Northern Tasmania Flood, April 1929

The 1934 Floods in Melbourne

The Great Roper River Flood of 1940

FLOODS

Few parts of the country are immune from flooding, whether it be localized flash flooding from intense thunderstorms, or more widespread and longer-lived inundations resulting from heavy rain over the catchments of established river systems. During significant floods lives can be lost, stock losses may be in the tens of thousands, and damage to homes, businesses, roads, etc can run into hundreds of millions of dollars. Lost production can add considerably to the costs, as can the intangible costs, such as effects on health. Overall, flooding is Australia’s costliest form of natural disaster, with losses estimated at over $400 million a year. On the positive side, floods have some beneficial aspects, such as cleansing excess salt from the soil and recharging underground aquifers.

Causes of flooding
In northern Australia, most of the big floods occur in summer or early autumn in association with tropical cyclones or intense monsoonal depressions. These systems can produce staggering quantities of rainfall - as much as 1,000 millimetres in a few days. Outside the tropics, coastal areas of eastern Australia mostly receive their flood rains from so-called "east coast lows" that develop over the Tasman Sea. In the southern states, flooding is mostly a winter-spring phenomenon, associated with unusually frequent low pressure systems and fronts. However some major events have occurred in the summer half-year as systems of tropical origin extend or move south. Some inland floods, notably those of Lake Eyre, may be initiated by rain falling many hundreds of kilometres away.

Flooding and La Niña/El Niño
Flooding, unlike drought, is often quite localized, and therefore not as closely tied to broad-scale controls like the El Niño-Southern Oscillation phenomenon. However the La Niña years of 1916, 1917, 1950, 1954 through 1956, and 1973 through 1975, were accompanied by some of the worst and most widespread flooding this century. It can safely be said that, over much of Australia, flooding is more likely than usual during La Niña years, and less likely in El Niño years.

Time-line showing major Australian flood episodes as a function of the Southern Oscillation Index. Extended periods of high SOI in 1916/17, the mid-1950s, and the early to mid-1970s, were periods of widespread, frequent flooding.

Title Image - Flooding at Mannum (South Australia) during the Murray River floods of 1956 (photo courtesy of the Adelaide Advertiser).