The main impacts of tropical lows and cyclones in the Derby region are heavy rainfall and associated flooding. Derby's cyclone risk with respect to wind is much lower than Broome and coastal Pilbara towns. The reduced risk is related to several factors including that there are less cyclones and less severe cyclones occurring in this region (see cyclone frequency map) and cyclones move over land and weaken before reaching Derby. For example Chloe in April 1995 crossed the coast to the north northeast of Derby as a category 4 cyclone but weakened rapidly before reaching Derby. Certainly those communities in more exposed coastal locations such as Beagle Bay and Cape Leveque have a much greater risk of a cyclone impact.
Since 1910 there is only one identified system, in March 1935, that has had at least a category 2 impact on the town. This cyclone caused significant property damage around the town. Nevertheless there is still the risk that a severe cyclone will impact Derby. The threat of a major impact lies from a strong system moving south either across the Dampier Peninsula as in March 1935 or down King Sound. The devastating impact that Tracey had on Darwin in 1975 is a graphic example of the potential impact along a coastline that has an infrequent severe cyclone occurrence. Certainly it is possible that intense systems such as Thelma or Chloe could have a major impact on Derby given the right steering.
It is possible that there have been more severe cyclones off the north Kimberley that have gone undetected. Prior to the satellite era it is likely that were many unidentified systems north of the Kimberley. A Thelma-type of system prior to the 1960s may have not have been identified or may have been analysed as a much weaker system given that it did not impact a Kimberley community.
Cyclones that impact Derby typically form over warm ocean waters north of the Kimberley. The typical initial steering of these systems is to the southwest but as they move further away from the equator they are more likely to take a southerly track. Cyclones move over land before reaching the Derby area and as a result they weaken below cyclone intensity or are at category 1 strength.
There are many other lows that move westwards across the Kimberley, and while they do not bring gale-force winds, they can produce widespread heavy rain. Those that move over the Fitzroy River catchment have caused major floods such as in January 2000.
See also the Interactive Tropical Cyclone Plotting web page to access tracks of historical tropical cyclones.
Figure 1. Tracks of notable cyclones affecting the Derby. Click on image to enlarge.
Figure 2. Monthly occurrence of cyclones affecting Derby. Click on image to enlarge.
With a catchment size of 85,000 square kilometres, the Fitzroy River is prone to regular floods. The peak flood levels at Fitzroy Crossing during floods such as those that occurred in 1914, 1986, 1991, 1993 equate to flow rates of 15,000 to 20,000 cubic metres per second, enough to fill 15 to 20 Olympic swimming pools in one second. The river can extend many kilometres in width along the floodplain between Fitzroy Crossing to its mouth in King Sound near Derby as shown in figure 3.
Widespread rainfall totals in excess of 100 mm over the Kimberley are common with tropical lows and cyclones. Such totals may occur even if systems that are well to the west as a result of moisture laden northwesterly monsoon winds. The amount of rainfall is not related to the intensity of the cyclone, rather to the location, size and speed of the system. Some of the largest flood events have been associated with monsoon lows below cyclone intensity, as in the case of the 1914, February 1993 and January 2000 events. Those that meander over inland areas for many days can cause widespread heavy rainfalls. In February 1956, a cyclone passed near Broome on the 18th and moved eastwards to the Northern Territory before returning over the Kimberley and redeveloping into a cyclone after moving offshore two weeks later. Flooding is enhanced if the low follows previous rainfall that has already saturated the ground and elevated river levels.
Figure 3. The Fitzroy River in flood at Noonkanbah station, 2002. Photo courtesy of Department of Environment.
There are no historical records of a significant storm surge at Derby. The combination of large tidal variations and the low probability of a significant cyclone over King Sound results in a low chance of a significant storm tide. However, it is certainly possible for a severe cyclone to move southwards. Indeed the worst possible scenario of a severe cyclone arriving at high tide and coinciding with floodwaters from the Fitzroy and other rivers into King Sound, would almost certainly inundate all of Derby. Fortunately, this extreme scenario has a very low probability of occurring.
Some Notable Cyclones Impacting Derby
|Tropical Cyclone||Impact Description|
|9 Jan. 1914*||A tropical low that moved across the top end and Kimberley caused phenomenal floods on the Fitzroy and Lennard Rivers on the night of the 6th, all previous flood marks being exceeded. Large numbers of stock were washed away by the Fitzroy River and carried into King Sound.|
|26 March 1935||Several houses were demolished and most houses lost some corrugated iron. The Port Hotel had all outbuildings and the garage blown down. This same cyclone devastated the pearling fleet at the Lacepede Islands north of Broome causing the loss of about 141 lives.|
|18-27 Feb.1956||The Fitzroy River rose past the record 1914 flood to 14.1 m, becoming a raging torrent from 300 m to 24 km wide. The town was marooned and communications lost for some time. The floods caused serious stock losses throughout the Kimberley.|
|Feb. 1993*||Two tropical depressions, both moving westwards from the Northern Territory combined to produce rainfall totals in excess of 500 mm in parts of the Kimberley. Damage to roads and fencing was extensive, and stock losses were high.|
* Non-tropical cyclone impact.