90 Years on from the Yarra flood – Governance lessons for Melbourne

Last Sunday marked 90 years since the 1934 Yarra flood in Melbourne, the most significant flood to have occurred since colonisation along the river. The flood inundated suburbs throughout the city, and in the wider catchment, with significant impacts to utilities, infrastructure, and the economy. 6000 people were made homeless and 36 people sadly lost their lives. With a catchment of just over 4000km2, large floods are undeniably a hazard. Rarity compounds complexity, and to mark the anniversary, Climate Resilience has applied three of the most frequent governance learnings from our learnings database; culture, hazard understanding, and stakeholder integration, to assess a potential sequence of events were the rainfall to be repeated.

Culture: Assumptions and hearsay deeply influence beliefs, and therefore culture of disaster planning. The 2023 dam failure in Norway, and the 2017 Oroville Spillway failure in the US are two from an extensive list where influence of “myth and legend” led to bad outcomes. Less discussed is the broader societal cultural influence, which is equally important. Take Brisbane; political discourse on Wivenhoe Dam and its ability to protect the city, a lengthy drought, backed up by years of floodplain development, meant there was genuine surprise that a river with a catchment area of 7000km2 would flood in 2011. Myth and legend had crept over 35 years. Brisbane was unprepared.

The Yarra context is similar to Brisbane; references to a wide, engineered flood plain that “protects” the city appear in online literature, and will be interpreted accordingly by many. Much of the engineering on the floodplain, although not all, took place prior to 1934. In addition, references in many online summaries focus on 2-day rain event whereas the rainfall occurred in less than 24 hours according to contemporaneous media. The 2-day total comes from manually read Bureau rain reports which are made at 9am each day. If a rain event continues past this time, recording occurs over 2 days. This short rain event, combined with the heaviest rain in mid-catchment areas (see Figure) demonstrates why Melburnians saw major flood impacts in less than 24 hours in 1934.

Public interaction with flood risk information is generally through frequency for land use planning, or insurance, such as the 1:100 annual chance flood. Such estimate's are problematic for disaster planning; their accuracy relies on the assumption that rainfall and flood frequency are the same, an assumption the national guidance advises is untried and untested.

Why is this important? Belief sets expectation, which in turn influences response as we saw in the Maribyrnong floods in 2022. Surprise generally turns to anger. It should be widely expected that a large flood on the Yarra will occur again, that it will unfold quickly, and significant impacts are possible. Is this the belief of the community in Melbourne?

Threat/hazard understanding: The weather maps this weekend were not dissimilar to 1934.? This gives us a contemporary insight into the broad set up - a deep low with rain areas fueled by tropical moisture dragged south. Such a set up is a risk for Victoria, as the climate effectively shifts to sub-tropical. However, at first glance the 1934 event doesn’t initially seem much to write home about. It doesn’t feature in records for top 3 daily, monthly, or annual totals for Victoria, but delving deeper, it still occupies 7 of the top 10 spots for daily rainfall

records in December as of 2023 (see table) This tells us two things; it was, and remains unusual for the time of year, and the area receiving the heaviest rain was widespread. ?In contrast, the 2011 flood event doesn’t rank in any of the top 10 rainfall lists for Victoria, annually or monthly. In short, nothing comparable has impacted the state since and understanding it's peculiarity is vital to planning for any repeat.

Underpinning all of this is climate change. A warmer world is a wetter world and means any future event will likely be more intense. The data below shows the climate has already warmed in Victoria. Simply adding up all the mean maximum temperatures shows a clear trend for the data at Wilson Prom. This means our understanding of hazard is reduced further, and the ability to accurately predict is diminishing, something the Bureau has already noted with respect to macro trends.

The tropics marching south, engineering assumptions, and climate change presents significant downside in the understanding of risk for Victoria. However, given we're discussing the 90th anniversary, it is correct to say the chance of such flood is clearly infrequent. The understanding of the threat is mixed but a flood that's very different from those considered for planning appears likely when it does occur.

Stakeholder Integration: Lessons from 2023 Auckland floods, and the recent floods in Valencia demonstrate the importance of clear roles and responsibilities assigned to planning and decision making. In the case of the Yarra catchment, review of available documentation suggests things can be improved. The Bureau service level specification (SLS) shows data agreements with Melbourne Water remain outstanding, and the references in State Emergency Service (SES) documentation to Bureau predictions at Spencer Street are not reciprocated in the Bureau SLS or the Bureau website where the Spencer Street gauge is not displayed, a gauge Melbourne Water refer to as Southbank. This is important as if decision makers are not clear, the public certainly won’t be when seeking information.

90 years on, Melbourne is a different city. The importance in learning lessons from others experience of natural hazards is growing in importance globally and can help planning, especially where events are outside of lived experience. We can say a flood on the large Yarra has the ability to unfold quickly, is likely to be fueled by tropical moisture, with confusion during the event for the public, who may well be surprised. Based on the outcome of similar occurrences elsewhere during significant events, a reputational crisis seems likely if a similar, or larger flood occurred today.

James is a non-executive director, former exec for dam safety in Qld, and founder of a natural hazard governance and advisory consultancy. www.climres.com.au/learnings-database/