Located on the most cyclone-prone part of Australia's coast, Onslow has a history dotted with cyclone events. The original settlement near the mouth of the Ashburton River was forced to relocate in 1925 because of changes in the river channels mostly because of flooding and a new town established near Beadon Creek.
Since 1910, a cyclone impact causing wind gusts in excess of 90 km/h at Onslow has occurred about once every two years on average. Half of these are category 1 impacts (wind gusts less than 125 km/h). However, the frequency is not evenly distributed. Between 1953 and 1963 Onslow suffered five severe cyclone impacts having wind gusts exceeding 170 km/h and a further three cyclones causing some damage, some flood related. No other Australian town has endured such a period of intense cyclone activity. Although the inadequacy of early wind records make it difficult to compare events, it is estimated that there have been eight severe cyclone impacts in terms of wind speed since 1910: 1934, 1953, 1958 (two), 1961, 1963, 1975 (Trixie) and 1999 (Vance). The 1963 cyclone had winds which were measured at 231 km/h while wind gusts to 247 km/h were recorded during the passage of TC Trixie in 1975.
Along the central Pilbara coast the cyclone season runs from mid December to April peaking in February and March as shown in the graph of monthly occurrence.
Since the 1960s the development of the mining and offshore oil and gas industries (e.g. Salt mining at Onslow and the oil and gas facilities on Barrow Island) has increased the damage potential of cyclones in the region. Substantial economic losses can be incurred even with the threat of a cyclone impact owing to lost production or disruptions to shipping activities. Fortunately modern structures are built according to cyclone wind ratings and are far less susceptible to damaging winds than those constructed in earlier times.
Figure 1. Aftermath of the 1934 cyclone in Onslow looking down Second Avenue towards the Beadon Creek Hotel.Photo courtesy of the Onslow Historical Society.
Figure 1. Tropical Cyclones in Onslow. Click on image to enlarge.
Cyclones that impact Onslow typically form over warm ocean waters to the north of the state. Although the typical initial steering of these systems is to the southwest, those that affect Onslow take a more southerly or southeasterly track as they move further south as shown in figure 2. Some cyclones such as Trixie (1975), originate from near the West Kimberley and take a west to southwest track toward the Pilbara. Fortunately cyclones that cross the coast more than 100 km to the east, usually have a minimal impact on Onslow, for example Orson 1989, Olivia 1996, John 1999 (1.5MB pdf) and Monty 2004. The strongest winds are offshore and as a result there is no storm surge.
See also the Interactive Tropical Cyclone Plotting web page to access tracks of historical tropical cyclones.
Figure 2. Tracks of notable cyclones affecting Onslow. Click on image to enlarge.
Figure 3. Monthly occurrence of cyclones affecting Onslow. Click on image to enlarge.
By not being on a major river, Onslow is not at risk of major flooding caused by rainfall alone. Localised flooding is certainly possible in susceptible areas especially near Beadon Creek and low-lying areas. Major flooding in Onslow is typically associated with storm surge, as discussed in the next section.
Heavy rainfall inland can cause flooding along the neighbouring river systems such as the Ashburton, Cane and Robe that can impact pastoral stations, mining activities and cause transport delays and damage to road and rail infrastructure. During Monty in 2004 two people were rescued from the roof of Yarraloola homestead on the Robe River and floodwaters cut road links along the North West Coastal Highway (see photos).
The flood potential of a system is not directly related to cyclone intensity but is associated with its track, speed and areal extent. Indeed rainfall totals in excess of 100 mm are common with tropical lows that move over land. In February 1997 a slow moving low moved over the west Kimberley, Pilbara and Gascoyne causing rainfall in excess of 400 mm in parts and one of the highest ever floods along the Ashburton River.
Storm surge is a major threat to Onslow. Storm surge is a complex function of cyclone intensity and motion, extent of maximum winds, bathymetry and coastline shape. The actual water level, called the storm tide is a combination of the storm surge and tidal variation. The worst case scenario is to have a severe cyclone pass near the town near the time of high tide, in which case the water level will be many metres above the highest astronomical tide. Given the significant tidal variations in the area, this is a rare occurrence. Even with an intense cyclone the highest surge is typically restricted to less than 80 km of the coast owing to the nature of the extent of a cyclone's maximum winds.
The graphic impact of TC Vance on the remote coast west of Onslow indicates how a storm surge can change the appearance of the coastline (see TC Vance (pdf report) ). Significant historical storm surge events have flooded parts of the town, particularly during the cyclones of 1934, 1958, 1961 and in 1999.
Figure 4. The altered coastline west of Onslow from TC Vance, 1999.
Some Notable Cyclones Impacting Onslow
|Tropical Cyclone||Wind Gust (km/h)||Impact Description|
|9 Jan. 1880||-||A cyclone passed near Yammadery Creek, between Onslow and Fortescue River, where the tidal surge was eight metres over the high-water mark. The Adalia was wrecked near Robe River and some of the crew drowned.|
|26 Dec. 1897||-||A violent cyclone swept over Onslow almost wrecking the whole township.|
|5-6 April 1909||-||Following a cyclone at Onslow in January a second storm hit the town causing damage to most boats and some buildings. Four luggers with all 24 of their crew were lost.|
|2 Feb 1918||-||The embankment of Alligator Creek was demolished for 85 m and rendered practically useless. A great part of the tramline was wrecked and it was impossible to cross the jetty.|
|28 March 1934||-||A severe cyclone struck Onslow on the 28th. Much of the new concrete jetty was destroyed at a cost of 50,000 pounds. All twenty- five of the buildings were damaged, half being completely destroyed. Estimated damage to housing was put at 15,000 pounds and the Beadon Hotel sustained 6,000 pounds worth of damage. The storm surge inundated the town and drove sea water through buildings. Despite the devastation, no lives were lost.|
|22 March 1953||184||A cyclone made landfall west of Onslow. Buildings were unroofed and the jetty was destroyed. The damage was estimated at 50,000 pounds.|
|2 Mar 1956||152||A near record tide pounded the jetty. A roadway and aboriginal camp were washed away. Buildings were awash and frail buildings blown away.|
|4, 15 March 1958||202||Two cyclones crossed near Onslow within two weeks. Wind gusts to 172 km/h unroofed some houses and communications were cut during the first cyclone on the 4th, but overall damage was not considered serious. However, the second cyclone on the 15th caused extensive damage. Half the jetty was washed away and the storm surge broke through the foreshore wall flooding the town. Onslow recorded a gust of 202 km/h and 283 mm of rain.|
|25 Jan. 1961||193||A severe cyclone made landfall at Onslow. Hurricane force winds demolished several buildings and storm surge inundated the town with 1.8 metres water.|
|12 Feb.1961||-||Less than three weeks after the severe cyclone a second event, that eventually crossed the coast north of Carnarvon, brought 274 mm of rain to Onslow causing further flooding in the town.|
|2 March 1961||139||A third cyclone in five weeks crossed near Onslow causing further damage to the jetty and to properties, and delaying reconstruction efforts around the town.|
|7 Feb. 1963||231||A severe cyclone with gusts recorded to 231 km/h made landfall at Onslow damaging all but six buildings in the town. Telephone poles were bent parallel to the ground. Even the weather instrument enclosure was destroyed. Heavy rain also caused flooding in the area, the town recording 356 mm of rain.|
|Joan, 10 March 1965||.||Joan crossed near Mardie on 10th. Power was cut off and an 8,000 litre tank was blown away from Onslow airport.|
|Trixie, 19 Feb 1975||247||Cyclone Trixie, one of the most severe cyclones to affect the Pilbara, passed over Mardie. Damage was reported all along the Pilbara coast. Mardie recorded a wind gust of 259 km/h* and Onslow reported 247 km/h, at that time the highest recorded wind gust in Australia. The Onslow Meteorological Office was destroyed and many houses seriously damaged. Onslow recorded 246 mm of rain.|
|Bobby, 26 Feb 1995||183||Bobby crossed the coast just east of Onslow before 1 am on the 25th. Seven lives were lost when two fishing trawlers were sunk off the coast from Onslow. Very heavy rain (Onslow 425 mm / 25-26th) caused widespread flooding in the west Pilbara, Gascoyne, Goldfields and Eucla regions. A motorist was drowned inland from Carnarvon while attempting to cross a flooded creek. Wind damage was relatively minor at Onslow.|
|Olivia, 10 April 1996||257 (Mardie)||Olivia crossed the coast near Mardie causing extensive damage, particularly to the offshore oil and gas industry around Barrow Island. Varanus Island recorded wind gusts to 267 km/h, the highest ever gusts in Australia at the time, and a minimum pressure of 927 hPa. Mardie reported gusts to 257 km/h. The inland towns of Pannawonica and Paraburdoo also received damaging winds.|
|Vance, 22 March 1999||182||One of the strongest cyclones to ever affect mainland Australia, Vance passed down Exmouth Gulf about 80 km west of Onslow. Damage to Exmouth was considerable. At Onslow the estimated 4 m storm surge inundated the lower parts of the town. Further west the storm surge was estimated at 5 m and caused severe coastal erosion and widespread denudation of vegetation.|
|Glenda, 30 March 2006||180 (Mardie) 156||Glenda was a small and intense system that developed very rapidly after moving off the northwest Kimberley coast. Glenda reached category 5 intensity but weakened as it approached the Pilbara coast. Although threatening to cross the coast near the populated Dampier/Karratha region as a Category 4 system close to the time of high tide, Glenda took a more SW course towards Onslow and weakened before finally crossing the coast near Onslow at 10pm on 30 March as a marginal category 3 system. Very destructive winds were recorded on the coast at Mardie prior to crossing|
*This was the limit of the recorder and so the actual maximum gust may be higher.