FAQs

Wollondilly Shire Council is seeking to improve the understanding of flooding across much of the Wollondilly Local Government Area, through a broad scale Wollondilly Flood Study. 

Council has engaged Worley Consultants (formerly Advisian) to carry out a Flood Study to improve the understanding of flooding across much of the Wollondilly Local Government Area. Worley has completed flood modelling using the latest topographic data and techniques to produce detailed flood mapping and a draft Flood Study report. The results of the Flood Study are used to identify the existing flood risk across the Shire and to inform the investigation of potential flood management measures.

The broad scale study area covers the eastern portion of Wollondilly Shire comprising 700 km², extending from Warragamba and Silverdale in the north to Bargo and Yanderra in the south. The study area comprises three main catchments; the Wollondilly River, Upper Nepean River, and Upper Georges River. 

Detailed studies are also being carried out for several towns and villages including Appin, Bargo, Silverdale, South Picton, The Oaks, Tahmoor, Thirlmere, Warragamba, and Yanderra.  

A computer flood model has been developed using the latest state-of-the-art TUFLOW software, which is considered an industry standard.  The flood model includes detailed topographic data for the drainage catchments and watercourses, Council’s land use zone mapping and details of major drainage culverts and bridges. 

The flood modelling considers a range of factors, including:

• the distribution of pervious and impervious land surfaces, which informs how rainfall collected over the catchment is converted to runoff that flows into rivers and creeks
• the distribution of vegetation, such as trees, bushes and grasses, to determine the “roughness” of the floodplain, which can affect flow depths and velocities
• potential blockage of hydraulic structures, such as bridges and stormwater culverts, caused by flood-borne debris
• flooding behaviour of the Nepean River
• local catchment runoff along rivers and creeks upstream from the influence of Nepean River flooding
• climate change testing, which accounts for a potential increase in storm rainfall intensity.  

Community input has been helpful in validating the flood modelling results. In two earlier rounds of consultation, we sought input from residents, organisations, businesses and property owners to obtain information on what they have observed and experienced of flooding in Wollondilly Shire, as well as their feedback about the draft broad scale flood mapping which was publicly exhibited in 2022. 

We are again seeking feedback from the community until 5:00pm on Monday 10 June 2024.

Council is undertaking the flood study as a step towards a Flood Risk Management Plan, which aims to reduce the worst effects of flooding. The flood management process is carried out in stages (see flowchart below). Council is currently in the Flood Study stage - where we are seeking to ensure our mapping is as accurate as possible.

Under the NSW Government’s Flood Prone Land Policy, management of flooding is primarily the responsibility of Local Government. The Wollondilly Flood Study is being undertaken under the guidelines of the NSW Government’s Flood Prone Land Policy, which aims to ensure that Council’s management of flood risk occurs with consideration to the social, economic and environmental impacts of flooding for the short, medium and long term. 

Council recognises that the local knowledge and personal experience of the people in the community are valuable to help identify issues surrounding flooding in the local area. 

The Flood Study needs to be based on reliable information and data. This is where many of you have already helped, and where you can still help.

We would like to know if the mapping is consistent with your own experience and observations, and invite you to fill in the feedback form, place a pin on the map, attend a drop-in session, or call us.

Insurance companies carry out a range of investigations in determining the price of their products. Insurance costs have likely already been affected by a range of factors such as recent natural disasters, a high number of claims, the high cost of re-building, and high interest rates. 

Council's draft Flood Study has not been provided to insurance companies. 

The broad scale Wollondilly Flood Study aims to assess the entirety of the Shire as accurately as possible, including detailed studies of several villages and towns. The project is following best practice, and constructive feedback from the community will help further strengthen the study.

It is possible that if Council's study provides greater accuracy than what insurance companies are currently using, it would create greater certainty and may help in stabilising and maintaining premiums. 

Mainstream (or riverine) flooding generally happens when the water in an established creek or river system rises and bursts its banks. It can happen quickly, but, it usually takes some time.

Overland flooding is different. It happens quickly when there is an intense burst of rain that cannot be absorbed or drained away fast enough, and the water becomes runoff travelling down to the creek or overflow in places where water doesn’t usually accumulate. 

Flooding in most village areas is overland flooding. There are some systems in place like drains that help manage this water, but they also have a limited capacity and are overwhelmed in a big storm event.