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Tuggerah Lake Entrance in flood 25 March 2021 (Photo: Central Coast Aero Club). 

“Flood plains are for floods”: so said Moss Cass, Minister for Environment in the Whitlam Government. The occasion was the devastating Brisbane floods of 1974. He recommended buy back of properties in harm’s way, and yes, a voluntary scheme was put in place. Guess what? More development took place after the Wivenhoe dam was built allowing for a repeat of devastation in the 2011 floods. Protocols in place to manage dam water levels were somewhat confusing between the role of the dam for flood mitigation and water supply.

Turning to March 2021 and again we have a federal government Minister expressing concern over building houses in flood-prone areas, this time along the Hawkesbury-Nepean. Over the last week debate has raged in various quarters about how to mitigate threats to property along this populated stretch of the Hawkesbury-Nepean Valley (HNV), including the role of what is viewed as a multi-purpose dam (Warragamba). But a very long record of flooding informs us of risk even as developments expand into known flood prone areas.

I have been recently diving into a fabulous historical account written by Grace Karskens on the HNV: People of the River—Lost worlds of early Australia (Allen and Unwin, 2020). Chapter 8 is all about “floods and flood mindedness” although in Chapter 1 on “old land, first people” a brief geomorphic history is presented (with good reference list; see also Appendix 4 on record of floods 1780-1821). Her book notes how the region’s Aboriginal people were long accustomed to floods and told settlers what happened before they arrived and dispossessed them of their lands. On page 257 she writes of an observation in March 1817 of people on board a ship off the NSW coast; they saw a strange sight: “Around 6 miles (almost 10 kilometres) out from land, a stack of wheat carrying several dead pigs rocked on the open ocean” Similar observations were made in August 1819 (p.257). She goes on to say, “the bobbing flotsam represented disaster for hundreds of Hawkesbury-Nepean River settlers” Similar observations are being made this past week of floating objects exiting east coast rivers in the deep brown buoyant plumes and being washed up on beaches, and as in 1817 and 1819 and later years, there are disastrous consequences to those who live today in this valley.

In July 2019, a new Hawkesbury-Nepean Flood Study was released: it is available on the NSW Flood Data Portal https://flooddata.ses.nsw.gov.au/flood-projects/hawkesbury-nepean-valley-regional-flood-study. This analysis builds on previous flood studies and provides detailed behaviour of floods past, present and future that should assist flood managers. In the introduction it states that “the historical flood records point to a pattern in which there are multidecadal periods with frequent and large floods interspersed with similar lengthy periods with infrequent and small floods” (p.v). In the report information is available on the magnitude of floods of past events. It appears that the 1988 flood event is of similar magnitude to the event of the past week. Detailed modelling of rainfall to runoff to flood levels is contained in this 2019 Flood Study as well as projections due to climate change, including effects of sea-level rise in lower reaches. One conclusion is that an increase in rainfall due to climate change of 9.1% would result in 1 in 100 Annual Exceedance Probability (AEP) with flood levels at Windsor increasing by the order of 0.71m (p.116).

Many questions arise in considering how settled flood plains such as in the HNV are to be “managed” in the future. Media have been deluged with opinions this week. There is considerable political and community attention on issues such as the height of the Warragamba Dam wall to mitigate flood risk, to improving evacuation routes, and even to buy back of properties in highly vulnerable locations. Most of these “options” have been canvassed in the past. But decision makers will continue to be confronted by an array of conflicting interests compounded by potential impacts and uncertainties associated with more extreme events exacerbated by global warming.

I am tempted to outline just a few of the complexities that must be integrated into future planning studies for flood mitigation in the HNV. There is one certainty: major floods of the scale of 2021 and bigger will occur in the future. Geomorphic, Aboriginal, and historical records attest to recurrence of extreme events. Even though the 2021 event involved the rather odd interplay of three weather systems, they were not of an intensity that could be foreseen as occurring with a “super storm” as the warmer atmosphere stores more water. We know enough about how La Nina periods can lead to clusters of storms which can allow soaking of soils prior to the culminating catastrophic event such as we saw on the NSW coast and flood plains in 1974. Should flood forecasting take more note of the emergence of such La Nina events and allow for water supply managers to lower dam water levels in advance to reduce overflow? Are we confident enough in the science to take such a step especially if the water supplies from desalinisation plants is sufficient to cover any shortfalls?

There remains an array of institutional problems that challenge flood managers, governments, and affected communities. In no specific order, one that has long been on my radar is the role of the federal government in assisting states and local councils in disaster mitigation. In 1976, I was involved in organising an Academy of Science, Academy of Social Science, Institute of Australian Geographers symposium on Natural Hazards in Australia (published by the AAS, Canberra, 1979). Minister Jim Killen wrote the foreword and optimistically stated his hope that its results “will make a very real contribution to our attitude towards the natural disasters that are a fact of Australian life”. One of the results detailed in Appendix 24.3 were a set of “policy suggestions” (p.355). In essence they point to a more formal role for the Commonwealth in disaster mitigation. Many organisations have advocated greater pre-disaster support since that symposium. Up until the recent Bushfire Royal Commission little action has been taken in that regard. Federal post-disaster assistance dominates the role of this level of government leaving planning to the states (for more details see Productivity Commission Report No 74, 2014 on Natural Disaster Funding Arrangements). One can only hope that recent initiatives in development of “resilient services” by the Commonwealth will provide more support to states dealing with what Killen so aptly put as disasters being a “fact of Australian life”.

Another institutional problem that concerns me is the way previous NSW governments, going back to the early 1970s, have placed development controls on a certain “flood planning level” including “once in a century” storm event (1 in 100 or 1% probability). These terms and their implications have been well outlined in an opinion piece put out by Andy Pitman and others in the UNSW Newsroom (24/3/21). But critical to planners at council and state levels is the issue of what controls to place over development in areas above that level up to the Probable Maximum Flood in the climate change era? Information in the recent HNV Flood Study makes one acutely aware of this issue.

 A further problem that attracts the attention of those like me involved in hazard risk assessment, is insurability of property in vulnerable locations such as flood plains. Karl Mellon has written extensively on this topic and predicts huge increases in uninsurable addresses in future decades along the east coast. Insurers will be well aware of the science. Andrew Hall, Insurance Council of Australia, on ABC RN (23/3/21) discussed these concerns as developments continue to spread in flood prone lands. Our governments do not provide socialised home insurance in ways seen in other countries. We tend to adopt the “moral hazard” approach offering support post event. Again Appendix 24.3 in the 1979 Academy report offered the Commonwealth a policy “suggestion” for a National Disaster Insurance Scheme; this was quickly rejected. Even private insurance gets very messy when damage claims arise from tidal inundation coupled with flooding. Insurers are not keen on covering actions of the sea. Thus the more risk prone we become, then the need for more integrated federal-state disaster mitigation action is required to address such problems.


Bruce Thom

Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2021. For correspondence about this blog post please email austcoastsoc@gmail.com