Flash floods

Mid-February 1972 in Victoria featured a spell of tropic-like heat, humidity and intense thunderstorms: on both the 15th and 16th storms dropped over 35mm of rain on Melbourne. Storms developed again on the afternoon of the 17th; but this time the development was particularly concentrated and slow-moving. In one hour, 78.5mm of rain fell on the city centre, demolishing the previous one-hour record of 47mm. Streets quickly turned into raging rivers, as some 100,000 tonnes of water was dumped on a square kilometre of the city. Basements of shops were flooded, and all trains were put out of action. Mere kilometres away, little or no rain fell: in an age without mobile phones, many suburban families were mystified by the storm that supposedly delayed their city working members. A similar storm over St Kilda (just south of the City) in February 1989 dropped over 100mm in an hour, triggering major flash flooding.

Sydney is no stranger to severe thunderstorms, and because large expanses of the urban area are paved, much of the rainfall becomes flood run-off. On the night of 8 November 1984, thunderstorms dumped extremely heavy rain over a sizeable area near the centre of Sydney: at Observatory Hill 196 mm fell in the three hours from 10pm. Flood waters raced downhill towards the Harbour, washing parts of houses away and sweeping cars off roads, or burying them in mud. As water coursed through houses and shops, stock and belongings were swept out; mud and rubbish were swept in. Thousands of homes were flooded, and hundreds of people evacuated.

Occasionally, intense rain persists for several hours over an area, especially where the topography acts to “anchor” storms. Precisely this happened along the Illawarra escarpment west of Wollongong on 18 February 1984. Staggering rainfall amounts, exceeding 800mm in 24 hours, fell on the escarpment - the heaviest falls registered in Australia outside the tropics or subtropics, and exceeding totals in the disastrous Clermont flood of 1916! In 10 hours (3am to 1pm), about 680 mm fell; in 12 hours, similar falls were estimated to have covered about 100 square kilometres. There was severe erosion on the escarpment: the railway from Moss Vale to Wollongong was undermined and left hanging in the air. As the flood descended from the escarpment, it swept across the Princes Highway and through the town of Dapto. Water overturned cars and damaged houses, and 600 people had to be evacuated. Remarkably, no-one was killed.

Arid areas are not immune. More than 300mm of rain fell in the western MacDonnell Ranges within the 24 hours to 9am 31 March 1988, with over 150mm at stations around Alice Springs, further east. The normally dry Todd River burst its banks, flooding large areas of Alice Springs, and isolating it from the south as road and rail links through Heavitree Gap were cut. Further south, the Hugh and Palmer Rivers also burst their banks, cutting the Stuart Highway. The flood in the Todd River was thought to be the second highest since European settlement in the 1870s. In the upper Finke, geological evidence suggested the flood there was the largest for some 850 years. Three lives were lost and there was considerable damage in the Alice Springs area.

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