Flood

Clermont, Queensland, December 1916

Southwestern Australia, July 1926

Northern Tasmania, April1929

The December1934 floods in Melbourne

The great Roper river flood of 1940

Southeastern Australia, June 1952

Hunter Valley, February 1955

The big wet, January 1974

Nyngan and Charelville, April 1990

Katherine floods, January 1998

While drought remains the main scourge for most of the country, there are times when too much water can also have devastating results. 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. Then normally quiet-flowing streams can spill out over thousands of square kilometres of surrounding country. On such occasions lives can be lost, stock losses may be in the tens of thousands, and damage to homes, businesses, roads, public utilities, property and equipment 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, which are difficult to measure, but significant nonetheless. Overall, flooding is Australia’s costliest form of natural disaster, with losses estimated at over $A400 million a year. On the positive side, floods have some beneficial aspects, such as cleansing excess salt from the soil, washing away man-made chemicals, recharging underground aquifers, and - more spectacularly - causing the desert to bloom.

Tropical 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. The official 24 hour rainfall record of 907mm was set on 3 February 1893 at Crohamhurst, northwest of Brisbane, causing devastating floods in Brisbane. However in January 1979, tropical cyclone "Peter" dumped 1,947mm in 48 hours at Bellenden Ker in North Queensland, as uplift of moisture-laden cyclonic winds by mountainous terrain further intensified already excessive precipitation. More recently (February 1999) cyclone "Rona" produced 1,870mm in 48 hours at the same location.

Flooding outside the tropics

Outside the tropics, coastal areas of eastern Australia mostly receive their flood rains from so-called "east coast lows" that develop from time to time over the adjacent Tasman Sea. Elsewhere in the southern states, flooding is mostly a winter-spring phenomenon, associated with unusually frequent or active extratropical depressions and fronts. However some major events have occurred in the summer half-year as systems of tropical origin extend or move south. Flooding over inland areas is usually associated with southward-moving tropical systems, but in the cooler months, may occur when well developed cloudbands extend across the interior from the oceans north and northwest of Australia. However some inland floods, notably those of Lake Eyre, may be initiated by rain falling many hundreds of kilometres away, and the flood peak may take months to move down-river into the interior.

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, though heavy rain and flooding often accompany the breakdown of El Niño in late summer or autumn.

Fig. 1: Flooding near Abbotsford, an inner city suburb of Melbourne, during a major flood in September 1916. (photo courtesy Emergency Management Australia)

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 coutesy of the Adelaide Advertiser).