On-Site Detention (OSD) systems are a non-negotiable requirement for most commercial developments in Australia. Their purpose is simple: to capture storm surges and release water slowly to prevent downstream flooding. However, the design reality is often far more complex.
For developers, a poorly designed OSD system is a silent budget-killer. It can lead to wasted leasable floor space, expensive construction variations, and—worst of all—a refusal of final certification at handover.
Drawing on our experience rectifying hundreds of failed systems, here are the 5 most common design flaws we see in commercial OSD projects and how to avoid them.
1. The “Maintenance Disconnect” (Impossible Access)
The most frequent failure we see isn’t hydraulic; it’s operational. Many designers treat OSD tanks as “set and forget” infrastructure, placing access grates in impossible locations or making them too small for safe entry.
The Flaw: Designing a tank that requires Confined Space Entry for every minor inspection, or placing access points under heavy machinery or permanent fixtures.
The Fix: Design for “Whole-of-Life” maintenance. We position access points for easy cleaning via suction trucks and ensure internal layouts allow for safe, efficient maintenance. If an asset is too expensive to maintain, it becomes a liability for the Facility Manager immediately upon handover.
2. Conservative Sizing (Wasting Valuable Space)
Generic calculators often result in conservative, oversized designs. While this satisfies the Council, it wastes your project’s most valuable asset: space.
The Flaw: Using simplified “Deemed to Comply” calculations rather than site-specific hydraulic modelling. This often results in OSD tanks that are 10-20% larger than necessary.
The Fix: Value Engineering. By using advanced DRAINS modelling, we can simulate the exact storage requirement based on site-specific hydrographs. This optimises the tank volume, reducing excavation costs and potentially freeing up space for additional car parking or Gross Floor Area (GFA).
3. The “Paper vs. Reality” Clash
A design that works on a 2D plan often fails in the 3D reality of a construction site.
The Flaw: Stormwater pipes clashing with structural footings, fire services, or sewer lines because levels weren’t coordinated during the design phase. This leads to expensive on-site variations and delays.
The Fix: Integrated civil and structural design. We ensure stormwater levels are coordinated with all other services before construction begins. We also verify that the Hydraulic Grade Line (HGL) actually works, ensuring water doesn’t back up into the building during peak flow.
4. Incorrect Discharge Control Pit (DCP) Configuration
The DCP is the “brain” of the OSD system. It controls the flow rate using a specifically sized orifice plate.
The Flaw: Incorrect orifice sizing or weir levels. If the orifice is too small, the tank overflows prematurely. If it’s too large, you exceed the Permissible Site Discharge (PSD) limit, leading to a failed certification.
The Fix: Precision fabrication. We design and install the orifice plate to the exact millimetre required by the hydraulic model, ensuring the system performs exactly as the Council approved.
5. Materiality & Watertightness Failures
How you build the tank matters as much as how you size it.
The Flaw: Using blockwork or porous materials in areas with high water tables, leading to ingress (groundwater filling the tank, reducing capacity) or egress (water leaking out and undermining foundations).
The Fix: Choosing the right construction method—whether in-situ concrete, pre-cast, or reinforced blockwork—based on geotechnical conditions. We ensure the tank is structurally sound and effectively waterproofed to prevent structural degradation over time.
Summary
An OSD system should be invisible to the asset owner—functioning silently and efficiently. When designed poorly, it becomes a constant source of cost and risk.
At Stormwater Services Australia, we bridge the gap between design and delivery. We don’t just model the system; our civil teams build it, ensuring that what is on the plan works in the ground.
Worried about your current OSD design?
[Book a Design Peer Review] with our engineering team to value-engineer your solution and ensure a seamless handover.
- MUSIC Modelling Explained: How to Meet Water Quality Targets for Your DA.
Here is a draft blog post for the Development & Commercial Property category.
This post demystifies the complex “Water Quality” aspect of a DA submission, positioning your firm as the expert who can balance strict Council compliance with commercial feasibility.
Blog Category: Development & Commercial Property
Target Audience: Architects (“Aaron”), Senior Project Managers (“Sarah”), Developers.
Target Keywords: MUSIC Modelling, WSUD, Water Quality Targets, DA Compliance, Stormwater Filtration, Bioretention Design.
MUSIC Modelling Explained: How to Meet Water Quality Targets for Your DA
If you are preparing a Development Application (DA) for a multi-unit residential or commercial project, you have likely encountered a Council request for a “MUSIC Model” or a “Water Quality Report.”
For many developers and architects, this is a black box. You submit your site plan, and an engineer tells you whether you pass or fail. But understanding how this modelling works is critical to controlling your construction costs and future maintenance liabilities.
Here is a plain-English guide to MUSIC modelling and how to use it to de-risk your DA.
What is MUSIC?
MUSIC stands for Model for Urban Stormwater Improvement Conceptualisation.
It is the industry-standard software used across Australia to simulate how stormwater pollutants run off your specific site. It calculates two things:
- The Pollutant Load: How much dirt (solids), nutrients (phosphorus/nitrogen), and litter your new development will generate.
- The Treatment Efficiency: How effectively your proposed stormwater system (tanks, filters, basins) removes those pollutants before the water leaves your boundary.
The “Big 4” Targets You Must Hit
While every Council has slightly different rules (often buried in their DCP or a specific WSUD guideline), most require you to achieve specific percentage reductions compared to a “do nothing” scenario.
If your MUSIC model doesn’t prove these targets, your DA will not be approved.
- Gross Pollutants (Litter/Debris): Typically 90% reduction. This is the easy part, usually handled by grates and pits.
- Total Suspended Solids (TSS): Typically 80-85% reduction. This refers to sediment and dirt.
- Total Phosphorus (TP): Typically 60-65% reduction. Often comes from decaying leaves, detergents, and fertilisers.
- Total Nitrogen (TN): Typically 45% reduction. This is the hardest target to hit. Nitrogen dissolves in water, so you can’t just filter it out with a mesh; you often need biological treatment (plants/soil) or specialised cartridges.
The “Treatment Train”: How We Achieve Compliance
To hit these targets, we design a “Treatment Train”—a series of devices that work together. A typical train in a MUSIC model might look like this:
- Rainwater Tanks: Capturing roof water for reuse (toilets/gardens) removes a huge volume of pollutants simply by keeping the water on-site.
- Gross Pollutant Traps (GPTs): Mechanical devices that physically trap litter and sediment.
- Secondary Treatment: This is where the modelling gets strategic. We might use:
- Bioretention Basins (Raingardens): Using soil and plants to naturally strip Nitrogen and Phosphorus.
- Filtration Cartridges: Underground canisters (like Ocean Protect or Stormwater360) used when surface space is limited.
The Cost of “Lazy” Modelling
Here is the trap for developers: It is very easy to design a compliant model that is incredibly expensive to build and maintain.
A generic engineer might just drag-and-drop a massive bio-basin into the model to hit the Nitrogen target. The result? You lose 50m² of developable land or car parking space.
Alternatively, they might specify an excessive number of proprietary cartridges. This hits the target, but saddles the future Body Corporate with a $20,000 annual maintenance bill for filter replacements—a liability that can affect buyer confidence.
The Strategic Advantage: Value Engineering
At Stormwater Services Australia, we use MUSIC as a design tool, not just a compliance checker.
We iterate the model to find the “sweet spot.” This might mean:
- Optimising Tank Re-use: Increasing the rainwater reuse demand (e.g., connecting to more toilets) to reduce the size of the required bio-basin.
- Hybrid Solutions: Combining a small, attractive raingarden with a reduced number of mechanical filters to balance CAPEX (construction cost) with OPEX (maintenance cost).
- Material Selection: Specifying the exact filter media porosity to maximise flow while maintaining treatment levels.
Summary
MUSIC modelling is the evidence base for your environmental compliance. It dictates what you have to build and what you have to maintain forever.
Don’t treat it as an afterthought. Ensure your engineering partner understands both the software and the commercial reality of the assets they are specifying.
Struggling to meet WSUD targets without losing land?
[Contact Our Design Team] for a review of your current MUSIC model and advice on value engineering.
