If you live or work in the Hawkesbury-Nepean Valley you are at risk of flood

There are three main floodplains in the Hawkesbury-Nepean Valley linked by the river. They are the Wallacia, Penrith/Emu Plains and the Richmond/Windsor floodplains.

If your house or work is within the probable maximum flood (PMF) area, you are within the floodplain. The PMF is what flood experts have determined is the largest possible flood for the area.

It’s vital to plan for natural disasters before they happen.

Like droughts, fires and storms, floods are unpredictable, damaging and dangerous. The Hawkesbury-Nepean floodplain was created by floods over thousands of years. More floods will happen and we need to be prepared.

Recent research points to a cycle of flood-dominated and drought-dominated periods across the region. While a flood occurred in February 2020, we have not yet emerged from this drought-dominated period which has included the Millennium Drought. A flood-dominated period is likely to follow this drier cycle.

Even in a long drier period, we can still experience a rain event that can cause a flood. That’s what happened in 1998, when an East Coast Low delivered enough rainfall to the large Warragamba Catchment to take Warragamba Dam from 56 percent full to spilling in around two weeks.

If you live in the Hawkesbury-Nepean Valley, or even in nearby suburbs, you might be affected by flooding the valley. Flood impacts can extend well beyond the streets and houses where floodwaters reach.

Even if your house or workplace might not be flooded, they could become isolated as other suburbs and streets flood around you. Significant floods can disrupt roads, rail lines, infrastructure, and essential services such as electricity, sewerage and water. A large flood will have a significant impact on everyday life within the region.  

There are two key elements to working out flood risk – how likely a flood is to happen and then the impact or consequence of that flood.

Chance (or likelihood)
Floods are often described in terms of the chance that floods of a certain size might occur.

This website describes this chance as a percentage or probability that this flood will occur in an 80-year lifetime.

Consequence
The Bureau of Meteorology has three categories for describing the consequences of regional flooding: Minor, Moderate and Major. The sorts of things that determine the consequences of a flood include how wide it is, how deep and the speed or velocity of the floodwaters. More information about flood categories is available on http://www.bom.gov.au/water/awid/

In assessing flood insurance, insurers take account of how often an individual property is expected to flood, how severe the flooding may be, and how deep the flood can get.

Fact sheets about flood insurance can be found at the Insurance Council of Australia and the Floodplain Management Australia website.

Most large floods in the Hawkesbury-Nepean Valley are caused by weather systems known as East Coast Lows. The East Coast Low weather systems develop off the NSW coast and can produce very high rates of rainfall and heavy rain.

The extent and depth of flooding is also influenced by the unique ‘bathtub’ effect in the valley. Most river valleys tend to widen as they approach the sea. The opposite is the case in the Hawkesbury-Nepean Valley. Narrow downstream sandstone gorges between Sackville and Brooklyn create natural choke points. Floodwaters back up and rise rapidly, causing deep and widespread flooding across the floodplain. Much like a bathtub with five taps (the major tributaries) turned on, but only one plug hole to let the water out.

There have been 131 Moderate to Major floods in the Hawkesbury-Nepean since European settlement.

If a Major flood similar to the 2011 Brisbane flood happened in the valley today (55% chance of occurring in an 80-year lifetime) it would pose a very significant risk to life. Around 60,000 residents would need to be evacuated, and more than 5,000 homes would be impacted by floodwaters.

The largest flood in the valley since European settlement happened in 1867. That flood reached around 19 metres above normal river height at Windsor, caused massive and widespread damage, and resulted in the loss of 13 lives. If a similar flood happened now the consequences would be catastrophic - many lives would be at risk and more than 90,000 people would have to evacuate.

Recovery from major floods like these can take years, and the personal and financial effects on people and communities can be devastating.

Floods are random, naturally occurring events. It’s impossible to predict when the next major flood will happen.

History has shown that serious floods can happen many times in a single decade, and not again for many years. For example, there were several major floods in the valley from 1956 to 1990, five of which were after Warragamba Dam was built.

The absence of floods like this in no way suggests this relatively flood-free period will continue. It’s not a matter of if the next major flood will happen, it’s a matter of when.

In recognition of the major flood risk in the valley, the NSW SES has prepared the Hawkesbury-Nepean Flood Plan to help manage and respond to floods in the region. The plan makes clear how agencies and organisations are to prepare, respond to, and recovery from floods. This plan is reviewed regularly.

Recent scenario exercises led by the NSW SES have tested the emergency services’ response to major flooding in the Hawkesbury-Nepean Valley.

Read the How to prepare for floods section.

View the Know how to get out - fact sheet which includes a map of evacuation routes for more information.

Make an emergency plan and include your animals. Download a copy of Keeping Your Animals Safe in an Emergency – in the Hawkesbury Nepean valley to find out more about:

• options for where to take your animals
• planning how to travel there
• animal identification
• preparing a ready to go kit
• having a Plan B
• promoting an animal-ready community.

• For emergency help in floods and storms, call the NSW SES on 132 500
• In a life‐threatening emergency, call 000 (triple zero)
• Keep local emergency numbers handy (near your phone, in your mobile and on your fridge)

• Learn about the warning signs of flooding in the Hawkesbury-Nepean Valley
• Listen to local and ABC radio for updates and advice
• Follow updates on social media such as on NSW SES Facebook or NSW Police Facebook or Twitter

The 2024 Hawkesbury-Nepean River Flood Study describes the existing flood behaviour of the Hawkesbury-Nepean River between Bents Basin near Wallacia and Brooklyn, including backwater flooding up tributaries such as South and Eastern creeks. It builds on and updates information from the 2019 Hawkesbury-Nepean Valley Regional Flood Study.

The 2024 Flood Study is used to help local residents and businesses understand their flood risk - providing high resolution flood information including new flood maps show flood extents, levels, depths, velocities and hazard.

Development of the 2024 Flood Study commenced in 2020 and was overseen by a technical working group including representatives from local councils and Government agencies. It was informed by 4 floods between 2020 and 2022.

The interactive mapping tool drew upon various flood studies to present information that was current at the time of its development (June 2024). Red lines show where information from one study has been joined to another for each flood size (refer to this explanation).