FAQs

A flood study helps Council understand how a flood behaves in a particular area so we can manage existing development and plan for future growth. It uses computer modelling to show where floodwaters go during heavy rain, how deep they can get, and how fast they might flow.

A flood study looks not only at current flood behaviour but also considers future changes, such as those caused by climate change, to help us understand how flooding might evolve over time.

As flood modelling techniques have improved, new technology allows us to more accurately understand flood behaviour within a catchment area. Flood models now map flooding from rivers and creeks, as well as local flooding from stormwater drains and overland flow.

Flood studies also help emergency services, such as the NSW State Emergency Service (SES) and Police, to better understand how flooding affects people’s safety. They also help landowners understand their own flood risk so they can be better prepared for future floods.

The flood risk management process is a series of studies undertaken to first understand and then to mitigate the risks of flooding to the community.

The first stage in this process is a Flood Study. Flood Studies help us understand how flooding behaves across an area. To achieve this, information about how past floods have behaved is collected and computer models are developed to replicate the flood behaviour. From this, maps of flood extents, depths, velocities and flood hazard are used to increase our understanding of flooding across the community. The Woronora River Flood Study will form the first stage of the floodplain management process.

The second stage of this process is a Floodplain Risk Management Study & Plan (FRMSP). FRMSPs are completed following the Flood Study. The FRMSP investigates and assesses the different ways that flood risk can be mitigated for a community. These options are assessed against a range of criteria including effectiveness, practicality, and environmental impacts. The options that provide the most benefit to the community then form the plan for Council to implement, with financial support from NSW Government.  

More information can be found in the NSW Government’s flood risk management manual and toolkit.

A previous Woronora River Flood Study was completed in 1991. However, a number of changes have occurred in the more than 30 years since the last study was completed. Therefore, this study will update the information that Council and the community have available using the latest data and technology.

A riverine area is land that lies along or is affected by a river or creek.

Overland flooding happens when intense, short duration rainfall overwhelms stormwater systems, causing water to flow across land making its way to creeks and rivers.

Riverine flooding is typically caused by prolonged or widespread rainfall that leads to rivers rising above their banks, though shorter events can still create the right conditions for riverine flooding.

The Woronora River Flood Study is only investigating riverine flooding of the Woronora River. Overland flooding that affects the Woronora River catchment is being investigated as part of the Sutherland Shire Overland Flow - Flood Study.

No immediate action is required. If you plan to develop your property, additional flood-related planning controls may apply. These controls ensure developments are safe and do not worsen flood risk to neighbouring properties. Many properties along the Woronora River are already subject to flood-related planning controls from the previous flood studies.

Insurance companies already have detailed information about flood risk across NSW, including the Sutherland Shire, and use this when setting insurance premiums.

Property values are influenced by many things such as house size, condition, location, demand, and the broader economy. Flood risk may matter more to some buyers than others, but it is only one of many factors that affect price.

If your property is identified as being at risk of flooding, it’s important to talk to your insurance provider about flood cover and what options are available.

In NSW, councils are responsible for local stormwater drainage and are primarily responsible for managing flood risk to their communities.

The NSW Government provides councils with technical and financial support to manage the flood risk through the Floodplain Management Program provided by DCCEEW (Department of Climate Change, Energy, the Environment and Water). The Program exists to understand and reduce the impacts of flooding and flood liability on communities, as well as individual owners and occupiers of flood prone property.

You can find more details about flood study requirements on the NSW Government’s website.

A 1% annual exceedance probability (AEP) flood represents a 1% chance of occurring in any year. This is a large but rare flood. It was previously known as the 100-year recurrence flood. Many parts of Australia have experienced 1% floods in the past few years and some areas such as Lismore experienced very rare floods. Just because you have had a 1% flood in the past does not mean that one cannot occur again or that it will not happen for another 100 years. Multiple 1% floods can happen any year. 

It is sometimes easier to think of this as rolling a dice with 100 sides during each major storm event - the chance of landing on the number 1 is always 1 in 100, no matter what happened in the previous events. Even if a 1% AEP event occurred recently, the odds remain the same for the next storm. 

A probable maximum flood (PMF) is the largest flood which can possibly occur in a given area. While extremely rare, a few floods in Australia have approached the magnitude of a PMF.  The mapping which shows the PMF extent shows the full extent of a floodplain and is mainly used to plan for emergency access and to guide the location of critical infrastructure like hospitals and evacuation routes.  Its purpose is to help ensure community safety in the most extreme scenarios — not to define everyday flood risk.

The Woronora River runs through the western part of the Sutherland Shire Local Government Area. Its catchment of around 174 square kilometres extends south of Heathcote Road and includes Woronora Dam, Forbes Creek and Still Creek.

This study focuses on flood behaviour over the 10.8-kilometre tidal section of the river, from the Pass of Sabugal (also known as The Needles) down to where it meets the Georges River.

The Woronora River flows through a steep valley with cliffs, bushland, mangrove forests and estuarine swamps. The river includes both tidal and freshwater sections.

Yes. The Woronora River has a long history of flooding, with significant events recorded since the 1930s. More recent floods occurred in 2000, 2012, 2013, 2016, 2020 and the largest in July 2022.

Several low-lying areas in Bonnet Bay and Woronora have experienced flooding, including during a major event in July 2022. 

A 1995 floodplain management study identified nearly 500 homes in Woronora, Deepwater Estate, Shackels Estate and Bonnet Bay that are at risk of over-floor flooding.

Council has developed the Woronora River Dredging Management Plan to set out a proactive, long-term approach to ensuring safe and reliable access throughout the Woronora River into the future. We are using this plan to monitor riverbed levels and conduct dredging in line with Council’s Dredging Policy, subject to funding from NSW Government.

The key recommendations in the Woronora River Dredging Management Plan were:

• Proactive assessment of the navigation channel, including surveying the riverbed levels every two years (or four years after dredging) to establish a minimum trigger for dredging
• Ongoing dredging at priority areas as needed, such as Gweneth Thomas Wharf and the Rural Fire Service facility.
• Navigational dredging (potentially every 7-10 years as determined by riverbed surveying) at the seven highly-sedimented problem areas identified along the river.

Dredging for flood mitigation purposes was previously examined in the Woronora River Floodplain Risk Management Plan but not recommended for inclusion due to the sheer volume of sediment that would need to be removed to meaningfully reduce flooding impact, the significant cost associated with this and additional environmental impacts to marine life and bank erosion.

As per Council’s Dredging Policy, the NSW Government has full financial responsibility for dredging of navigational channels as the bed of the river is owned by the NSW Government. 

Council is available to manage dredging activities on behalf of NSW Government and residents, subject to all works associated with dredging activities being fully funded. 

We will continue efforts to secure grants from the NSW Government to improve navigation of the river. Council is aware of the NSW Government's 'Boating Infrastructure & Dredging Scheme' that will fund the development of new and improved public boating infrastructure and enhanced public access to NSW waterways. The Scheme replaces the previous grants program 'Boating Now'. 

Find out more about the Scheme on the Transport for NSW website.