Natural hazards in Australia: Past and future trends in floods.

The possibility of changing flood risk due to climate change is an issue of concern for engineers, governments and emergency services. However, it is not yet clear how flood risk will change and how these changes may vary across Australia. This presentation approaches this issue by considering the complex interactions between flood-causing variables, achieved by synthesising existing research on the trends and likely changes in the causes of floods. These causes include the possible changes to meteorological variables – in particular extreme rainfalls – as well as impacts of changing catchment conditions including catchment wetness and land use modifications due to vegetation changes and/or urbanisation. While there has been research into trends in daily rainfall extremes for Australia, there has been less work on sub-daily rainfall. Australian daily rainfall extremes show limited evidence of trends when the annual maximum series is considered although there is significant variability in these time series and changes in seasonality are not accounted for in this type of analysis. Sub-daily rainfalls show clearer evidence of generally increasing trends. The interaction between these generally increasing trends and changes to catchment wetness, which are more likely to be linked to annual rainfall and evaporation trends, is a question of particular interest. This presentation focuses on how the trends in each of these aspects can be combined together to better understand the observed changes in flood risk. Using this improved knowledge, projections of changes in flood-producing mechanisms can be combined to establish likely future flood risk. Open research questions will be highlighted to focus the future efforts of the research community. Three flood case studies, namely the 2007 Pasha Bulker storm, 1956 Murray River floods and the flood characteristics of the Fortescue Marsh area in the Pilbara, are used to highlight the complexities of flood behaviour and to illustrate the open research questions. This work has been undertaken as part of submitted Special Issue in Climatic Change, organised by the OzEWEX working group on Trends and Extremes (http://www.wenfo.org/ozewex/wgs/ wg4-trends-and-extremes), with related activities occurring across a range of Australian natural hazards, including bushfires, drought, coastal extremes, heatwaves, frost and storms. The aim of this Special Issue is to establish future research directions for the Australian science and engineering communities.