Figure 1. Intensity Frequency Duration Curves for Perth.

Mechanisms

Heavy rainfall in the Perth region can be caused by a range of meteorological mechanisms, but are usually associated in some way with one of the following:

1. A mid-latitude low and cold front embedded in the westerlies (see winter storms).
The most typical pattern for heavy rain is for a front to interact with a tropical moisture source to produce a band of cloud stretching from the northwest to southeast as shown in Figure 2 (July 2001). A slow-moving low to the south or southwest can result in this rainband causing extensive heavy rain or even a succession of rainbands as several fronts sweep across southern Western Australia. These rainbands have the potential to cause widespread rain although it is common for particularly heavy rain to occur in a strip owing to strong convergence in a northwest to southeast orientation.

2. A tropical low or cyclone off the west coast and moving south (see tropical cyclones ).

3. Thunderstorms (see warm season thunderstorms).

Figure 2. Satellite image of a rainband responsible for heavy rain in Perth in July 2001 (Courtesy of the Japan Meteorological Agency).

Flooding

Typically, flooding occurs in the form of:
1. Localised flash flooding as a result of heavy rain over a short period.
2. Inundation of low-lying land after prolonged rain. Improved drainage has reduced the risk of flooding in these areas that were once prone to seasonal flooding.
3. Flooding caused by a rise in river height on the Swan and Canning Rivers. Although uncommon, major river floods have the most significant impact on Perth.
4. Inundation of low-lying coastal areas caused by high tides caused by storm surge.

A heavy rain event does not automatically result in a rise in river levels sufficiently high to flood surrounding areas. While properties may be affected by flash-flooding, more significant flooding on the Swan and Canning Rivers is associated with heavy rainfall over prolonged periods. This is demonstrated in Figure 3 which shows rainfall over a seventy day period for four different rain and flood events in 1926, 1945, 1963 and 2001. The rainfall event in July 2001 recorded the heaviest daily rainfall at 99 mm. However, this rainfall event did not produce a major flood in Perth as just 184 mm fell in the previous seven weeks. Major floods did occur in 1926, 1945 and 1963 as heavy rain over the proceeding period had already raised river levels. In 1945 a total of 842 mm of rainfall fell over a seventy day period compared to just 184 mm prior to the 2001 event.

 

Figure 3. Rainfall over seventy days for the major floods in 1926, 1945 and 1963, and the rainfall event of July 2001 showing the importance of accumulated rainfall. Although Perth registered a daily fall of 99 mm in 2001, relatively dry antecedent conditions prevented significant flooding.

Indeed rainfall can be important as much as twelve months prior to the event. The 1963 flood, for example, influenced the consequences of rain the following year. The Collie flood in 1964 was rated as a one in eighty year event, yet the rainfall which fell was only rated as a one in twenty year event.

The requirement for preceding rain events to saturate the catchment and raise river levels provides an explanation as to why flooding on the Swan River is primarily a winter-time phenomenon. Those uncommon warm-season rainfall events generally do not cause major floods on the Swan River because of the river's capacity to discharge large volumes of water when existing river levels are low. This is not the case for some other towns such as Moora and Greenough that are located on smaller river catchments more susceptible to short-term heavy rain. Rainfall associated with tropical cyclone Elaine, for example, flooded the town of Moora in March 1999.

Very exceptional rainfall events can however cause flooding on large river catchments such as the Swan River. In March 1934, the Swan River reportedly rose 5.8 m in less than eight hours at Guildford in response to heavy rain (134 mm in just four hours at Clackline) associated with a tropical cyclone. In January 1982, 250 mm of rain fell in association with tropical cyclone Bruno, resulting in a flood with a 100 year ARI on the Blackwood River in the Southwest.

Even if summer rain events do not cause a major flood on the Swan River, they can still cause considerable damage and disrupt activities in the city. The impact of one such event is illustrated by the newspaper headlines relating to rainfall on 22 January 2000 (Figure 4).

Figure 4. The heavy rainfall event of 22 January 2000. While not causing a major flood on the Swan River, the event did considerable damage and disrupted activities in the city.

Historical Trends

Historical rainfall patterns in the southwest have been the subject of intense study in recent times owing to a sustained reduction of annual rainfall in the last forty years. Studies associated with the Indian Ocean Climate Initiative (IOCI) showed that heavy rainfall events decreased both in frequency (the number of events) and intensity (the amount of rainfall in each event) particularly since the 1960s (Haylock and Nicholls, 2000). Using the Manjimup site south of Perth, the ARI for daily rainfall of 50 mm increased from 1.5 years (0.0018 AEP) in the 1930-65 period, to 7.5 years (0.0004 AEP) in the 1966-2001 period (IOCI, 2002). Such changes are less dramatic in Perth, but nevertheless the number of heavy rain events has decreased noticeably. Figure 5 shows the frequency at 15 year intervals of daily rainfall exceeding 50 mm for Perth since 1883. This highlights the sudden decrease in heavy rainfall events in the 1960s.

Figure 5. Frequency of daily rainfall exceeding 50 mm at Perth in 15 year intervals 1883 -2002.

This drier period agrees with the flood record on the Swan River where the last major floods occurred in 1963 and 1964. Although there have been significant individual rain events in the last forty years they have not caused major floods. The most serious of these was in July 1983, when heavy rain over the Swan-Avon catchment flooded the upper Swan Valley and threatened some properties in Bassendean and Guildford.

Storm Surge

Sustained strong westerly or northwesterly winds can cause a build up of water on the coast that is referred to as storm surge. This enhances the water level above the normal tidal variation. This includes water levels on the Swan River subject to tidal variation so that parts of the river foreshore may be inundated near high tide. A storm surge also exacerbates flooding on the coastal plain by not allowing the river water to escape out to sea. The extent of flooding in Perth is dependent upon:

1. The intensity of the surge itself which relates to the strength and duration of onshore winds.
2. The timing of the peak surge in relation to the predicted high tide.
3. Existing river levels from rainfall in the catchment area.

Most storm surge events are associated with sustained westerly gales caused by intense lows and cold fronts in the cool season (refer cool season storms). During the 23 May 1994 storm, the tidal elevation measured at Fremantle showed a storm surge of 0.98 m. Fortunately the potential for more serious and extensive flooding was not realised as the peak surge occurred near the time of predicted low tide. A strong westerly gale on 20 July 1910 caused damage along the west coast to as far north as Geraldton. The Fremantle North Mole was damaged while on the Swan River all the surrounding low-lying lands and many of the jetties were submerged. More recently, gales associated with a low passing near Busselton on 16 May 2003 caused a storm surge of 0.8 m at Fremantle at close to the time of high tide as shown in Figure 6. The actual tide was 0.5 m above the highest astronomical tide and significant coastal erosion occurred. Figure 7 shows water overflowing onto Riverside Drive near the city centre. Fortunately existing river levels were low owing to dry conditions in the previous months, otherwise flooding would have been significantly worse.

 

Figure 6. Actual tide above the predicted tide at Fremantle, 15-16 May, 2003, showing the impact of a storm surge during a cool season westerly storm (Data courtesy of the Department of Planning and Infrastructure, Western Australia).

Figure 7. The effects of the storm surge on Riverside drive, 16 May 2003 (Courtesy of Lee Evelegh).

Wave action adds to coastal erosion. Intense winter lows to the south generate significant swells. During the May 1994 storm the open-water swell was estimated at eight to nine metres.

In addition to cool season events tropical cyclones can also cause storm surges in Perth. Cyclone Alby in April 1978 generated northerly gales responsible for large waves and a storm surge, which caused substantial coastal erosion along the Lower West coast. At Fremantle the surge was about 0.6 m, causing a high tide of 1.8 m, about 0.5 m above the highest astronomical tide. Very dry conditions and northerly, rather than westerly winds negated the flood potential on the Swan River. Areas further south at Bunbury and Busselton more prone to a northerly surge were flooded in low-lying areas on the coast.