Flooding is one of the costliest extreme events to affect Australia. It can arise from heavy rainfall directly over the area affected; rainfall in headwater regions that bursts banks and leads to flooding, sometimes far downstream; sea water delivered by spring tides, storm surges or tsunamis; or combinations such as high tides preventing fresh water flowing into the ocean and instead spreading out from river channels across normally dry land. Rising water levels due to rainfall events or coastal storm tides can damage agriculture, buildings and infrastructure; disrupt supply chains; displace people; and threaten human life. Floods also cause erosion and deposition in natural environments.

However, although they can be catastrophic, floods are also frequently life giving. In Australia, some species depend on floods for their lifecycle. Floods running outside riverbanks across normally dry habitats can distribute seeds and other propagation material, fill temporary waterholes and stimulate population explosions in some species. Floods are essential for groundwater recharge in some systems; in natural landscapes and some agricultural contexts, flood is encouraged, although potentially managed, and may be celebrated.

In highly human-modified environments, flood is often seen only as a damaging force, and extensive preparations are made to prevent, redirect or minimise the effects of flood. The modifications made to the managed landscape may themselves contribute to the nature and impacts of flood events (see case study: 2016 Tasmanian floods review).

Rainfall events and flash floods

When we think of rainfall-related floods, we typically think of the flood events that occur after intense (Figure 4) or prolonged periods of rain, causing saturated ground, full watercourses and out-of-bank flow. Rainfall events themselves can be extreme events. Intense rainfall and subsequent floods can have impacts over huge areas, and their effects may persist for long periods. Predictions suggest that heavy-rainfall events are likely to become more intense with climate change (CSIRO & BOM 2020, ESCC Hub 2020).

Storms and floods in March 2021 across eastern Australia (from a low-pressure system, but not technically an east coast low; BOM 2021d) caused extensive damage that led to natural disaster declarations for 63 local government areas (LGAs) in New South Wales. Residents, councils, businesses and primary producers in 64 New South Wales and 12 Queensland LGAs received support following the events (NDNQFRRA 2021), described as 1-in-100-year events. Prolonged rainfall associated with the 2019 northern Australia monsoonal trough caused the most significant flood event for 50 years in Queensland’s longest river, the Flinders, resulting in floodwaters 700 kilometres long and 70 kilometres wide (IGME-Qld 2019) (see case study: Northern Australia monsoon trough floods 2019).

The scale of some Australian river systems and the topography of inland Australia mean that rainfall events relatively close to the coastline of the Australian mainland may eventually cause flooding many hundreds or thousands of kilometres away. Such floods may be beneficial, such as the out-of-bank flow necessary to stimulate regeneration of the river red gums (Eucalyptus camaldulensis) of the Murray–Darling system, and filling of Kati Thanda–Lake Eyre by rainfall events that have occurred in the tropics, sometimes 2 months after the rain fell (Tweed et al. 2011). But such events can also be damaging, and even deadly, because fast-moving water arrives unpredictably far from where it fell.

The plains to the south of the Gulf of Carpentaria are usually dry and form a biogeographic ‘arid barrier’ between areas of distinct species composition. Phylogenetic divergence (where 2 populations of the same species become more genetically dissimilar) as a result of this barrier is recognised in terrestrial mammals, birds, reptiles and insects. Alternative hypotheses have been proposed to explain these distributional patterns, including that subdecadal flooding selectively removes shrubby vegetation and understorey animals (Crowley & Preece 2019). In such events, floods may extend over thousands of square kilometres to depths of several metres and take weeks to retreat. Inundation drowns or displaces nonflying animals, and disrupts feeding and roosting sites for flying animals. The scale of floods makes recolonisation slow, and the drying clay soils crack, posing physical challenges for woody-rooted plants.

Flood dynamics also affect in-stream fauna. Australian dryland rivers such as the Diamantina and Cooper may exhibit the greatest variability of any rivers in the world (Puckridge et al. 1998). Not only is the flow in these systems highly variable, it is also highly unpredictable; consequently, the systems support low fish species richness and highly dynamic bird populations (Jardine et al. 2015). Floods in these rivers contribute to the occasional filling of Kati Thanda–Lake Eyre, and to groundwater recharge in some systems, such as the Finke (Miles et al. 2015), but not to the Lake Eyre Basin itself (Tweed et al. 2011). However, such floods are limited to the rivers themselves and immediate surroundings. Extreme rainfall events in the Simpson Desert are linked to a higher likelihood of bushfire 2 seasons subsequently (Verhoeven et al. 2020), but there is insufficient out-of-bank flow to stimulate sufficient biomass growth to carry fire when the rainfall events are far distant.

Figure 4 Flash flooding in Johnston Creek, Rosebank, New South Wales, 9 February 2020
Case Study Northern Australia monsoon trough floods, 2019

In January and February 2019, a monsoon trough and embedded tropical lows delivered record-breaking rainfall across 39 local government areas (LGAs) in northern and western Queensland, totalling 56% of the state or 100 million hectares (IGME-Qld 2019). Some areas received more rain than their average annual rainfall, and there was significant flooding, impacting large areas of pastoral holdings, as well as several towns and cities.

Townsville city flooded after the event, classified as a 1-in-500- to 1-in-1,000-year occurrence, with the highest rainfall totals in the region exceeding 2,000 millimetres over 10 days. The record floods included waters released from the Ross River Dam upstream of Townsville, which peaked at 244.8% of capacity. Considerable local sentiment was expressed that the dam releases exacerbated the impacts; however, reviews by Townsville City Council and a contracted assessment by BMT Eastern Australia Pty Ltd found that the dam’s Emergency Action Plan worked, and that releases did not exacerbate – and potentially limited – flooding impacts (IGME-Qld 2019). Damage in Townsville was estimated at $600 million, 90% of which was borne by domestic customers.

Across north-western Queensland, extensive flooding in pastoral areas caused losses of 500,000 beef cattle and 30,000 sheep. Across all 39 LGAs, the estimated social and economic cost was $5.68 billion, of which $3.15 billion was direct costs (Deloitte 2019).

Flooding at this scale has enormous consequences for communities, businesses and infrastructure, but it also has significant positive events for some natural ecosystems. Floodwaters from this event reached the Kati Thanda–Lake Eyre system and filled the northern lakes, triggering massive responses in wildlife, from freshwater fish and frogs to birds. Floodwaters flowing north into the Gulf of Carpentaria also carried large quantities of organic matter and detritus, which triggered a population boom in some coastal fisheries, resulting in some compensation for hard-hit regional communities (Brown 2019).

Case Study 2016 Tasmanian floods review

After a notably dry period leading to record low water storage levels across Tasmania, a period of extended rainfall started in May 2016 that delivered more than double the normal monthly rainfall across the south, central and north-eastern parts of the state (Figure 5). This rainfall saturated soils across most of Tasmania, such that further rainfall in June rapidly resulted in run-off. Intense rain over 72 hours on 5–7 June delivered several hundred millimetres of rain, causing inundation of farmland and houses, damage to infrastructure and significant deposition of debris. A comprehensive review of the floods (Blake 2017) highlighted several contributing factors to the damage caused, acknowledging that the storm was a 1-in-100-year event.

Timber debris was recognised to be a natural feature of floods originating in forested catchments, although the presence of abandoned Managed Investment Scheme plantations contributed to the available timber debris. Drought conditions in preceding years may also have contributed to the number of dead and distressed trees at risk of mobilisation, and widespread major bushfires earlier that year led to increased run-off and erosion.

Willow trees had been introduced and planted to aid bank stabilisation in some areas. Without adequate management, these can increase in density to levels where their branch and root systems block rivers.

Drowned livestock posed an additional hazard; both sheep and cattle carcasses provided challenges for retrieval and disposal downstream for landowners.

Figure 5 Three-day rainfall totals across Tasmania, to 9:00 am on 7 June 2016

mm = millimetre

Source: Blake (2017)

Coastal erosion and inundation

Sea level rise caused by global climate change is a chronic change that is facing all coastal communities. Although sea level rise itself is not an extreme event, it can exacerbate the impact of extreme events, such as storms and heavy rainfall, in coastal regions (see the Coasts chapter).

Natural landscapes are at risk of increased saltwater incursions as sea levels rise and the effects of king tides and storm tides are exacerbated. Examples are as follows:

  • Modelling of the coastal floodplains of Kakadu National Park suggests that all freshwater floodplains will be subject to saltwater inundation within the next century (Bayliss et al. 2018). Although the frequency and duration of saltwater inundation will vary across the entire floodplain, any saltwater flushing will affect the most sensitive species, and progressively longer or more frequent inundation will affect a greater proportion of the floodplain flora.
  • Bayliss et al. (2018) pointed out the potential cascading effects of changed vegetation composition affecting the viability of sites for dry-season feeding by magpie geese (Anseranas semipalmata).
  • Houston et al. (2020) recorded a decline in the Critically Endangered Capricorn yellow chat (Epthianura crocea macgregori) associated with the loss of a key habitat plant due to persistent hypersalinity on coastal flats on Curtis Island, Queensland.
  • Australian fur seal pups in Bass Strait are at risk of mortality from storm tides; increasing sea levels mean that less severe storms will cause mortality, or dispersal of individuals to higher ground or new colonies (Maclean et al. 2018b). New colonies may place Australian fur seal foraging areas in greater competition with major fishery locations.
  • In Torres Strait, the Bramble Cay melomys (Melomys rubicola) is presumed extinct, having been unrecorded in extensive surveys between 2011 and 2014 (Waller et al. 2017) (see the Biodiversity chapter). The extinction is attributed to a severe reduction in the species’ food resources caused by saltwater inundation from sea level rise, and increased frequency and intensity of storm tides. Waller et al. (2017) describe this as the first mammalian extinction caused by human-induced climate change.

Management interventions can exacerbate the problem. In the Gippsland Lakes, traditional lands of the Gunaikurnai people, an artificial entrance to the ocean that was created in the 19th century to improve access for boats enabled saltwater ingress; declines in freshwater inflow because of diversions exacerbated this in the 20th century, and increases in sea level and the incidence of storm tides have worsened the impacts of chronic salinisation (Boon et al. 2016). Long-term changes in native flora and fauna have reduced the value of recreational and commercial fisheries, Indigenous cultural values, recreational amenity and ecological function.

Engineering solutions to reduce rather than increase saltwater and freshwater exchange can also be problematic. Surge barriers may be erected in estuaries to prevent storm tides from flooding managed landscapes, but these may pose a challenge to fish movements. In a Western Australian study of an obligate estuarine fish (a fish that depends on the estuary environment for its lifecycle; Beatty et al. 2018), fish became trapped upstream of the barrier in summer and autumn at times when flow rates and water quality decreased and water temperature increased, and fish kills were common. The barriers effectively fragment habitat, potentially reducing species’ access to resources, and reducing diversity and abundance. In this instance, the barriers also reduced the capacity of fish to escape suboptimal conditions. Careful consideration of the movement dynamics of species within surge-protected estuaries can inform design decisions to minimise impacts on biodiversity and ecosystem functioning.

Coastal inundation is also a challenge for Australia’s highly populated coastal plains. Sea level rise will increase the frequency of storm and storm tide events. This means that coastal inundation and erosion events will occur more often, resulting in long-term coastal recession. In natural environments, this may be accommodated by the entire coastal complex moving inland, but, in areas where hard landscaping and linear infrastructure prevent recession, habitat may become fragmented or disappear (Lavorel et al. 2015). In urban areas, erosion can mean loss of properties and infrastructure, as was recently experienced in the New South Wales Central Coast area. There is now emerging evidence of measurable increases in the frequency of inundation associated with sea level rise (Hague et al. 2020).