Severe Flooding, Associated Weather Systems and Death Tolls, in Southeast Australia Since the Mid-19th Century

Millions of Australians live in a 1500 km stretch of coastal catchments in south-east Australia from Brisbane, through the rapidly growing Gold Coast and Northern Rivers district, through Sydney, to the state border. Major flooding in this region causes death, economic loss and major disruptions to the lives of its inhabitants.

Using a range of historical sources ranging from hydrological and meteorological data, through to newspaper articles, we identified over 250 severe floods since 1860.  We considered a flood to be major if it caused inundation of a river within approximately 50 km of the coast or if there was non-riverine flooding over land near the coast, extending 20 km or more along the coast. Some of the most extreme events identified occurred in the 19^th century and early-to-mid 20^thcentury.  If such storms occurred today, they would have catastrophic impacts due to the massive increase in urban development in the study region since that time.

The El Niño–Southern Oscillation (ENSO) is shown to strongly modulate the frequency of severe coastal flooding and the number of associated death tolls. For example, the average numbers of coastal floods and deaths associated with freshwater drowning in La Niña years are 92% and 220% higher than the corresponding averages in El Niño years, respectively. The average number of deaths per flood in La Niña years is 3.2, which is 66% higher than the average in El Niño years. Fortunately, death tolls have been relatively low since 1960, despite an increase in the number of floods and a very marked increase in population density. This presumably reflects improvements in weather forecasting, emergency services and an increased awareness of the risks associated with flooding in the broader community.

There has been a statistically significant, 50% increase in the frequency of major floods since the late 19th century. While this is consistent with an expectation that anthropogenic global warming may lead to a projected increase in flood risk in response to global warming over the coming century in many regions, further research is needed to clarify the relative importance of possible contributors, including anthropogenic global warming and land use change.