Tropical Cyclones in Western Australia - Impacts

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Impacts - wind, storm surge, flooding, major historical impacts

The impact potential of each cyclone is unique varying according to many factors such as track, intensity, rainfall potential and size. The main impacts of a cyclone are wind damage, storm surge, and flooding as a result of heavy rain.

1. Wind

The severity of a tropical cyclone is described in terms of categories ranging from 1 (weakest) to 5 (strongest) related to the zone of maximum winds as shown in this table.

It is important to note that as wind speed increases the power of the wind to do damage increases exponentially. Hence a category 5 severe tropical cyclone (with wind gusts > 280 km/h) has the potential to do around 250 times the damage of a Category 3 severe tropical cyclone (with wind gusts of 170 km/h). This underscores the importance of the category system.

2. Storm Surge

Storm surge is a large mound of water that accompanies a tropical cyclone as it comes ashore. The intense winds of the cyclone pile up the ocean into a dome of water that is pushed onshore as the cyclone strikes the coast. The low pressure of the cyclone adds to the height of the mound of water, though this is a secondary effect. When the height of a storm surge is discussed it does not take into account the height of the large waves on top of the mound of water.

The combination of storm surge and astronomical tide is known as 'storm tide'. The worst impacts occur when the storm surge arrives on top of a high tide. When this happens, the storm tide can reach areas that might otherwise have been safe.

The graphic below shows the actual tide (blue) and the astronomical tide (red) at Exmouth during cyclone Vance (1999). The black line is the storm surge component that peaked at 3.5 m. At this time the predicted tide was 1.4 m so the resultant peak storm tide was 4.9 m. If the peak surge had of occurred at the time of high tide the actual tide would have been 6.0 m or 2.6 m above the highest astronomical tide (pink line).

Data courtesy of WA Department of Planning and Infrastructure.

For more details see Storm Surge Information.

3. Flooding

For inland areas, particularly near watercourses, the greatest impact of most cyclones is flooding from heavy rainfall. This can cause damage to buildings and fences near watercourses in addition to road and rail networks. Transport delays can cause significant economic costs particularly to the mining industry. Some of the more remote communities may be isolated for extended periods before water levels fall and roads and bridges are repaired. While most towns and communities are located away from flood-susceptible areas, some such as Nullagine may experience flooding in the town.

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. Those systems that meander over inland areas for many days can cause widespread heavy rainfall. In February 1997 a slow moving low moved over the west Kimberley, Pilbara and Gascoyne producing rainfall in excess of 400 mm in parts and one of the highest ever floods along the Ashburton River. The large Fitzroy River floods of 1914, February 1993 and January 2000 were also associated with monsoon lows below cyclone intensity. Flooding is enhanced if the low follows rainfall that has already saturated the ground and elevated river levels.

One of the more recent significant flood events associated with a tropical cyclone was a result of cyclone Steve in March 2000. This was notable because it caused heavy rain in excess of 100 mm along its track across north Queensland, Northern Territory, Kimberley, Pilbara and Gascoyne as shown in the weekly rainfall map below. Flooding even occurred as far south as the Esperance region where roads and bridges were washed away (see impacts from Steve).

Inundation of plantations along the Gascoyne River following TC Steve, March 2000. Click on image to enlarge.	Severe erosion along South River Road in Carnarvon following TC Steve, March 2000. Click on image to enlarge.

Major Tropical Cyclone Impacts in Western Australia since 1975.