The Flood Risk

The Hawkesbury-Nepean Valley has the highest single flood exposure in NSW, if not Australia. It is important to note that nothing can stop all flooding in the Hawkesbury-Nepean Valley entirely - it will always flood in the valley. Due to the unique landscape of this highly populated region, floods in the valley pose a significant potential risk to people’s lives, livelihoods and property.

For example, if a flood similar to the 2011 Queensland floods happened in the valley today, many lives would be at risk, around 60,000 people would need to be evacuated, and thousands of homes and businesses would be damaged, costing more than $2 billion. The costs of the social impacts of major floods could be as much as the direct damages to people’s homes, businesses and other regional infrastructure.

This high flood risk is due in part to the ‘bathtub’ effect of the floodplain. Most river valleys tend to widen as they approach the sea but the opposite is the case with the valley. Its major river tributaries and narrow ‘choke points’ through natural sandstone gorges between Sackville and Brooklyn cause floodwaters to back up and rise widely and deeply. Much like a bathtub with five taps turned on, but only one plug hole to let the water out.

We need to be prepared for floods so we can better respond and recover from floods in the future.

Do you know your flood risk?

The first step to becoming more prepared for floods is to better understand your flood risk. Visit the Hawkesbury-Nepean flood mapping tool and enter in your suburb to find out your flood risk.

Please note: this tool is just for the Hawkesbury-Nepean Valley.

Why do we need to prepare for flood now?

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 are inevitable and we all need to be prepared.

Recent research points to a cyclic pattern of flood-dominated and drought-dominated periods across the region. This would suggest we are currently in a drought dominated period which has included the Millennium Drought. A flood-dominated period is likely to follow this drier cycle.

Floods can happen during droughts – as happened in the Hawkesbury Nepean Valley in February this year. On this occasion, flooding happened when Warragamba Dam was less than 43% full, level 2 water restrictions were in place, and bushfires were still burning in parts of the catchment.

Currently the Warragamba Dam can rapidly fill and spill. In the February 2020 flood, the dam increased by around 30% in four days – that’s around 600 billion litres of inflows, or more water than greater Sydney uses in a year.

What causes floods in the Hawkesbury-Nepean Valley?

Rainfall events known as East Coast lows are the primary weather systems behind large floods in the Hawkesbury-Nepean Valley. The extent and depth of flooding is also influenced by the unique ‘bathtub’ effect of the floodplain.

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 turned on, but only one plug hole to let the water out.

What is the risk of flooding?

It’s important to think about flood risk both in terms of their likelihood and the consequences when they happen.

Likelihood

Floods are most often described in terms of the chance that floods of a certain size could happen.

The terms ‘1 in 100 flood’, or ‘100-year flood’ refers to a flood that has a 1 in 100 (or 1%) chance of happening or being exceeded in any one year. It does not mean a chance of happening once every hundred years. For example, every year there is a 1 in 100 chance (or 1% chance) there would be a flood reaching around 17 metres or higher above normal river level at Windsor.

Said another way, it means a person living to 80 years of age has a 50% chance of experiencing this type of flood during their lifetime. Another way of thinking about this size flood is that there would be around a one in four (or 25%) chance of it happening during the period of a 30-year mortgage.

Consequence

The Bureau of Meteorology has three categories for describing the consequences of regional flooding: minor, moderate and major. More information about flood categories is available on the Bureau of Meteorology website.

What are the impacts of floods in the Hawkesbury-Nepean Valley?

There have been around 130 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 (approx.1 in 100 (or 1%) chance per year event) it would pose a significant risk to life. Around 60,000 residents would need to be evacuated, and more than 5,000 homes would be impacted.

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, more than 12,000 homes would be impacted, and around 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.

When will the next major flood happen in the valley?

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

The Hawkesbury-Nepean flood in February 2020 was a relatively minor flood in the history of flooding in the region. 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 1964, two of which were after Warragamba Dam was built. Following flooding through the 1980s, the last major flood was in 1990.

The absence of major floods since 1990 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.

What if a flood happened now in the Hawkesbury-Nepean?

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 allocates responsibilities to agencies and organisations to prepare, respond to, and recovery from floods. This plan is reviewed regularly.

What can people do to prepare for flood?

It’s vital for people who live and work in the valley to be aware of their flood risk and be prepared.

Information on flood emergency management and what people can do to prepare for a flood can be found on the NSW State Emergency Service website. It is vital that people be alert to flood warnings and respond to evacuation orders.

How do people know if they are in a flood-prone area?

Local councils are primarily responsible for managing flood-prone land in their local government area. Residents can contact their local council to request flood information related to their property.

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