Tropical Cyclones Affecting Carnarvon and Shark Bay

Tracks | Flooding | Storm Surge | Notable TCs | TC Herbie

It is estimated that a cyclone impact causing wind gusts in excess of 90 km/h in the vicinity of Carnarvon and Shark Bay occurs about once every five years on average. This is less than half the frequency for communities along the Pilbara coast reflecting the decreased risk southwards along the west coast. However, the frequency is not evenly distributed. Since Herbie in 1988 there has only been one cyclone (Frank, 1995) to have caused gales at either Carnarvon or Shark Bay, but there were six cyclone impacts between 1960 and 1970.

In general, cyclones weaken as the move southwards over cooler water and to latitudes where the atmospheric winds are unfavourable for cyclones. Indeed, winds speeds over 170 km/h have only been recorded once, during the March 1960 cyclone when Carnarvon recorded winds of 178 km/h. However, the inadequacy of early wind records makes it difficult to compare events. Other major impacts have been in 1916, 1921, 1934, 1937, 1979 (Hazel) and 1988 (Herbie).

Amongst the earliest accounts of tropical cyclones in Western Australia are those of 1 March 1839, when George Grey's expedition reached Dorre Island off Carnarvon. They reported a sudden drop in wind from the southeast followed by swift onset of northwesterlies, indicating that the eye passed over them. Even the experienced George Grey wrote: 'We all quailed or fell before it , for it come with sudden and indescribable violence'.

Whereas cyclones pose a threat to coastal communities, and in particular the Carnarvon banana plantations and market gardens, pastoralists in the region generally welcome the associated "cyclonic rains" especially after a long dry spell. Inland areas often benefit from systems that have crossed the Pilbara coast and are weakening as the move over the region. However, serious flooding may also result, particularly if heavy rains fall in the catchment areas of the major rivers such as the Gascoyne. Indeed aside from the threat to banana plantations the greatest risk from cyclones at Carnarvon is from potential flooding on the Gascoyne river. The semi-enclosed bays exposed to strong winds increases the risk of Shark Bay to storm surge.

Cyclones have occurred from December to May although they are most frequent in February and March as shown in the graph of monthly occurrence.

Tracks

Cyclones that impact Carnarvon typically form over warm ocean waters to the north of the state. The typical steering of these systems is to the southwest and they tend to take a more southerly track as they move further south as shown in the tracks of all cyclones that have moved within 100 km of the town. Fortunately cyclones that cross the coast to the north, usually have a minimal wind impact on Carnarvon. These systems usually produce heavy rain inland.

See also the Interactive Tropical Cyclone Plotting web page to access tracks of historical tropical cyclones.

Figure 2. Tracks of notable cyclones affecting Carnarvon/Shark Bay. Click on image to enlarge.	Figure 3. Monthly occurrence of cyclones in Carnarvon. Click on image to enlarge.

Flooding

Potentially the greatest cyclone-related threat to Carnarvon is flooding along the Gascoyne River. The heaviest rainfall is not necessarily associated with the most intense cyclones but rather is associated with the cyclone's track, speed and areal extent. Flood events are normally associated with tropical cyclones having crossed the coast to the north and moving inland causing heavy rain over the considerable Gascoyne River catchment area. In 1960 heavy rain from a tropical low over inland areas submerged large areas of country along the Gascoyne River and flooding half of Carnarvon. The extent of flooding is also strongly linked to previous rainfall and existing water levels. In 2000, rainfall events in January, February and then from cyclone Steve in March resulted in major floods along the river.

In the Shark Bay area localised flash-flooding is certainly possible in susceptible areas especially near creeks and low-lying areas. This can make roads impassable for periods.

Figure 4. Inundation of plantations along the Gascoyne River following TC Steve, March 2000. Click on image to enlarge.	Figure 5. Severe erosion along South River Road in Carnarvon following TC Steve, March 2000. Click on image to enlarge.

Storm Surge

Storm surge is a major threat around Shark Bay in particular. In 1937 high seas and an estimated 4.3 m tide, associated with a cyclone passing offshore, resulted in 14 km of the Hamelin Pool - Denham road being covered with water. In February 1921 a storm surge pushed sea water 2.9 m above the highest tide mark at Denham, resulting in 20 cm of water in the Post Office. The whole town was inundated and from 3:30 pm until midnight on the 20th, the population had to shelter in the School and Police Station.

Historically storm surge has affected Carnarvon, such as in March 1934. However, since then flood mitigation works has decreased the risk of storm surge risk to the town.

Some Notable Cyclones Impacting Carnarvon and Shark Bay