A flood study is a technical investigation of the way floods behave within a river catchment. It is a vital part of floodplain management. The aim of a flood study is to define existing flood behaviour, particularly the chance and severity of different sized floods happening as well as depths and extents of the flooding.

The Hawkesbury-Nepean Valley covers over 670 square kilometres of floodplain in Western Sydney, stretching from Brooklyn to Wallacia and Wisemans Ferry. It includes parts of the Hawkesbury, Hills, Blacktown, Penrith, Central Coast, Wollondilly, Liverpool and Hornsby local government areas.

The Hawkesbury-Nepean Valley is likened to a ‘bathtub’ – referring to the multiple taps (tributaries) that direct water into the river and single ‘plug hole’ – the river outlet at Lower Hawkesbury. The catchment is also characterised by steep gorges, tight bends and open floodplains – creating perfect conditions for floodwaters to back up and inundate the Valley.

The extreme depth of floods in the Valley means that many properties and large numbers of people may be impacted by floods. This includes needing to evacuate at short notice before the roads out are cut off or prepare to stay home, if safe to do so, and have enough supplies to do so.

The NSW Government through the NSW Reconstruction Authority. The study was produced by some of the country’s best flood experts. It has been through an independent peer review process and also drew upon the knowledge of experts at floodplain councils.

The NSW Government is committed to ensuring it uses and provides the most up to date information for flood risk management, evacuation, land use and emergency planning. Latest information and technology, significant improvements in the science of flood modelling, increased understanding about climate change, and changes within the Valley led to development of the new study.

The Flood Study builds on and updates the 2019 Regional Hawkesbury-Nepean Valley Flood Study. It looked at more than 20,000 different scenarios and used the latest modelling techniques to provide high quality data so we can better understand the impact of different, future flood events. It has been informed by and tested against the most recent floods that have impacted the Valley, including information supplied by community members. The Flood Study has been reviewed by leading hydrologists from the University of NSW and University of Queensland and flood experts within local councils.

Building on the last flood study published in 2019, the key findings of the 2024 Flood Study confirmed that most flood exposure in the Valley happens in Penrith or Windsor. Compared to the 2019 flood study, the 1 in 100 chance per year level remains the same at Windsor but has increased by 0.5m at the Victoria Bridge (Penrith) gauge.

The floodplain is defined by a Probable Maximum Flood, or PMF, which represents the largest flood that could possibly occur within a catchment. It has a 1 in 100,000 chance per year probability.

While the PMF is considered an extremely rare event it is used to determine the number of people needing to evacuate in an extreme flood event. This is so that emergency planning considers and prepares for all scenarios.

The PMF is also used to support decisions about location and design of critical infrastructure such as hospitals, nursing homes, schools and emergency service facilities.

It’s important to understand that much larger floods have occurred in the Hawkesbury-Nepean Valley than we have seen in recent years, notably in 1867 when the floodwater was nearly 6 metres higher than the levels reached in July 2022 at Windsor.

While larger floods are rare, they can and do occur, and will become more likely with climate change.

Under international and national disaster risk reduction frameworks, it is essential the NSW Government plans for these risks and educates the community about them.

Regional flood studies provide a broader perspective, often spanning multiple local government boundaries, while council flood studies focus on local details. A regional flood study can be used to inform regional floodplain management strategies, emergency response planning, and land-use policies at a broader scale. A council flood study guides local land-use planning, development controls, and floodplain management measures specific to that council area. Both play crucial roles in enhancing community resilience to flooding.

The study provides advice on risk so that local planners can make informed decisions to protect life and property. It does not make any recommendations on individual developments. Proposed developments must still meet all relevant local planning controls in local environment plans and development control plans set by the local council. You should check with your local council if you want specific information about a particular property.

The Flood Study defines existing flood behaviour, particularly the chance and severity of different sized floods happening and provides advice on risk.

It does not evaluate or make recommendations about land values.

Some people worry their insurance will increase because of the release of a Flood Study or that they won’t be able to insure their homes. The insurance industry undertakes its own investigations and does not rely on our work alone to determine flood risk. In some instances, broad models used by insurance companies may be updated with more accurate information. Individual insurance companies use a range of mechanisms to determine premiums and where they will offer insurance.

We are working with the insurance sector and the Australian Government to consider risk reducing actions which may make insurance more affordable. Our own analysis, using similar methods used by insurance companies to calculate Average Annual Losses, shows some small decreases and some increases to premiums are indicated by the updated Flood Study data.

A flood study describes different sizes of floods, from more likely and smaller floods through to the highest possible, extreme flood. These flood events are expressed in terms of likelihood that a flood of a certain size will occur. For example, a 1 in 100 chance per year flood means that there is a 1 in 100, or 1%, chance that a flood of that size will occur (or be exceeded) in any year. This means that, while unlikely, it is still possible to experience a 1 in 100 chance per year flood in consecutive years.

The July 2022 flood reached a height of 13.9 metres at Windsor. A flood this high has a 1 in 20, or 5%, chance of happening each year. Much higher floods have occurred in the Valley, with the largest flood on record in 1867 reaching 19.7 metres at Windsor – nearly six metres higher than the July 2022 flood.

There are evacuation maps available on the NSW SES website at https://www.ses.nsw.gov.au/yourfloodplan

All flooding cannot be prevented. Flooding is a natural part of how rivers work and will always be a possibility when large amounts of rain falls. However, there are things we can do to reduce the impacts and risks of flooding such as managing or delaying flows, stopping it reaching certain areas, or by making space for it. The Hawkesbury-Nepean Valley Disaster Adaptation Plan is currently being developed and will look at a broad range of mitigation and adaptation measures.

Warragamba Dam is a water supply dam and is not built or operated to provide flood mitigation. Under current water policy, WaterNSW is not authorised to release water early to mitigate downstream flooding, so cannot lower the storage based on weather forecasts. WaterNSW is only authorised to make controlled releases when the dam reaches full supply level, to draw the storage down by up to 1 metre for operational purposes.

Releasing volumes of water large enough to provide meaningful benefits ahead of a forecast flood could cause significant downstream flooding and damage in low lying areas. Pre-releases from the dam could also reduce evacuation time and increase the risk to life as roads could be cut by the released floodwaters.

Options to change to operating conditions at Warragamba Dam for flood mitigation are being explored as part of our work to develop the Hawkesbury-Nepean Valley Disaster Adaptation Plan.

A range of flood risk management and mitigation options are being investigated as part of development of a Disaster Adaptation Plan looking at flooding in the Valley. Information from the 2024 Flood Study will also be used for emergency management including evacuation planning, land use planning and infrastructure investment prioritisation. Educating local communities about flood risk and what they can do before or in the event of a flood also helps with preparedness and recovery.

Many people in our communities have been impacted by past flood events. It’s important to note a flood study is a model of potential floods and must consider the largest, albeit rarest possible floods. Affected individuals can contact specialist support services including Lifeline on 13 11 14 (24 hours/7 days a week). Your local Council may also have recovery or support services working with communities.

If you have any questions or would like more information about the 2024 Flood Study:

T: 137788 (Service NSW, Monday to Friday 7am-7pm) E: hnv@reconstruction.nsw.gov.au