New ARR guidelines are a wake-up call to prepare for the coming storms

The Australian Rainfall & Runoff (ARR) guidelines are crucial for anyone involved in designing and managing infrastructure that is subject to the effects of rainfall and runoff. The recent update to version 4.2 introduces the latest climate science to help practitioners account for observed and predicted changes to storms due to a changing climate.

Understanding the context of climate change

Global temperatures are rising, and Australia is experiencing drier conditions between April and October. We are also experiencing changes to the magnitude and frequency of rainfall and runoff events.

Historically, ARR relied on the assumption that the climate was stationary. However, with the latest ARR releases, climate change considerations were introduced and refined. Version 4.2 consolidates a meta-analysis of climate science for rainfall and runoff, establishing a new procedure for accounting for these changes in our designs.

Key messages from the new guidelines

The fundamental takeaway is that historical rainfall data can no longer be used to estimate future storms and floods. Instead, these observations must be adjusted to reflect the impacts of rising global temperatures. A secondary message is that climate change introduces an increasing level of uncertainty that we must navigate.

Several approaches are used by hydrologists to estimate runoff, including flood frequency analysis and continuous simulation. However, there are no agreed methods for adapting these for climate change. Version 4.2 introduces a new method for adapting rainfall and runoff parameters used in event-based simulation, which we must now integrate into our practice.

Significant changes in design storms

Design storms are statistical representations of historical storm events, usually represented as Intensity-Frequency-Duration (IFD) curves. In Australia they were last updated in 2016, from weather station data dating back to the 19th century.

The warmer atmospheric conditions we are experiencing can hold higher levels of moisture, leading to significant changes in rainfall intensity. As global temperatures increase further, so does the likelihood and magnitude of storm events.

The new guidelines present an approach to convert historical rainfall depths into climate change-adjusted values, for different global temperature change projections. They also provide guidance on dealing with runoff changes associated with drier antecedent soil conditions and sea level rise along our coastline.

The modified rainfall depths are alarming, with increases up to 88% by the end of the century, and an uncertainty band up to 170%.

The frequency of storm events is increasing. By 2070, under Shared Socioeconomic Pathway 3 (SSP3), the historical 1 in 100-year 1-hour storm will occur as frequently as 1 in 13 years in Perth and Brisbane, and 1 in 20-years in Melbourne.

The magnitude of storm events is also increasing. Under SSP3, the 1 in 100-year 1-hour storm in 2070 will have the same magnitude as the historical 1 in 600-year storm in Perth, and similar orders of magnitude in other capital cities.

The frequency and depth of short duration storms are predicted to increase more than long duration storms.

Impacts on infrastructure

These changes in frequency and magnitude will have profound implications for the design and resilience of our infrastructure. No infrastructure exposed to rainfall and runoff will be safe, but certain infrastructure classes will be affected differently.

Small catchment assets like stormwater collection and conveyance systems will be affected more than large catchment assets like dams, waterway bridges and culverts, and flood levees.

Community expectations on the performance and serviceability of infrastructure have increased over time whilst performance and serviceability could be declining due to climate change. The ARR guidelines serve as a crucial reminder to address this growing disparity.

Preparing for climate impacts

We must take proactive steps to prepare for climate impacts on both existing and new assets. For existing assets, undertaking a risk assessment can help prioritise assets requiring intervention.

For existing assets requiring intervention or new assets, we’re faced with at least five choices. If we live in a state of climate change denial, we will simply design our assets for the historical design storms. At the other end of the spectrum is the climate change pessimist who thinks we are on the worst-case global warming trajectory and need to design our assets to be ready for this eventuality.

I recommend the middle ground, where we consider a range of possible futures and design our assets to be resilient or adaptable over time. An example of a resilient asset is a road that floods more often but isn't damaged and is trafficable immediately after the flood. During the flood there might be an alternative route. An example of an adaptable asset is an urban detention basin with space set aside for future expansion if needed, or future housing if not needed.

Final thoughts

The new ARR guidelines mark a significant shift in how we approach infrastructure design and flood management. By integrating these changes, we can better prepare for the uncertainties of a changing climate, ensuring the resilience and adaptability of our projects.

If you’d like to chat more about the new ARR guidelines and the potential impact climate change can have on infrastructure, please reach out so we can catch up.

You can find the ARR Guideline (4.2) here. https://www.arr-software.org/arr-web/

And if you are particularly interested in how the guidelines impact Western Australia Stormwater networks, you can find the webinar presentation I delivered to Stormwater WA last year on this topic here.

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