Difficult time for Victorian communities managing impact from catastrophic fires last week to intense rainfall causing flash flooding this week.
Many dangers are associated with flash floods including rapid stream rises, characterised by high velocity and debris laden floodwater. Extreme flash floods are often described as walls of water or inland tsunamis. They are very dangerous occurring with little notice making it difficult to reach individuals at-risk. People are often reliant on their own observations of heavy rain and stream rises rather than formal warnings.
Previous Natural Hazards Research Australia research found at least 130 flash flood fatalities were recorded between 2000 and 2017 across New South Wales, Victoria, the Australian Capital Territory and southeast Queensland. A 2017 study led by Lucinda Coates found most of these deaths were a result of people attempting to cross streams.
Many Australian communities are at risk of flash flooding, including densely populated regions such as southeast Queensland; the Hunter Valley including Newcastle; New South Wales Central Coast; Sydney, including the Northern Beaches, Parramatta, Ryde and Fairfield, the Illawarra; Melbourne, the Otway Ranges; Hobart; and Adelaide.
The risk of flash flooding is increasing. A warmer atmosphere can hold more water, increasing the frequency of intense rainfall. Urban development exacerbates flash flooding risk by increasing the coverage of impervious surfaces, meaning greater runoff during intense rain. Recent fires in catchments can also increase flash flood risks.
We have experienced flash flood disasters before such as in Toowoomba and Grantham in 2011, water levels rose 9 m in an hour and a half with 21 fatalities. And in Eugowra in 2022, about 150 people were airlifted from their homes with two fatalities.
Responsibility for flash flood warning lies with the states and territories in partnership with local government. The Bureau of Meteorology provides warnings of the potential for heavy rainfall that may lead to flash flooding.
Some local governments including Moreton Bay and Brisbane in Queensland and Newcastle and Parramatta in New South Wales have developed local-specific warning services, while most other high-risk areas have no specific local warning services. Melbourne Water provides flash flood intelligence to VIC SES for Melbourne catchments.
During yesterday’s flash flooding, a rain gauge in the Mt Cowley area west of Lorne recorded more than 175mm of rainfall in six hours. Catchments along the Great Ocean Road are small and steep with narrow floodplains making them susceptible to flash flooding. The area had only recently been experiencing a significant bushfire threat days before with warnings of catastrophic conditions.
The regional flood strategy identifies the risk of flash flooding and caravan parks in the area. A large flood had occurred in 1985 when 40 caravans were washed into the sea at Wye River. Luckily most vans were empty at the time. The river had been said to have risen more than a metre in 30 minutes.
The impacts this time could have been much worse and are a reminder how dangerous flash flooding can be. A similar event occurring at night would have presented much greater risks to life.
The event highlights the vulnerability of caravan parks and camping grounds that are commonplace along Australian rivers and streams. Large populations of tourists unfamiliar with local hazards and emergency arrangements are particularly vulnerable and caravans and tents can easily be washed away.
Flash flood warnings can be associated with high degrees of uncertainty. Flash floods are more difficult to predict than slower rising riverine floods. Research has found that key terminology employed in Bureau flash flooding severe weather warnings is often misunderstood. Research is finding communities often may not follow warning instructions, relying on local knowledge and information networks to make protective decisions for themselves and loved ones. Provision of warnings is challenged by erosion of trust in institutions and the communication of extreme events without historical precedence.
Much has been done to enhance Australia’s flood management, but how can we better prepare for flash floods? Some ideas:
- Improve flash flood warning systems: identify and fill warning system gaps for high-risk areas, including the provision of community engagement campaigns regarding flash flood risks. Flash floods can be deadly, and communities need to understand what to do.
- Ensure vulnerable facilities have emergency plans and are connected to warning systems: sites such as caravan parks should be connected to local flash flood warning systems and have plans to keep residents safe when warnings are issued.
- Treat flood risk to high-risk roads: most flash flood fatalities involve motorists. Roads should be risk assessed as a standard component of floodplain risk management to identify priority road sites for improvement measures to maximise the safety of motorists during flash flooding. Centre research into the influence of road characteristics on flood fatalities provides some evidence to inform risk assessment methods.
- Land-use planning: Sensitive land-uses for example schools, childcare centres, hospitals and nursing homes, should be avoided in areas prone to flash flooding where there is no ability to safely evacuate in advance. Caravan parks where residents are at high risk should also be avoided.
- Assisted relocation: In the highest risk areas where there is a significant risk to life assisted relocation programs should be considered to remove vulnerable structures from the floodplain.