Tropical Cyclones Affecting Onslow

Tracks | Flooding | Storm Surge | Notable TCs | TC Bobby | TC Vance (pdf report)

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. It has since experienced major cyclones in 1934, 1958, 1961, 1963. The 1963 cyclone had winds which were measured at 231 km/h.

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).

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.

Tracks

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.

Flooding

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

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.

Some Notable Cyclones Impacting Onslow