Adapting to climate change: revising our approach to estimating future floods, Discussion Paper for the Australian Climate Change Adaptation Research Network for Settlements and Infrastructure.

Abstract

It is becoming increasingly clear that human‐induced climate change will impact on almost all facets of the hydrological cycle. Modelling and observational studies are finding evidence of change at the planetary scale, including large increases in atmospheric water vapour; changes to various circulation patterns resulting in shifts in the spatial distribution of precipitation; an increase in the frequency and intensity of extreme precipitation events; an increase in evaporation and changes to soil moisture; and the melting of snow and ice and an increase in ocean heat content which both are causing mean sea levels to rise [see detailed review in Bates et al., 2008]. At least in terms of the global scale, such changes support the expectation of an increase in flood risk. Despite this evidence there is considerable uncertainty about: (1) the absolute magnitude of change to key flood‐producing variables such as extreme rainfall; (2) the much more significant changes of continental and regional‐scale hydroclimatology which are masked by global averages and which may not be as well simulated by general circulation models; and (3) the role of physiographic catchment characteristics in decreasing or augmenting flood risk at the local scale. Thus, while it is now widely accepted that stationarity – the assumption that the future climate will mirror the past climate – may no longer be regarded as the ‘central, default assumption in water‐resource risk assessment and planning’ [Milly et al., 2008], the identification of an alternative framework for flood estimation remains elusive. The objective of this discussion paper is to describe some of the principal issues associated with accommodating climate change into Australian flood estimation practice. Given that the vast majority of Australia’s historical effort to manage and mitigate flood risk has been underpinned by the assumption of stationarity, the development of an alternative framework that accounts for future changes will form a critical input into how we adapt to future risks. The emphasis of this paper is therefore on summarising the science linking climate change to flooding, and reviewing some of the tools which can be used to quantify future flood risk, as these will form a necessary input in any future adaptation effort. A brief overview will also be provided of official guidance that is currently available on incorporating changes to rainfall and sea level in planning and design, although it should be noted that much more detailed guidance is expected as part of the upcoming revision to Australian Rainfall and Runoff (ARR http://www.ncwe.org.au/arr/index.html). There are a range of broader questions related to how to adapt to the changes that might take place, including the relative roles of engineering solutions (such as levees, reservoirs, storm water drainage, seawalls and so on), planning controls (such as land‐use zoning), and numerous other potential tools that can aid in reducing or better living with flood risk. Further details on these issues will be reserved for future discussion papers. However as this paper will argue, any research on adaptation options must be done in parallel with research which improves our understanding of the hydrology which causes floods to occur in the first place, to ensure that adaptation efforts are deployed wisely and make the best possible use of finite resources in addressing this complex problem. This paper is largely drawn from a discussion paper entitled “Implications of Climate Change on Flood Estimation” [Westra, 2010], which was prepared on behalf of the Department of Climate Change and the ARR technical committee, and was designed to inform the current round of ARR revision projects related to incorporating the impacts of human‐induced climate change into flood estimation practice. That paper was presented at a workshop in Sydney on 30 November 2010, and is currently assisting to inform the next phase of research into how flooding might change under a future climate.