Brisbane Floods Offer Opportunity to Reassess Adaptation Options with Climate Change

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CLIMsystems Ltd has just completed a preliminary assessment of the extreme rainfall event that led to the recent Brisbane floods. The work included an assessment of the 1974 flood which is often referred to as the most extreme event in living memory. The 2011 event is compared to the 1974 event and is also explored again over 100 years of rainfall records for this part of Australia and with climate change models in the SimCLIM software system.

Key features of the Brisbane River catchment including sub-catchments, location of key rainfall stations and dams.

CLIMsystems Ltd has just completed a preliminary assessment of the extreme rainfall event that led to the recent Brisbane floods. The work included an assessment of the 1974 flood which is often referred to as the most extreme event in living memory. The 2011 event is compared to the 1974 event and is also explored again over 100 years of rainfall records for this part of Australia and with climate change models in the SimCLIM software system.

The recent floods in Brisbane have led to heavy financial, social and political costs. To put past events in context one can easily see that the macro management of the Brisbane catchment has been one of flood, inquiry of a sort, and then implementation of a potential mitigation strategy. The flood of 1893 led to the completion of the Somerset dam in 1953. The flood of 1974 led to the construction of the Wivenhoe dam in the early 1980s. While the clean-up continues and the cost of the floods is tallied, the powers that be will be thinking ‘how can we minimise the risk of such an event happening again’. The answers will not be easy to find nor will they be free from debate and difficult political decisions. Queensland Premier Anna Bligh has commented that the year-long commission on the floods will examine all aspects of the flooding including the timing of water release from dams, landuse planning, and warning systems. The importance of such an inquiry cannot be understated. The public wishes to better understand what happened, what the underlying events were that led to so much damage and disruption to so many people’s lives. While looking back at what has happened is important, what is equally vital is exploring the range of scenarios that could shape the future. Few would argue that the ways of the past, however they are found to have operated, my not have a place in current planning and decision making in light of the recent extreme event. For one thing, the evidence seems to be mounting that the underlying cause of the floods – an extreme rainfall event – is part of important changes in the global, regional and local physical environment that must be seriously considered when defining mitigation measures and plans for the future.

Floods have always occurred and the rainfall events that drive them will continue to impact Queensland and much of Australia. The cyclone season is now upon the nation and Queensland is staring at another cyclone for the Northern part of the State this week. What should be considered are the implications of more extreme rainfall events for a range of interrelated environmental, social and economic services. For the case of Brisbane, the inquiry could consider the state of the land cover over the entire basin, the relationships between landcover, hydrological change and catchment response to various types of rainfall events both in terms of timing, for example, short and very severe but often times highly localized such as the terrible Toowoomba flash flood which coincided, in this case, with a multiday event of seven days duration. Each extreme rainfall event has different characteristics and hence requires careful consideration for the types of response to be advocated. Similarly, and as noted, there is a propensity to respond to flood events through the building of durable infrastructure such as dams. This may indeed be a logical response to the most recent floods, but durable infrastructure of this sort is often designed to provide a hundred or more years of service. In such cases the science of climate change should without doubt now be brought to bear on such decisions.

Let us look at the current extreme rainfall event in the context of history and a climate changed future. It is generally agreed the recent Brisbane catchment rainfall and flooding event were catastrophic with loss of both life and assets. Through the application of software modeling tools developed by CLIMsystems in New Zealand we were able to examine the 1974 and 2011 events and to draw some important findings. First of all the 1974 and 2011 floods were driven by very different types of rainfall events. The 1974 floods were related to cyclone Wanda that led to heavy rainfall in the eastern and southern portion of the catchment over a four day period. The event of 2011 was of seven days duration and led to rainfall being concentrated in the upper two-thirds of the Brisbane catchment. We have discovered that rainfall over this part of the catchment over the seven days was about 40 percent greater that the rainfall over the same area over a shorter period in 1974. In the case of eight rainfall stations we examined that are well distributed across the upper part of the catchment seven had rainfall over the seven day period that exceeded what would be expected to be a one in 50 year rainfall event. More interesting is the fact that when the most recent event and the last thirty years of extreme rainfall events are looked at in the context of over 100 years of rainfall records, such extremes have increased in magnitude. For many of the 17 rainfall stations we examined there has been an important and unmistakable shift in frequency of extreme rainfall events. For example, expected return periods for the extreme rainfall events at Blackbutt Post Office for a seven day rainfall event of 439 mm shifted from 128 years, for the entire rainfall record that started in 1900, to just 67 years in the most recent thirty year period. Similarly, the Lowood Don Street station showed a similar shift from records that began in 1887 from where an extreme event of 550 mm, which would normally be expected on a return period 258 years, narrowed to just 69 years in the last thirty years. We explore these records in 30 year or longer blocks in order to be statistically robust in our analysis.

When we turn to the future and the potential impact of climate change we see a continuation in the trend toward more extreme events. Our preliminary analysis which is based on the application of twelve general circulation models (GCM) run as an ensemble (run all together and their means collated) and for a worst case scenario with a slightly less than worst case scenario for greenhouse gas emissions through to 2100 shows continued intensification of extreme rainfall events. For a seven day event in 2100 the models show up to a quarter more rainfall could fall – just think about what effect this would have on the high-water mark during a flood like that just experienced in Brisbane, absolute chaos and an even more severe loss of property and life. A copy of the preliminary results of the examination of extreme rainfall events as they relate to the recent Brisbane floods and climate change can be found at the CLIMsystems website.

The implications that stem from the likelihood of even more extreme rainfall falling in the future need to be considered by the Queensland flood inquiry. Whatever plans that are made to reduce risk to life and property, whether for short and high intense rainfall events or longer multiday events, should factor in the probability of considerably heavier rainfall events. More modeling is called for so that the range of uncertainty in the possible scenarios of change can be found and brought to the table for consideration. The amount and timing of such extreme events and plans to mitigate their negative effects drives the disasters that ensue. The linkage of climate change patterns with flood and hydrological models, with landuse models, with dam design are all possible. The citizens of Queensland will be expecting that as Premier Bligh has said, no line of inquiry will be avoided in getting to the bottom of the flooding issues faced by Queenslanders. For these reasons we need to insure that climate change is at the forefront of such planning and decision making.

Dr Peter Urich is an Adjunct Associate Professor at the University of the Sunshine Coast, A Research Associatee at the International Global Change Center at the University of Waikato and is the Managing Director of CLIMsystems, a New Zealand-based company that specializes in climate change risk assessments and modelling and maintains a staff of highly qualified climate scientists. The company has developed methods for examining extreme climatic events such as the rainfall associated with the recent floods in Southeast Queensland. As specialists in climate change risk and adaptation the company also has expert capacity for developing scenarios of climate change. The company is known for its innovative and cutting edge scientific approaches and the use of Intergovernmental Panel on Climate Change General Circulation Model outputs for practical application for policy and decision making.


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