Natural Hazards

Natural hazards are extreme events in the earth's physical systems which impose sever and unexpected stresses upon society. These stresses include death, injury, property damage, and loss of production and income. Fortunately South Australia does not rank prominently among the world's hazard-prone regions. Bushfires pose the most dramatic threat to life and property, although these have been limited in impact. Droughts in both agricultural and pastoral areas have more widespread effects and have produced more serious financial losses than fires. Earthquake damage to property since European settlement has been slight, and no lives are known to have been lost, even though the seismic zones in South Australia are some of the most active areas in Australia.

Apart from droughts few dramatic events punctuate South Australia's rather benign climatic record. Long-sustained periods of intense rainfall are rare, although very intense thunderstorms, often with hail, lasting only a few minutes can cause damage to crops and freshly cultivated soil. Local flooding associated with these intense thundershowers is rarely of long duration. Flooding in the River Murray is caused by climatic events beyond the State, such as abnormal snow melt in the Snowy Mountains. Sustained and violent wind storms are rare in South Australia although winter gales occasionally cause minor damage to foreshore works along the exposed southern coats.

Bushfires

Southeastern Australia shares with southern California and the French Riviera the unenviable reputation of being among the world's most fire-prone regions. A highly combustible vegetation cover and occasional summer weather combinations of high temperatures, strong winds and low humidity produce conditions for potential disasters. Each summer serious bushfires occur in South Australia while hundreds of minor outbreaks are suppressed by the South Australian Country Fire Services. Sometimes the combined effects of the weather and the condition of the flammable vegetation produce conflagrations of such intensity and rapid spread that fire-fighting services are virtually powerless to contain them.

Data on the number of bushfires and the estimated extent of land burnt are recorded for local government areas by the Country Fire Services. The maps depict the number and extent of all bushfires reported in the three years 1981-82 to 1983-84 and include the devastating Ash Wednesday fires of 16 February 1983. There is a close correlation between the number of fires and the density of population. The density of flammable vegetation is also correlated with the more densely settled areas of higher rainfall. By far the greatest number of reported fires occurred on the metropolitan fringes, especially in the Mount Lofty Ranges. The South-East and the Port Lincoln district also reported an abnormally large number of outbreaks. Most reported fires are attributed to human agency.

The map showing the area affected by bushfires is dominated by a number of major fires which burnt out of control, sometimes for several days. The severe damage to pastures and pine forests in the lower South-East on 16 February 1983 is apparent. Some of the other large bushfires in the three-year period occurred in conservation parks or on unoccupied Crown Land where lightning strikes sometimes start fires which are difficult to contain.

Uncontrollable fires tend to occur when a particular combination of circumstances exists: a long dry spell which has reduced the vegetation to a tinder-dry state, followed by hot, dry and gusty northerly winds stimulated by the rapid eastward movement of a low-pressure system and associated cold front. The map shows aspects of two of South Australia's worst bushfire episodes, in January 1939 and February 1983, periods which were disastrous also in Victoria.

10-14 January 1939

Autumn and winter rainfall in 1938 had been near normal, promoting the usual flush of grass growth in the Mount Lofty Ranges, but drought set in during September and potential fuel was well cured by early January. Early that month a 'blocking' high-pressure system became established over the Tasman Sea and persisted for two weeks. Temperatures over south-east Australia remained continuously high and daily maxima reached the highest levels ever recorded. Adelaide experienced an extraordinary run of ten days on which the maximum exceeded 36°C. On 12 January the Stevenson screen temperature at Adelaide was 46.1°C, a record which still stands. Bushfires raged for five days from 10 to 14 January; winds were mainly from the north-west at speeds recorded at up to 45 kilometres per hour in Adelaide, but hills residents spoke of trees being uprooted and of strong dust eddies away from fires areas, suggesting near-gale conditions. A cool change with thunderstorms finally arrived in Adelaide on the night of 14 January.

The extent of the 1939 fires in the Mount Lofty Ranges has been mapped from newspaper accounts and the recollections of hills residents who were interviewed in 1984. The areas burnt cannot be identified with the precision possible from the aerial photographs taken immediately after the 1983 fires. The 1939 outbreaks in the hills had four major features which distinguished them from later fires such as Ash Wednesday 1983. First, they burnt on wide fronts over several days, blazes which had seemed under control flared up days later. Second, they burnt much of the remaining native forest in the Mount Lofty Ranges. The largest fires began in or near forested areas and swept across extensive tracts of cleared farmland. Third, although damage was assessed at £650,000, including the destruction of ninety houses, population density in the hills at the time was low and occupations were rural. No lives were lost, although some isolated communities of woodcutters and miners had lucky escapes. Fourth, local volunteer fire services were quite inadequate. Six thousand city volunteers fought the fires with branches and wet bags - even knapsack sprays were rare. Given the confusion and difficulties in communications, it is surprising that there were so few serious injuries.

Most damage claims were for fencing, haystacks, orchards, pigs and poultry. The most dramatic episodes were the battles to save the townships of Mount Torrens, Macclesfield and Meadows. At one stage fires raced towards Meadows from three different directions. The blaze traversed the main street which was piled high with residents' furniture. Nearer Adelaide the fires swept through the southeastern section of Belair Recreation Park (then named The National Park), including the Japanese cherry garden; razed house in Crafers, Stirling and Upper Sturt; and burnt down from Mount Osmond to the tramway terminus at Glen Osmond. In the South-East, extensive areas of grassland and 1000 hectares of pine forest were destroyed near Naracoorte, Penola and Mount Gambier on 13 January, and a child was killed at Penola.

In the aftermath of the fire the State Government paid compensation to affected householders and set about improving the organisation of fire services. The Emergency Fire Services organisation was set up as a branch of the Police Department, providing training and equipment for volunteer fire organisations in rural areas. A central Bushfires Advisory Committee was established to advise the Minister of Agriculture on fire protection strategies. Eventually in 1977 all the groups concerned with fire control in country districts and some urban-fringe communities were brought together in the one organisation - the South Australian Country Fire Services.

16 February 1983

There were serious fires in the Mount Lofty Ranges in 1955, 1957 and on Ash Wednesday, 20 February 1980, but all these were eclipsed by Ash Wednesday 1983. On 16 February 1983 a series of fires broke in South Australia and Victoria. They burnt out more than half a million hectares of forest, farmland and townships, and killed seventy-one people in the two States. In South Australia twenty-eight people died as a result of the fires, sixty-seven were hospitalised, and livestock losses were estimated at between 257,000 and 3300,000 head.

In 1983, summer temperatures were not as high as in 1939 but in 1982 most of South Australia had experience the driest year on record and Adelaide had received only 61% of its average rainfall. Following some cool days, the Adelaide temperature rose rapidly on 15 February to 40.8°C. An exceptionally deep layer of hot dry air over the continent began to move south, while a strong cold front from the Southern Ocean was moving north-east. On the night of the 15th a new front formed near Kalgoorlie, Western Australia, which intensified the following morning and began to move rapidly eastward over South Australia and Victoria. The combination of temperature, humidity and wind ensured that any outbreak of fire would burn virtually out of control over large areas.

During the morning of 16 February the settled areas of the State experience strong north to north-westerly winds which brought a thickening haze of dust to Adelaide and the Mount Lofty Ranges. Major fires, burning in a general south-easterly direction, began in the period 11 a.m. to 1 p.m. to the west of Clare in the Mid North, in at least four separate areas in the Mount Lofty Ranges, and in the Lower South-East.

The leading cold front moved easterly at 90 to 100 kilometres per hour, reaching the fire areas in mid-afternoon (Adelaide 1522 hours, Mount Gambier 1555 hours, and Clare 1600 hours). The sharp discontinuity between the hot dry continental air and the cool oceanic air produced rapid changes in pressure, temperature and relative humidity as the front passed. The light rain which fell had minimal impact on the fires, and the strong gusty south-westerly winds drove the fires in a north-easterly direction, producing the tongues of burnt-out areas visible on the maps. In the South-East wind speeds were higher two hours after the passing of the front than before its passage and greatly contributed to the destruction of pine forests and livestock on farmland. In addition, the fierce heat of the fires created intense local wind systems which caused fire whirlwinds, snapped trees and unroofed houses.

Within a period of twelve hours the fires in South Australia had burnt about 1600 square kilometres, causing damage variously estimated from $200 to $400 million. Some 10,000 kilometres of farm fencing was destroyed. 500 vehicles were burnt and two timber mills were lost. About 75% of the burnt area and resultant estimated financial loss was in the South-East. Of the 385 houses destroyed or seriously damaged, just over 60% were in the Mount Lofty Ranges.

Three years after Ash Wednesday 1983 signs of the conflagrations were hard to discern at the superficial level. The fire-damaged pine forest yielded a considerable harvest of salvageable timber. Copious autumn rains quickly followed the fires of February and three good seasons restored pasture growth and aided regeneration of burnt bushland. Electricity powerlines were alleged to have ignited some of the Ash Wednesday fires and the Electricity Trust of South Australia has taken steps to reduce the potential for ignition, including the installation of 176,000 spacers on its distribution lines in fire-prone areas. A government working party has recommended a high priority for the preparation of detailed fire-hazard maps, the establishment of a Bushfire Prevention Council, and the progressive underground replacement of overhead powerlines in the most vulnerable areas. On the metropolitan fringe of the Mount Lofty Ranges, property values dropped in the wake of the fires. But houses were rebuilt, generally on the same sites and often in materials and designs of high fire risk, and the market recovered. People seeking attractive environments have short memories for occasional disasters, especially where the recurrence interval on a particular site may be as long as half a century. New regulations under the Planning Act, proposed early in 1986, will give councils in high fire-prone areas greater control over the citing, design and materials of new buildings.

Drought

There is no agreed definition of drought, although in South Australia's experience it means a period of rainfall deficiency causing significant reduction of harvests, livestock losses and, in the past, shortage of urban water-supplies. Advances in agricultural technology and engineering works to conserve and distribute water-supplies may, over time, reduce drought impacts.

Before the 1860s, droughts were scarcely noticed as most settlers were living in areas with reliable rainfall. The drought of 1864-65 which led to the drawing of Goyder's 'line of rainfall' was the first shock to the complacency of pastoralists and farmers. Severe agricultural drought occurred again in 1884-86, 1895-98, 1901-03, 1911-15, 1927-29, 1943-46, 1959, 1961, 1967, 1976-77 and 1982. Improved methods of farming have reduced the impact of drought on grain yields. The lowest South Australian wheat yield on record, 0.09 tonnes per hectare, occurred in 1914, the year when Adelaide recorded its lowest growing-season rainfall. In 1967, the second lowest rainfall season, the wheat yield was 0.63 tonnes per hectare.

The incidence of droughts shows no regular pattern. During drought years the upper air is of predominantly dry continental origin and the daily meteorological surface charts display a high frequency of high pressure cells over southern Australia through the growing season (map for 8 June 1982). Cold fronts typically weaken as they approach South Australia, being deflected by 'blocking highs' over the Tasman Sea.

A measure of the geographic distribution of drought frequency in South Australia can be inferred from the map of rainfall variability. The index measures the degree to which the annual rainfall of a place may depart from its long-term average.

A low index value implies little variation from the long-term average and therefore a low probability of either drought or extreme rainfall. The southern coasts and southeastern region of South Australia, with an index value of less than 0.5, share with southern Victoria, western Tasmania and the south-west tip of Western Australia the most reliable rainfall on the entire continent and thus the lowest drought frequency. The variability index value increases progressively inland, the value of 1.0 approximately matching the inner margin of grain farming.

The season of 1982-83 was South Australia's most severe recent drought, an experience shared with all States except Western Australia. In New South Wales and Victoria the agricultural impact was even more severe. The national wheat harvest was only half the previous season's, and although the national wool clip fell by a mere 3%, farm incomes were 24% lower than the previous year.

In 1982, all rainfall stations in South Australia received less than their 30-year average. The falls varied from more than 70% of the average in some coastal areas and the central Mount Lofty Ranges to less than 30% in an area between Loxton and the Victorian border. The impact of the drought in terms of the estimated value of farm production differed markedly within South Australia. Not all areas suffered financially from the drought and indeed there were some areas which gained in productive value. The drought impact map illustrates, by local government areas, the percentage change in the estimated value of farm production in the drought year 1982-83 compared with the previous year 1981-82 (a year of generally above-average rainfall in the settled districts of South Australia).

For South Australia as a whole the value of farm production in 1982 was 16.4% less than in the previous season. The worst shortfalls were in the traditionally marginal grain farming regions of the Murray Mallee, the Upper North and northern Eyre Peninsula. The District Council of Brown's Well suffered the most serious decline with a decrease of 75% on the value produced in 1981-82. Best off in the drought year were the district councils of Willunga, Meadows and Tea Tree Gully, which recorded increases of 30% or more. The irrigated districts of the Riverland were not completely 'drought-proofed' by their access to River Murray water, and experienced slight declines in production. Many southern areas, although receiving only three-quarters of their annual rainfall, enjoyed increased production values because the prices of vegetables and some fruit and dairy produce increased, and because some districts escaped the normal waterlogging of pastures in winter.

Of approximately 21,000 farm establishments in South Australia, 6000, or 28%, were situated in district councils experiencing an increase in productive value in the drought year. A further 4000 establishments, or 18%, were located in councils which suffered losses of less than 15% on the previous year. The brunt of the drought impact was borne by 2000 farm establishments, 9% of the State's total, in council areas which lost 45% and more of their value of production compared with the previous year.

Earthquakes

On 28 July 1837, scarcely six months after the founding of the new colony, Adelaide residents were startled by a loud rumbling noise. The earth shook and trembled. Reassured by Aborigines, who could remember only two similar shocks, the new immigrants resumed the task of founding a settlement.

At 3.40 a.m. on 1 March 1954 many inhabitants of Adelaide were awakened by a loud noise which was followed by shaking sever enough to crack walls and loosen plaster from many buildings. For most residents it was their first experience of a major earthquake. The epicentre was on or very close to the Eden Fault in the southern suburbs of Darlington and Seacliff. The maximum intensity was established at 8 on the Modified Mercalli Scale of 1 to 12. The magnitude of the Richter Scale was estimated as 5.5, compared with 6.8 at Meckering, Western Australia, in 1968, an earthquake which nearly ruined two small towns. The Adelaide earthquake caused no injuries. Although widespread, the damage to individual buildings was slight. No less than 30,000 insurance claims were made and payments totalled £4.5 million (about $50 million in 1986 values).

Among Australian cities, Adelaide share with Perth the dubious distinction of being the most dangerous place seismically, although on a world scale the risk is slight and the estimated recurrence period of a 1954-sized earthquake is about 100 years. South Australian earthquake epicentres occur in two main seismic zones. The major zone, within the Adelaide Geosyncline, extends from Kangaroo Island through the Mount Lofty Ranges and Flinders Ranges to Leigh Creek. The second seismic zone is on Eyre Peninsula where some epicentres may be associated with the Lincoln Fault. There are two other zones, one near Kingston SE, and the other in the Simpson Desert in the far north - possibly the most active seismic zone on the Australian continent.

The largest known South Australian earthquake occurred at an epicentre near Beachport in the South-East on 10 May 1897. It has been assigned an intensity of 9 and a magnitude of 6.5. On 19 September 1902 an earthquake of somewhat smaller magnitude occurred at Warooka on southern Yorke Peninsula. Both the Beachport and Warooka shocks were clearly felt in Adelaide.

Measurement of Earthquakes

The size of earthquakes is measured in a variety of ways; two of the most common are used on the map.

Magnitude relates to the energy released and is determined from the amplitude and the period of the seismic waves. Individual earthquakes are shown by blue circles drawn to show their inferred rating on the Modified Richter Scale, an open-ended logarithmic scale.

Intensity relates to the severity of the earthquake as experienced by an observer. The best known measure is the twelve-stage Modified Mercalli Scale, used on the map to show zones of risk or probability of recurrence. An earthquake of intensity 6 is felt by all, cracks plaster and move furniture. At intensity 8 some chimneys and masonry walls fall, cracks appear on wet ground, and the steering of cars is affected.

Floods

Although flooding is not a notable hazard to life and property in South Australia, the State is not completely exempt from the risk. For example, on 2-3 March 1983 the thunderstorm rains that broke one of the most severe recent droughts caused flood and storm damage estimated at about $10 million in Adelaide suburbs, the Barossa Valley and its neighbourhood. Areas subject to particular flood risk are the trench of the River Murray and low-lying portions of the Adelaide Plains.

Locks generally maintain the River Murray at a navigable level. Abnormal flows in the South Australian section of the river occur two to three months after unusually heavy rains or abnormally rapid snow melt in the headwater mountains. There have been four exceptional foods this century when water rose more than 5 metres above pool level at Morgan. These occurred in 1931, 1973 and 1974. The most destructive flood since European settlement occurred in 1956. This followed an unusual coincidence of high rainfall in the River Darling catchment in Queensland and abnormal autumn rainfall in the River Murray catchment. Throughout July and August there was a frenzy of activity in strengthening old levees, building new sand-bag embankments around towns, pumping stations and irrigation areas, and removing possessions to high ground. The flood peak reached Renmark on 24 August and passed downstream during the next fortnight. The flood volume, nearly five times the normal river flow, caused damage estimated at about $48 million (1985 prices) to farms, orchards, roads and houses. It is estimated that the return-period of a flood of 1956 dimensions is about 140 years and of the 1931 flood about 40 years. In most floods the levees protecting the dairy pastures on the lower River Murray flats have held, but occasional flooding gives benefits of a 'top dressing' of fertile silts and the flushing of salts.