Appendix A – Additional Guidance to Support Flood Risk Management

Appendix A – Additional Guidance to Support Flood Risk Management

Flood Risk

Between 2011/12 and 2021/22 there were 123 flooding related drownings across Australia – 63% of which were as a result of driving through flood waters.

Flood risk is a combination of the likelihood of a flood event and the consequences of when it occurs. Flood risk analysis involves understanding:

• the probability of flood events
• the population at risk
• the average annual damages associated with different events

Floods put people who live, work or travel in a floodplain at risk of social disruption, financial loss, disease, injury or possibly death.

There are different ways to measure the population at risk, depending on the detail required. In general, the larger the population at risk, the more people who need to be warned and, if necessary, evacuated.

The population at risk is not just about total numbers however, the relative vulnerability of the people at risk is also important.

Exceedance Probability

Floodplain managers tend now to refer to the probability each year of a certain size flood being equalled or exceeded; they refer to this as the percentage Annual Exceedance Probability (AEP).

The likelihood of the occurrence of a flood of a given or larger size occurring in any one year, usually expressed as a percentage. For example, if a peak flood flow of 500 m3/s has an AEP of 5%, it means that there is a 5% chance (i.e. a one-in-20 chance) of a flow of 500 m3/s or larger occurring in any one year.

The term AEP reinforces the fact that there is an ongoing flood risk every year – regardless of how recently there was a similar flood.

Damage of Floods

Floods of different sizes cause different amounts of damage. These can be broadly categorised as:

• Direct tangible damages, including to the structure and contents of buildings, agriculture, livestock or infrastructure
• Indirect tangible damages, including to community wellbeing, economic activities and social activities and can include the costs of emergency response, clean-up, community support and/or lost production and any disruptions to transport, commerce and/or employment
• Intangible damages, including things that are difficult to quantify in monetary terms, despite their significance. These can include trauma, stress, loss of cultural heritage, biodiversity and threatened habitats.

For a given flood-prone area, the damage caused by floods of various magnitudes can be averaged to determine the Average Annual Damage (AAD).

AAD provides a basis for comparing the economic effectiveness of different structural and non-structural mitigation measures, allowing the costs of mitigation to be compared with its benefits.

Evaluating flood risk

Floods are forseeable disasters with tools available to predict and analyse their magnitude, frequency and impact.

Negative impacts of flooding can be reduced by using credible data about flood behaviour such as:

• Flood heights
• Flood extents
• Flood probabilities

Flood studies

Flood studies can be conducted to fill gaps in knowledge and help communities consider flood management options. Flood studies should:

• Model the hydrologic inputs – including rainfall and runoff – that lead to floods of different sizes and calibrate these models against historic floods
• Model the hydraulic behaviour of floods – including flood heights, extents and velocities as they vary with time – and calibrate these models against historical floods
• Understand the varying hydraulic nature of the floodplain being studied
• Understand the varying flood hazards within the floodplain
• Assess the scale of potential flood damages for the existing community
• Assess the potential for flood damage on areas of the floodplain that may be considered for future development
• Analyse risk treatment options
• Consult with local communities to take advantage of local knowledge
• Consult with local Aboriginal and Torres Strait Islander communities to ensure cultural values are considered in assessing and mapping flood risks
• Assess the consequences of floods of different sizes
• Capture the local community’s experience and knowledge of floods.

Outcomes of flood studies

The outcomes of flood studies can be used to:

• Inform stakeholders including the waterway managers, local government authority, local community and Local Water Safety Committee on flood risk
• Inform the Local Water Safety Plan and/or Local Emergency Management Plans
• Provide Guidance for land use planning management and building regulation
• Inform the flood warning system requirements
• Provide options and/or recommendations for effective flood mitigation works

Flood Preparation and Mitigation

Ideally drowning prevention planning should consider the risk of flooding and its effects on people, property and plant, livestock and other natural assets. Features considered should include:

• the exposure of people
• transport routes through potential flood-prone areas
• the availability of barriers and/or early warning systems to reduce the likelihood of persons becoming at risk due to flooding
• the exposure of sensitive environmental receptors
• all people and property located within the boundaries of flood-prone area
• hazardous materials being transported or removed from the area that are under the responsibility of the owner, operator or relevant party
• any other areas or activities under the control or influence of the owner, operator or relevant party not covered by separate emergency arrangements.
• It is important to identify significant community and environmental features surrounding the waterway. These should include features where large numbers of people gather (e.g. sporting complexes, function centres, casinos, bars, restaurants), sensitive land uses (e.g. schools, hospitals, child care facilities, nursing homes) and sensitive environmental receptors. Sensitive environmental receptors may include:
• surface waterways (e.g. creeks, rivers, stormwater drainage systems, access to sewerage system)
• sub-surface aquifers
• soil (considering characteristics such as soil permeability, which controls the rate at which leachate from contaminated land will reach ground water reservoirs) natural buffers, wildlife corridors, State forests/national parks.

References

• Australia Institute for Disaster Resilience. (2017). Managing the floodplain: A guide to best practice in flood risk management in Australia (3rd ed.). Australian Government Attorney-General’s Department, Barton, Australia.
• Department of Environment, Land, Water and Planning. (2016a). Victorian flood data and mapping guidelines. Victoria State Government, Ovens, Australia.
• Department of Environment, Land, Water and Planning. (2016b). Victorian flood plain management strategy implementation snapshot 2016-2020. Victoria State Government, Ovens, Australia.
• Department of Infrastructure, Planning and Natural Resource. (2005). Floodplain development manual: The management of flood liable land. New South Wales Government, Sydney, Australia.
• Department of Natural Resources and Mines. (2015). Review of flood risk management arrangements in Queensland: Final report. PricewaterhouseCoopers, Brisbane, Australia.
• Jacobs. (2015). Victorian Murray floodplain environmental water management plan (Final Report). Jacobs Group (Australia) Pty Limited, Melbourne, Australia.
• Queensland Fire and Emergency Services. (2017). Queensland state natural hazard risk assessment 2017. State of Queensland, Brisbane, Australia.
• State of Queensland. (2015). 2015 Callide Creek flooding review (Vol. 1: Report). Office of the Inspector-General Emergency Management, Brisbane, Australia.