is a mandatory requirement for almost every commercial project in Australia.
The challenge is that OSD tanks are often treated as immovable objects in the design process—massive concrete boxes that dictate the layout of basements and car parks.
However, the size of an OSD tank is not fixed in stone. It is a variable derived from hydraulic variables. By moving from a standard “Deemed to Comply” calculation to a sophisticated “Performance Solution,” we can often reduce the required volume of your tank, unlocking space and saving construction costs.
Here is how we optimise OSD designs to balance strict Council compliance with commercial reality.
The Problem: The “Safe” (and Expensive) Calculation
Most local Councils provide a “Deemed to Comply” spreadsheet or simple formula for calculating OSD size. It usually looks something like this: Site Area x Factor = Required Storage Volume.
Many civil engineers, under time pressure, will simply plug these numbers in and put a tank on the drawings.
The issue? These formulas are deliberately conservative. They assume the worst-case scenario for every variable to ensure the Council’s drainage network is never overwhelmed. The result is often a tank that is 15% to 20% larger than it strictly needs to be to meet the actual hydraulic performance requirements.
The Solution: Tuning the “Permissible Site Discharge” (PSD)
To optimise a tank, we need to understand its relationship with the Permissible Site Discharge (PSD).
- PSD is the maximum rate (litres per second) that Council allows you to release into their street drains.
- OSD is simply the “waiting room” for water that exceeds that rate.
The Golden Rule: The faster you can legally discharge water (up to the PSD limit), the less water you need to store.
If an engineer designs a system that discharges below the PSD limit (due to a restrictive pipe size or incorrect orifice plate), the water backs up in your tank, forcing you to build a bigger “waiting room.”
How We Optimise Using DRAINS
We use DRAINS hydraulic modelling software to simulate the exact behaviour of your site during a 1-in-100-year storm. We optimise the design by:
- Precision Orifice Sizing: We tune the Discharge Control Pit (DCP) orifice plate to the exact millimetre. By maximizing the flow rate closer to the legal PSD limit, we drain the tank faster, reducing the required storage volume.
- Modelling Pipe Storage: A comprehensive model counts the water stored inside the large-bore pipes and pits leading to the tank. This “in-pipe” storage can sometimes account for several cubic metres of volume, allowing us to shrink the main concrete tank structure.
- Hydrograph Routing: We analyse the timing of the peak flows. Often, the peak runoff from the roof arrives earlier than the runoff from the driveway. By modelling this time lag, we can prove to Council that the simultaneous storage demand is lower than a simple calculator suggests.
Above-Ground vs. Below-Ground: Finding “Free” Volume
Another optimisation strategy is moving storage out of the deep basement and onto the surface.
While deep excavation is expensive (requiring shoring, rock-breaking, and waterproofing), shallow surface storage is often “free.”
- Driveway Storage: We can design driveways and hardstands to safely hold a shallow depth of water (e.g., 50-100mm) during a major storm event.
- Landscape Storage: Depressed grassed areas or OSD basins can hold volume without requiring reinforced concrete.
By utilising these surfaces for the “rare” high-volume storage, we can significantly reduce the footprint of the expensive underground tank.
The Bottom Line
Every cubic metre of concrete you don’t pour saves money. Every square metre of basement you reclaim from a tank is a potential car space or storage cage that adds value to the property.
Don’t settle for the first OSD size you are given. If your engineer hasn’t run a detailed DRAINS model to test the PSD limits, you are likely building a tank that is bigger than it needs to be.
Is your OSD tank eating up too much space?
[Book a Value Engineering Review] with Stormwater Services Australia. We will review your current plans to see if the storage volume can be legally and safely reduced.
