Update to ARR Version 4.2 – Climate Change Considerations

At JOCES, we are committed to staying in the loop when it comes to industry developments and engaging in relevant discussions that shape the future of infrastructure and urban development.  

Over the last couple of months, our team has actively participated in key events, including the Development Engineering Forum hosted by IPWEA NSW & ACT and the Watercom webinar on Climate Change Considerations in Stormwater Modelling. These discussions have provided valuable insights that help us better serve our clients and support the broader engineering community. 

Key Insights from ARR 4.2 Updates

In addition to the Development Engineering Forum, JOCES also attended a Watercom webinar on Climate Change Considerations in Stormwater Modelling, which focused on the latest updates from Australian Rainfall and Runoff (ARR 4.2). It was conducted by Hossein Ansari, with a Q&A panel led by the CEO and senior executives by Watercom, the creator of DRAINS software.

These updates are crucial for making sure that infrastructure is resilient to the impacts of climate change, and that they reinforce the need for adaptive design approaches in our engineering solutions.

In the discussion, there were THREE findings that our team found very interesting: 

1. Depending on the size of the project, there should be no major impacts expected on pit and pipe sizing. Pit and pipe drainage systems are relatively easy to adjust – sizing up 1 to 2 pipe sizes can significantly increase pipe capacity to accommodate climate change scenarios without significant increases in cost.

2. The main impact of climate change is on OSD (On-Site Detention) design is most apparent in the increased detention volume required. As rainfall intensities rise under future climate scenarios, the volume of stormwater runoff that needs to be captured and gradually released also increases. Although outlet control structures generally remain unchanged, the storage volume must be expanded to accommodate the higher inflow rates. In order to maintain post-development peak discharges at pre-development levels—as defined by the 2016 IFD (1% AEP)—detention basin design volume requirements rise substantially when accounting for future climate projections. These findings underscore the significant and non-linear influence that climate change can have on OSD system performance, particularly under long-term, high-emissions scenarios.

3. For the most conservative design outcomes: Pre-development flow rates are to be designed following the IFD 2016. The Post-development flow rate is to be designed with SSP Factors applied in the rainfall data for long-term detention sizing. 

The ARR 4.2 updates also highlight the importance of integrating Shared Socioeconomic Pathways (SSP) in design, which helps to future-proof stormwater management systems. By adopting these updated guidelines, infrastructure can better withstand the challenges posed by changing climate conditions.

Photo by Pixabay by Pexels

What Does This Mean for Our Clients?

Being a part of these discussions may also be for professional development, but more than that, it’s an intentional effort of making sustainability a part of what we do while serving our clients’ needs. 

For Council

1. It’s more important than ever for councils to update their engineering design specifications to align with the latest ARR climate change guidelines. Outdated stormwater design standards could result in undersized detention basins, creating significant infrastructure challenges in the future. 

2. Flood studies will need to be updated regularly to incorporate the new rainfall data and projections. 

3. Overland flow routes must be planned to ensure detention structures can handle overflow from under-designed systems. 

4. Councils should also consider introducing a transition period as new guidelines are rolled out to help manage the shift in standards. 

For Consultant Stormwater Engineers

1. Consultants should stay informed and collaborate with councils to ensure transparency as new guidelines are developed and implemented. This collaboration will help ensure that stormwater systems are robust and effective in the long term. 

2. Designs must accommodate future climate projections by considering updated basin and tank sizing. 

3. Freeboard allowances should be reviewed to ensure that designs meet the new climate scenarios. 

4. Consultants are encouraged to document and justify the SSP scenario used in their reports, especially for long-term infrastructure assets.  

Continuing Education and Knowledge Sharing

We believe in continuous learning and staying informed on the latest industry developments. Attending events like the Development Engineering Forum and the Watercom webinar enables us to provide our clients with the most up-to-date, reliable information for their stormwater and infrastructure projects. 

For those interested in the ARR updates, we highly recommend checking out these free and accessible tools: 

Watercom’s Climate Change Rainfall Modifier

➡️ https://watercom.com.au/resources/climate-change-rainfall-modifier/

WMA Water’s Climate Change Calculator 

➡️ https://ccc.wmawater.com.au/ 

The insights gained from these events have been invaluable in shaping our approach to sustainable and climate-resilient infrastructure. At JOCES, we are committed to implementing these updates and ensuring that our designs continue to meet the highest standards of quality and resilience. 

We encourage all councils, engineers, and industry professionals to stay informed, collaborate, and adapt to the evolving climate change considerations to ensure that infrastructure projects not only meet current needs but also deliver lasting benefits for future generations.