Climate Change Factors in Stormwater Design: What NSW Expects

A few years ago, a stormwater report for a duplex or a small subdivision in Sydney rarely mentioned climate change. Today it is increasingly common to see a DA condition or a council RFI asking the engineer to demonstrate that the drainage design has considered future rainfall intensification — or to model the 1% AEP storm with a climate change uplift applied. For builders and developers, this can feel like a moving target: a design that sailed through one council gets questioned by the next, and nobody on site is quite sure what “climate factors” actually means for a pipe or a detention tank.

The short version is this: the rainfall data your engineer designs to describes the past, the drainage asset you are building will operate for the next 50 to 100 years, and the science says rainfall intensity will increase over that period. Councils, as the long-term owners of drainage risk in their LGAs, are starting to insist that new designs account for it. Here is what the current guidance says, where it is heading, and what it means in practice for your next project in Greater Sydney or regional NSW.

Why Climate Factors Are Turning Up in DA Conditions

Councils approve drainage designs, but they also wear the consequences when those designs are overwhelmed — flooded properties, damaged public assets, and liability questions that surface decades after the compliance certificate was issued. A warmer atmosphere holds more moisture, and the consistent finding across Australian and international research is that short, intense rainfall events — precisely the storms that govern urban drainage and on-site detention — intensify fastest as temperatures rise.

Layer onto that the recent memory of major NSW flood events, updated floodplain risk management practice, and state-level planning policy that pushes climate risk consideration into local decisions, and the result is predictable: development engineers reviewing your stormwater plans are asking what happens to the design when rainfall is 10 or 20 per cent more intense than the historical record. Some councils have formalised this in their DCPs and engineering specifications. Others raise it case by case through RFIs. Either way, it is no longer safe to treat climate consideration as optional on NSW projects.

The Current Guidance: ARR 2019’s Interim Climate Change Approach

Australian Rainfall and Runoff 2019 — the national design rainfall guideline — includes an interim climate change guideline in Book 1. The word interim was deliberate: the authors acknowledged the science was moving quickly and the chapter would need updating. The interim approach works like this:

• Design rainfall intensities are increased by approximately 5 per cent for every degree of projected warming.
• The degree of warming is taken from climate projections under the Representative Concentration Pathways (RCPs) — the greenhouse gas scenarios used in earlier IPCC assessments — for a horizon matched to the design life of the asset.
• The uplifted intensities are then run through the normal ARR 2019 design process: ensemble temporal patterns, pre-burst, losses and all.

In practical terms, the engineer selects a scenario and time horizon — say a higher-emissions pathway out to the latter part of the century for an asset with a long design life — derives the corresponding temperature increase, scales the IFD intensities accordingly, and re-runs the model. The mechanics are straightforward in DRAINS: the uplift is applied to the rainfall inputs and the ensemble is run again, giving a direct comparison between the historical-climate design case and the future-climate sensitivity case.

It is worth being clear about what the interim guidance is not. It is not a precise prediction of future rainfall at your site. It is a defensible, nationally consistent way of stress-testing a design against a plausible future — which is exactly how councils tend to use it.

An Updated Chapter Is Coming

The interim guidance is now being rewritten. The Department of Climate Change, Energy, the Environment and Water (DCCEEW) and Engineers Australia have been running a project since late 2022 to update the climate change considerations chapter of ARR — Book 1, Chapter 6 — with industry consultation through 2023 and into 2024. The updated chapter is expected to be released later in 2024.

While the final content is not yet published, the direction of travel is reasonably clear from the consultation process. The update is expected to align with the IPCC’s Sixth Assessment Report, which moved from RCPs to Shared Socioeconomic Pathways (SSPs) as its scenario framework, and to provide more refined scaling guidance — with the research pointing towards higher intensification rates for short-duration storms than the flat interim figure implies. For urban stormwater and OSD design, where short durations usually govern, that is a significant detail to watch.

What should you do in the meantime? Design to the current interim guidance, document the assumptions clearly, and be aware that projects with long approval timelines may see the guidance change between DA lodgement and construction certificate. A well-documented climate assumption is easy to update; an undocumented one means starting again.

How NSW Councils Differ — and Why You Must Check

There is no single NSW-wide rule for applying climate factors to site drainage, and council practice varies widely:

• Some councils mandate a specific percentage uplift on design rainfall for the 1% AEP event, written into their DCP or engineering specification, particularly for flood-affected land or major storage design.
• Some require a climate change sensitivity assessment — the design is sized on current rainfall, but the engineer must show the system’s behaviour under uplifted rainfall and demonstrate the consequences are acceptable.
• Some reference ARR 2019’s interim guidance generically, leaving scenario and horizon selection to the engineer’s judgement.
• Some are silent — for now. Silence in the DCP does not always mean silence in the RFI, especially on sites near overland flow paths or flood planning areas.

The percentages, scenarios and trigger thresholds are council-specific and change as DCPs are reviewed, so the only reliable approach is to check the current DCP and engineering standards for the LGA — or engage a stormwater engineer who works across Sydney councils daily and knows where each one sits. Assuming the last council’s rules apply to the next council’s site is one of the most common ways projects collect avoidable RFIs.

What Climate Factors Mean for Your Design

OSD volumes and outlets

On-site detention is sized around a permissible site discharge (PSD) and a site storage requirement (SSR). Apply a rainfall uplift and the post-development inflows to the tank increase; if the PSD stays fixed — as it usually does, because it reflects downstream capacity — the storage requirement grows. Whether the council requires the tank to be sized for the uplifted case, or merely checked against it, varies. Either way, it pays to know early: an extra increment of storage is trivial to accommodate at concept design and painful to retrofit once the basement, driveway and landscape plans are locked.

Freeboard and habitable floor levels

Climate uplift raises flood levels and hydraulic grade lines, which flows through to freeboard — the safety margin between design water levels and floor levels or critical infrastructure. On sites affected by overland flow or mainstream flooding, an uplifted 1% AEP event can change minimum floor levels. Discovering that after the architectural section is finalised is an expensive conversation.

Overland flow paths

Every competent design assumes the pit-and-pipe system will be exceeded eventually and provides a safe overland flow path for the excess. Under uplifted rainfall, more frequent and deeper overland flow is the expectation, so path widths, depths and velocity-depth safety criteria deserve a second look. This matters most on steep sites, battle-axe lots and infill development where the flow path threads between buildings.

Pipes and pits

Minor system components are typically designed for frequent storms, and a modest intensity increase can tip a marginal pipe size over the line. The cost difference between pipe sizes at construction is small; the cost of augmenting an undersized system later is not. Where the hydraulics are borderline, sizing for the future case is often the cheaper decision over the asset’s life.

Sensitivity Testing: Cheap Insurance for a Long-Lived Asset

Even where a council does not mandate climate factors, running the model with uplifted rainfall is good practice and inexpensive — the model already exists, and the additional runs take hours, not weeks. The output answers a question every developer should want answered: what happens to this design in the world it will actually operate in?

Sometimes the answer is reassuring — the OSD has inherent headroom, the overland flow path copes, and the design is robust as drawn. Sometimes it reveals a cliff edge: a tank that surcharges into a basement stair, or a floor level with no margin. Knowing which situation you are in costs very little at design stage. A drainage system built in 2024 will still be doing its job in 2074 and quite possibly 2124. Designing it only for the rainfall of the past century is a bet that the next century will look the same — a bet the evidence says you will lose.

For certifiers, documented sensitivity testing also makes sign-off cleaner: a report that states its climate assumptions, shows the uplifted case and demonstrates acceptable performance is far easier to defend than one that never mentions the subject.

Frequently Asked Questions

Is a climate change uplift legally required for stormwater design in NSW?

There is no single blanket mandate covering all development. Requirements come from individual council DCPs, engineering specifications and DA conditions, which vary across the state. Some councils mandate an uplift for certain design cases; others require a sensitivity assessment or are currently silent. ARR 2019’s interim guidance provides the accepted method wherever climate consideration is required, so always check the specific council’s current requirements.

How much bigger does climate change make an OSD tank?

It depends on the site, the uplift applied, the council’s PSD and the storm durations that govern storage — there is no universal percentage. The honest answer comes from running your site’s model with and without the uplift and comparing the storage requirement. On many sites the increase is modest and easily absorbed at concept stage, which is precisely why the check should happen early.

What is the ARR 2019 interim climate change guideline?

It is the current national guidance for incorporating climate change into design rainfall. It scales rainfall intensities by roughly 5 per cent per degree of projected warming, with the warming taken from RCP-based climate projections for a horizon matching the asset’s design life. An updated chapter, developed by DCCEEW and Engineers Australia with industry consultation, is expected later in 2024.

Should I wait for the updated ARR climate chapter before lodging my DA?

No. Councils assess against the guidance current at the time, and the interim approach remains the accepted method until the update is released. The sensible course is to design to the interim guidance now and document assumptions clearly, so the design can be efficiently reviewed if the updated chapter lands mid-project.

Need a stormwater design that will stand up to council climate change requirements? Contrive Consultants prepares OSD and drainage designs across Greater Sydney and NSW with ARR 2019 climate factors and sensitivity testing built in, tailored to each council’s DCP. Explore our stormwater engineering services or get in touch to discuss your project — call +61 497 848 111 or email info@contriveconsultants.com.au.