A tank that is too small can transfer flood risk downstream. A tank that is too large can consume valuable development area, add unnecessary construction cost and still fail approval if its outlet is not correctly controlled. To size onsite detention tanks properly, the design team must establish the site’s pre-development and post-development hydraulic response, then demonstrate compliance with the applicable authority requirements.
Onsite detention, usually referred to as OSD, is not simply a storage-volume exercise. It is a controlled discharge system. Its performance depends on the relationship between catchment runoff, available storage, outlet hydraulics, overflow arrangements, maintenance access and the downstream drainage network. A defensible design accounts for all of these elements from concept through to as-built verification.
What determines onsite detention tank size?
The required OSD volume is generally driven by the increase in runoff created by development. Replacing pervious ground with roofs, driveways, hardstand and paved areas reduces infiltration and causes runoff to reach the drainage system faster. The detention system temporarily stores part of that additional flow and releases it at a restricted rate.
Local council or consent authority requirements establish the design basis. In Sydney, Newcastle, the Central Coast and other NSW jurisdictions, OSD requirements can differ materially between local government areas and individual development consents. In Queensland, requirements may instead be framed through local planning schemes, flood criteria and lawful point-of-discharge conditions. The correct standard is the one applying to the site, not a generic calculation reused from a previous project.
The primary inputs usually include the site area, surveyed levels, existing and proposed impervious areas, catchment boundaries, rainfall data, downstream drainage capacity and nominated design storm events. The allowable site discharge is then compared with the estimated post-development runoff. The gap between those two flow conditions informs the storage and discharge-control design.
A tank may be below ground, integrated into a car park, formed as an above-ground basin, or distributed across several storage elements. The most suitable arrangement depends on site constraints, constructability, maintenance obligations, flood behaviour and the authority’s acceptance criteria. Storage volume alone does not establish compliance.
Start with the correct hydrologic model
A reliable OSD design begins with an accurate representation of how water moves across and through the site. This requires more than measuring the development footprint from an architectural plan. Levels, overland flow paths, roof drainage connections, upstream runoff, legal points of discharge and downstream surcharge risk all influence the outcome.
For many developments, hydrologic and hydraulic modelling is undertaken using recognised methods and software appropriate to the authority requirements. DRAINS is commonly used to model minor drainage systems and detention behaviour, while other modelling tools may be necessary where site flooding, major overland flow or complex downstream interactions are relevant. MUSIC may support WSUD and water-quality treatment design, but it does not replace hydraulic assessment of an OSD system.
The model should test the critical storm durations rather than assuming one rainfall duration will always govern. Short, intense storms can control peak discharge. Longer events may control storage volume, particularly where outlet flows are heavily restricted. A design based on only one assumed duration may appear efficient on paper but underperform during the actual critical event.
Pre-development assumptions require the same level of scrutiny as post-development inputs. If existing site conditions, runoff coefficients or catchment areas are overstated or understated, the permissible discharge target may be incorrect. That can create approval delays, redesign costs or a system that does not deliver the required flood mitigation outcome.
Do not overlook tailwater and downstream constraints
The outlet from an OSD tank does not operate in isolation. If the receiving pipe, pit, kerb outlet or downstream system surcharges during a storm, water levels at the discharge point may rise. This tailwater condition can reduce outlet capacity and cause detention storage to fill differently from the original calculation.
Where downstream capacity is constrained, the design may require a revised outlet configuration, additional storage, separate major-flow management or a broader drainage solution. It may also expose a limitation in the existing network that needs to be addressed through the development approval process. Ignoring tailwater is not a conservative shortcut. It can invalidate the assumed discharge performance.
Storage volume and outlet control must work together
An OSD tank performs as a system of connected components. Storage is provided within a defined operating level range, while an orifice, weir, pipe or proprietary flow-control device limits the rate of discharge. As water levels rise in the tank, the hydraulic head at the outlet changes. The design must demonstrate that this relationship does not exceed the allowable discharge during the nominated storm events.
The outlet is often the most sensitive part of the design. A small change in orifice diameter can materially alter flow rate. An outlet that is too restrictive may require excessive storage or lead to prolonged drawdown. An outlet that is too large can exceed the permissible discharge rate and shift flood risk beyond the property boundary.
Practical operation also matters. Very small orifices may be hydraulically effective in theory but vulnerable to blockage from sediment, leaf litter or debris. This is particularly relevant for commercial sites, industrial hardstand and developments with landscaped catchments. The designer must balance hydraulic compliance with maintainability, and may need to incorporate screening, sediment controls, access provisions or a different outlet arrangement.
Freeboard should be included above the maximum design water level to manage uncertainty, wave action where relevant and performance during more severe events. The tank must also have an identified overflow path that directs water away from buildings, electrical infrastructure, vehicle access points and neighbouring properties. No detention system should rely on an assumption that the design storm is the largest event the site will ever experience.
Designing for construction and long-term access
An approved design can still become an underperforming asset if construction changes the tank geometry, pipe invert levels, outlet dimensions or access arrangements. OSD construction requires clear set-out information, nominated levels, details for flow-control structures and inspection requirements. Substituting materials or altering pipework without reassessing the hydraulic model can compromise compliance.
For underground tanks, access is a central asset-management issue. Maintenance personnel need safe, practical entry points or inspection access to sediment collection areas, screens, outlet structures and pumps where installed. If these components cannot be inspected and cleaned, the detention function will degrade over time.
The design should define who is responsible for maintenance after handover. For strata, commercial, industrial and institutional sites, this should be supported by an operations and maintenance plan, asset register, inspection frequency and documented cleaning requirements. A system installed beneath a car park may be out of sight, but it remains a regulated drainage asset with a performance obligation.
Verification protects approvals and future liability
Before practical completion, the constructed OSD system should be checked against the approved design. This may include as-built survey information, verification of storage dimensions and levels, confirmation of outlet sizes, inspection of overflow routes and evidence that access provisions have been installed as documented.
This process is especially valuable where future ownership will transfer to a body corporate, facility manager or institutional asset owner. Clear records establish what was approved, what was built and how the asset is intended to operate. In disputes involving flooding, drainage failures or alleged non-compliance, that evidence can become critical.
Common reasons OSD tank designs fail
The most frequent failures are not always dramatic structural defects. They are often design and documentation gaps that become apparent only during approval review, construction or a significant rainfall event. Common examples include:
- using generic council criteria rather than the site-specific approval requirements
- omitting upstream or external catchment flows from the model
- sizing storage without verifying outlet hydraulics across critical storm durations
- installing an outlet that is inaccessible or prone to blockage
- failing to provide a safe major-flow overflow path
- treating the tank as a one-off construction item rather than a maintained asset.
Each issue can be avoided through coordinated drainage design, hydraulic modelling, construction oversight and planned maintenance. The earlier these disciplines are brought together, the less likely the project is to face late-stage redesign or conditions that cannot be practically delivered on site.
When standard tank sizing is not enough
Complex sites require a broader assessment. This includes developments with basement levels, constrained legal discharge points, flood-affected land, large industrial roofs, contaminated runoff, redevelopment of ageing assets or known downstream drainage limitations. In these cases, OSD may need to be integrated with flood modelling, WSUD treatment, pump-system assessment, civil works and compliance documentation.
Forensic investigation may also be required where an existing detention system has contributed to flooding, has unknown capacity or differs from available drawings. Measuring the existing asset, reviewing approvals and modelling its actual hydraulic response can establish whether the issue is inadequate storage, blocked controls, incorrect levels, downstream surcharge or a wider catchment problem.
The right detention volume is not the biggest tank that fits within the site. It is the storage and control arrangement that meets approval conditions, protects downstream assets, can be built accurately and remains inspectable throughout its operating life. Treating OSD as a managed infrastructure asset from the first design decision gives project teams a far stronger basis for compliance and long-term performance.
