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4 March 2022 Expert commentary

Flood risks under climate change

Senior hydrologist Dr Francis Chiew says

"Our climate and streamflow are highly variable, with many different drivers of this variability including the El Nino and La Nina cycle. The effects of climate change will be superimposed on this natural variability. We can attribute the contribution of climate change on some extreme events with confidence, including the role in heat extremes on land and in the ocean. 

"However, due to the complex drivers of extreme rainfall and floods, and the highly variable nature of rainfall, we are less confident about attributing the relative contributions of different factors right now. That analysis will take a few months. We know that under a warmer climate, flood risk in general is likely to increase. 

"Floods are the costliest natural disasters in Australia, averaging $8.8 billion per year. Cascading societal impacts are evident in Queensland and NSW, such as deaths, injuries, mental stress, thousands of houses damaged, thousands of people evacuated, major infrastructure damage and service disruption, limited food and fuel, and contaminated drinking water."

Senior climate researchers Dr Michael Grose and Kevin Hennessy say

"The IPCC Sixth Assessment Report (Working Group I and Working Group 2) indicates that the frequency and intensity of extreme rainfall has already increased over most land regions, and extreme rainfall will generally become more intense under a warmer climate. This will increase flood risk in cities, in built-up urban areas, and in small catchments, where extreme rainfall over hours to a day can quickly become flash floods.

"It's more complex in rural areas and for larger river basins, which are driven by multi-day rainfall events and by the preceding catchment conditions. For example, in southern Australia, the change in flood risk will be the net result of contrasting effects of increases in extreme rainfall and generally drier catchments under climate change. Nevertheless, the IPCC report gives medium confidence that risks from river flood will increase with climate change."

Adaptation progress

Senior climate researcher Kevin Hennessy says

"The federal government has a National Climate Resilience and Adaptation Strategy, all states and territories have climate adaptation plans, most local councils have climate adaptation plans, and the private sector is developing introduction of climate risk and disclosure regimes. However, recent fires and floods highlight those regional vulnerabilities remain large, so more needs to be done to improve our resilience.

"The IPCC Working Group 2 report found that adaptation progress is uneven. Barriers include lack of consistent policy direction, competing objectives, different risk perceptions and values, knowledge constraints, inconsistent information, fear of litigation, up-front costs, and lack of engagement, trust and resources.

"The good news is that a range of incremental and transformative adaptation options and pathways is available. Key enablers include shifting from reactive to anticipatory planning, integration and coordination across levels of government and sectors, inclusive and collaborative institutional arrangements, government leadership, policy alignment, nationally consistent and accessible information, decision-support tools, along with adaptation funding and finance.

"Local 'adaptation champions' and tailored engagement processes can enhance learning. Knowledge brokers, information portals and alliances can help communities, governments and businesses to better access and use climate change information.

"Recent initiatives like the Australian Climate Service, the National Recovery and Resilience Agency and Future Drought Fund, are positive steps to helping address risks from floods, fires, droughts, heatwaves and other hazards. These are important because of the cascading and aggregated impacts from multiple hazards on ecosystems, people, infrastructure, services and supply chains."

Environmental impacts of flooding

Senior environmental scientist Dr Klaus Joehnk discusses challenges caused by flooding

“Large floods are a threat to water supply and safe drinking water, straining water treatment plant operation by increased sediment load and potential contaminants.  

“Potential sewage treatment plant overflow as well as the inundated sewer system will increase pathogen concentration in the water masses. (See a recent article in Drinking water can be a dangerous cocktail for people in flood areas (

“Like in the floods after bushfires, the large-scale erosion will lead to increased sediment loads in the flood waters carrying with it increased concentrations of nutrients but also other contaminants such as metals.

“Shoreline vegetation will be smothered with sediments, increased turbidity reduces underwater light climate and thus impacts plant photosynthesis. This large pool of sediment load flowing into coastal systems will have similar negative impacts, increased turbidity, sediment deposition across a larger coastal area and increased nutrient concentrations. Those plumes can be easily tracked by satellite imagery.

“While terrestrial animals might be negatively impacted, fish and other aquatic animals may benefit from the increased connectivity of flood inundated areas.  

“Inundation of large, vegetated areas always bear the risk of blackwater forming, that is high levels of dissolved organic carbon in water associated with decreased oxygen and potential hypoxic conditions killing fish. This is not so much of a problem in a well-connected system where fish can avoid low oxygen zones.”

Senior ecologist Dr Tanya Doody discusses benefits of flooding

“Floods are important to reconnect creeks and wetlands that come off main river channels, to supply fresh water to top up or fill wetlands, which in turn supports flora and fauna in the vicinity. Floods can often initiate breeding of various fauna such as fish, frogs and birds. More frogs, for example, then provide a food source for reptiles and other larger fauna. 

“Floods and flooding rains are also critical in areas with soil salinisation as water can flush the salts to deeper areas of the soil profile and provide fresh water sources for the surrounding vegetation. Most vegetation is not especially tolerant to salt and will become unhealthy without periods of freshwater availability. Likewise, any additional water that becomes available to vegetation, especially trees will provide benefits in reducing water stress and growth of new leaves to improve canopy vigour. 

“Vegetation debris accumulates on the soil under trees and bushes and floods are required to push this food source into water bodies where microorganisms digest the debris and make the nutrients available to other organisms in the water via the food web. So small fish eat microorganisms, and large fish eat microorganisms, small fish and other food sources such as tadpoles. 

“Many understory plants and bushes require water/floods to germinate. This is especially important for species that provide food sources for Indigenous communities such as Nardoo (Marsilea drummondii) which is used to make a form of bread. Many plant species provide critical food and medicinal resources for indigenous communities.” 

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