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.
