A dual orifice on-site detention tank uses two outlets to control discharge across both small and large storms, while a single orifice system relies on one. Dual orifice designs match predevelopment flow rates more accurately across the 20% to 1% AEP range, reduce overflow risk, and align with the Upper Parramatta River Catchment Trust Handbook fourth edition still adopted by Western Sydney councils.
If you are designing or reviewing an OSD tank for a subdivision in NSW, the choice between a single orifice and a dual orifice control sets the tone for the entire stormwater strategy. Get it right and the system meets council policy without complaint. Get it wrong and the project stalls at certification.
This guide walks through both options, explains how high early discharge (HED) pits sit in the picture, and looks at why the UPRCT Handbook still drives OSD review across major Sydney Councils.
What is on-site detention and why councils require it
On-site detention (OSD) is a stormwater system that temporarily holds runoff from a development and releases it to the public drainage network at a controlled rate. The aim is simple. Match the post-development discharge to predevelopment levels so downstream pipes and creeks are not overloaded.
Councils impose OSD as a standard condition of consent on most infill developments, including duplex builds, dual occupancies and multi-lot subdivisions. The discharge rate, storage volume and tank configuration are all set by the Council’s stormwater policy.
Where OSD fits in the subdivision works approval pathway
For DA-approved subdivisions, OSD is part of the civil works that sit under a Subdivision Works Certificate. The certifier reviews the OSD design against the council’s policy before construction starts, then inspects the built system before issuing the certificate. Without that sign-off, the Subdivision Certificate cannot be issued and lots cannot be released.
Single orifice OSD tank design
A single orifice OSD uses one fixed-diameter outlet to control flow out of the tank. As the tank fills during a storm, water passes through the orifice plate at a rate governed by the head of water above it.
How a single orifice controls flow
The orifice plate is usually a stainless steel plate with a precisely cored hole. Most NSW councils require a minimum 200 x 200 mm flat plate, 3 mm thick, with an orifice diameter of at least 40 mm. Some councils refuse to approve diameters smaller than 25 mm to reduce blockage risk. Liverpool Council, for example, sets out these specifications in its On-Site Stormwater Detention Standard.
Discharge through the orifice follows standard hydraulic theory. Flow rate scales with the square root of the head, which means a small tank releases slowly and a full tank releases faster.
Where single orifice systems struggle
The fundamental problem with a single orifice is that one fixed diameter cannot accurately throttle flow across a wide range of storm events. NSW design guidance, including the City of Ryde Stormwater DCP, recognises that it is impractical to use a single outlet of fixed diameter to restrict flows for events ranging from the 20% AEP to the 1% AEP storm.
If the orifice is sized for the 1% AEP event, smaller storms barely fill the tank and discharge is too low. If it is sized for the 20% AEP event, large storms overwhelm the orifice and trigger reliance on the high level overflow.
When single orifice is still acceptable
For small infill sites with limited catchment area, a single orifice can still meet some council policies, especially where the design event range is narrower or the policy uses a single design storm rather than a range.
Dual orifice OSD tank design
A dual orifice OSD uses two outlets at different invert levels. The result is a control system that responds to the size of the storm rather than treating every event the same.
The two-stage outlet concept
The two-stage outlet is widely accepted in NSW councils as the practical solution to the orifice sizing problem. The first stage outlet limits discharge to the predevelopment 20% AEP flow. The second stage outlet, set higher up in the discharge control pit, restricts flows for larger events up to and including the 1% AEP storm. The MidCoast Council Site Stormwater Drainage Guidelines reflect the same approach.
First stage outlet for frequent storms
The lower orifice handles the smaller, more frequent storms. Because it sits at the base of the pit, it activates as soon as runoff reaches the system. Sized correctly, it discharges at predevelopment minor storm rates and avoids overfilling the tank during nuisance events.
Second stage outlet for major events
The upper orifice or weir engages once water rises above its invert. From that point, both outlets work together to manage the larger volumes generated by major storms. This combined discharge keeps peak outflow within the council’s permissible site discharge for the 1% AEP event.
Practical benefits over single orifice
Dual orifice designs deliver three advantages that matter on a real project.
Tighter compliance across the storm range. The system actually achieves predevelopment flow at both minor and major events, not just one design storm.
Smaller storage volumes. Because the system uses both outlets effectively, the required Site Storage Requirement (SSR) is often lower than an equivalent single orifice setup.
Fewer review queries. NSW certifiers and council engineers see two-stage outlets as standard practice. A clean dual orifice design moves through review faster than a borderline single orifice.
High Early Discharge (HED) pits: pros and cons
A high early discharge pit is a discharge control device that aims to release the full permissible site discharge as soon as the tank starts filling. It sits between the catchment and the OSD storage, intercepting low flows directly to the council drain before water enters the tank.
How a HED pit works
A HED pit accepts runoff at the inlet, releases the permissible discharge through the low-level outlet to the council system, and only diverts excess into the OSD tank once the council outlet is at capacity. To work properly, at least 85% of the contributing catchment must drain directly to the HED pit. Anything less and the high early discharge benefit is reduced.
NSW councils require HED pits to be cast in-situ concrete. Precast or masonry brick pits are not permitted. For HED pits up to 1200 mm deep, the minimum internal opening is 900 x 900 mm to allow inspection and silt trap maintenance.
Pros of high early discharge
Smaller storage volumes. Under the UPRCT method, the SSR for an off-line OSD storage that achieves full HED is 470 m³ per hectare. Without HED, SSR climbs because more volume must be held back to prevent peak overshoot.
Faster system recovery. The tank empties quicker after small events, ready for the next storm.
Lower nuisance overflow. Because low flows go straight to the public system, the tank is not constantly cycling for minor rainfall.
Cons and design risks
Tighter construction tolerances. Cast in-situ concrete and detailed access provisions add cost and inspection time on site.
Maintenance load. Silt traps, mesh screens and flap valves require regular cleaning. Skipped maintenance kills the high early discharge benefit and pushes the system back toward standard operation.
Catchment dependency. If the building layout cannot get 85% of the area to the HED pit, the design has to either accept a lower benefit or revert to a standard discharge control pit.
Compliance certification. Inspection of the constructed pit is critical. Issues found at the Compliance Certificate stage can hold up the SWC if remediation is required.
The UPRCT Handbook Version 4: why it still matters
The UPRCT On-Site Stormwater Detention Handbook fourth edition was published in December 2005. It was prepared by Cardno Willing (NSW) Pty Ltd with Haddad Khalil Mance Arraj Partners and Brown Consulting (NSW) Pty Ltd, on behalf of the Upper Parramatta River Catchment Trust. Principal contributors were Dr Brett C Phillips, Dr Allan Goyen, Mr Steve Arraj and Mr Robert Peterson.
The Trust itself was eventually wound up and its functions transferred to NSW Government water agencies, but the handbook continues to be referenced by councils across Western Sydney as the technical standard for OSD design.
What the handbook actually covers
The handbook sets out the design parameters, discharge curves and storage tables that engineers use to size OSD systems for the catchment. It provides the formal method for calculating Site Storage Requirement and Permissible Site Discharge, including adjustments for HED.
Site Storage Requirement and Permissible Site Discharge
PSD is the maximum allowable discharge leaving the site, expressed in litres per second per hectare or in litres per second for a specific lot. SSR is the minimum tank volume needed to keep discharge at or below PSD across the design storm range.
Across Sydney, common SSR values fall between 300 and 500 m³ per hectare. Under the UPRCT method with full HED, SSR can sit around 470 m³ per hectare for off-line storage. Where full HED is not achieved, or PSD is reduced, SSR has to be adjusted upward.
HED design parameters under the handbook
The handbook gives specific values for site reference discharge and site storage requirements at both lower and upper design events. Engineers use these values directly in the calculation sheet that accompanies the handbook to confirm tank sizing.
Which NSW councils were early adopters?
The handbook was developed in conjunction with four foundation councils inside the Upper Parramatta River catchment. These four adopted a common OSD policy and were the first to mandate the UPRCT method.
The four foundation councils
The Hills Shire Council, Blacktown City Council, Holroyd City Council and Parramatta City Council each formally adopted the UPRCT method for areas within the catchment. The shared policy meant that engineers working across these councils could apply a consistent design approach. The City of Parramatta still hosts the fourth edition of the handbook on its website.
How Council boundary changes affected the policy
The 2016 NSW council mergers reshaped local government in the catchment. Baulkham Hills became part of The Hills Shire Council. Holroyd was absorbed into Cumberland Council, with parts moving to Cumberland City Council and the City of Parramatta. Parramatta City Council expanded into the City of Parramatta. Blacktown City Council remained largely unchanged.
Despite the boundary shifts, the UPRCT OSD method survived. Successor councils kept the handbook in their stormwater controls, often referencing it directly in their development control plans.
Where the UPRCT method is enforced today
City of Parramatta, The Hills Shire, Cumberland and Blacktown still require OSD design under either the third or fourth edition of the UPRCT Handbook for sites in the Upper Parramatta River catchment. Other Sydney councils outside the catchment, including Ryde, Hornsby, Liverpool and Bayside, use their own stormwater policies but draw on similar two-stage outlet and HED concepts.
How OSD compliance affects your Subdivision Works Certificate
OSD is a routine reason a Subdivision Works Certificate is delayed. Across NSW Councils we work with at Southwell Certifiers, the same issues come up at review.
What the certifier checks
The Subdivision Certifier confirms that the design reflects the council’s stormwater policy, that orifice sizes, tank volumes and HED parameters match the calculation report, and that constructed elements match the approved drawings. Inspection covers concrete pour records for the HED pit, orifice plate dimensions, pipe inverts and the discharge connection.
Common reasons OSD compliance fails review
Orifice plate too thin or wrong material. Stainless steel and 3 mm thickness are not optional in most councils.
HED pit precast where cast in-situ is required. This often gets picked up at the construction stage and requires demolition.
Incorrect SSR calculation. Designs that do not reflect the actual catchment area or impervious coverage trigger amended drawing requests.
Missing maintenance provisions. Silt traps, screens and access covers are routinely inspected before the certifier signs off.
Frequently Asked Questions
What is the difference between a dual orifice and single orifice OSD?
A single orifice OSD uses one fixed-diameter outlet to control discharge from the tank. A dual orifice OSD uses two outlets at different invert levels, with the lower outlet controlling minor storms and the upper outlet engaging during major events. Dual orifice designs match predevelopment flows more accurately across the design storm range.
What is a high early discharge pit?
A high early discharge (HED) pit is a discharge control device that releases the full permissible site discharge to the council drain as soon as runoff arrives, and only diverts excess into the OSD tank once the public system is at capacity. To qualify for full HED benefits, at least 85% of the catchment must drain directly to the pit and the pit must be cast in-situ concrete.
Is the UPRCT Handbook v4 still used in NSW?
Yes. The Upper Parramatta River Catchment Trust was wound up, but the fourth edition of the OSD Handbook from December 2005 remains the technical reference for OSD design across Western Sydney. City of Parramatta, The Hills Shire, Cumberland and Blacktown councils all reference the handbook in their stormwater controls.
Which Sydney councils require OSD under the UPRCT method?
The four foundation councils were Baulkham Hills, Blacktown, Holroyd and Parramatta. Following the 2016 council mergers, the UPRCT method continues to apply through their successor councils, namely The Hills Shire, Blacktown City, Cumberland and the City of Parramatta. The method applies to sites within the Upper Parramatta River catchment.
Does my OSD design need a approval with SWC?
Yes. For DA-approved subdivisions, OSD construction is reviewed and inspected under a Subdivision Works Certificate. The Certifier confirms compliance with the Council’s stormwater policy and approved drawings before signing off. Without the SWC, the Subdivision Certificate cannot issue and the lots cannot be released.
Conclusion
Dual orifice OSD design is the practical solution to a real engineering problem. A single fixed orifice cannot handle the full storm range without compromising on either minor or major event control. Two-stage outlets, often paired with HED pits, deliver tighter compliance, smaller tanks and faster certifier review. The UPRCT Handbook fourth edition still drives OSD policy across Western Sydney, and getting your design aligned with it from the start saves time at every stage of the approval pathway.
If you are planning a subdivision in NSW and want clear advice on the certification pathway for your OSD and civil works, Southwell Certifiers can help. To discuss your project and receive a no-obligation fee proposal, contact us on (02) 8734 5676, email admin@southwellcert.com.au, or request a fee proposal.
