Why is the Flood Study being reviewed and what’s changed?

From time to time, flood models are reviewed and predicted levels may change. For the Collins Creek Flood Study, we’ve considered Council's revised Blockage Policy. We have improved information, such as recent data from land and waterway surveying. We've considered an extended network of pits and pipes and used more detailed modelling techniques. We’ve also extended the mapping to capture additional flood-prone areas that were not previously mapped.

We’ve found that Council's revised blockage policy typically provides lower flood levels upstream of key culverts. The consideration of pits and pipes also resulted in some areas where there was shallow flooding before to no longer be flood affected, as the model recognises that the pits and pipes will carry some of the floodwaters. There are a few new areas that have been captured by the flood mapping, as the model was extended to include additional areas of the catchment. Generally, 60%, of the areas that were identified as flood prone by the last flood studies haven't had a change in flood levels, 25% of the area had reduction in flood levels and about 15% had an increase in flood levels (that includes the newly mapped areas). On the difference maps presented in the report, the areas with reduction are shown in bluish colours and the area of increase are shown with warm colours (yellow, orange).

What about historical floods?

At the start of the Collins Creek Flood study review, we sent out more than 6,000 brochures and questionnaires asking the community about the floods they have experienced in the past. We received over 500 responses. Many people reported having experienced a flood in the past and specifically in August 1998. Those residents that have reported flooding are captured by the flood extend of our flood maps which gives us additional confidence in our results. The 1998 flood in this area was not as big as a 1% Annual Exceedance Probability (AEP) flood, which would explain why some areas are within the mapped flood extent while residents haven't been impacted by a flood to date. A 1% AEP flood means there is a 1% chance of a flood of this size or larger occurring at a particular location in any given year.

What happens next?

After we’ve completed the flood study, we’ll review the Floodplain Risk Management Study to look at what the risks/damages from floods might be and what we could do to mitigate (reduce) those risks. Next, we’ll review the Floodplain Risk Management Plan, to give us a prioritised plan of flood mitigation measures proposed for the catchment. Then we’ll roll out the Plan! Every 5-10 years, we go back to the beginning of this process and start with a review of the flood study to consider new survey data, policy changes, recent major flood events and changes in the catchment such as flood mitigation works or new development.

What flood mitigation work is Council doing in this catchment?

We’ve purchased the two houses that were identified in the Collins Creek Floodplain Risk Management Study and Plan for Voluntary Purchase.

We’re progressing the detailed design of structures that help stop debris from blocking drains at Gordon Hutton Park (Woonona) and Albert St (next to Memorial Drive, Bellambi), with construction expected in the next 2 years.

We’ve progressed the design of a swale in West St (Russell Vale) and are in the process of negotiating access with the owner of the land so that the swale can be constructed.

The Ursula Road Flood Mitigation Scheme in Bulli is progressing well. Investigations have shown that increasing the culverts under Franklin Avenue and/or installing a deflection levee on the northern bank of Whartons Creek would reduce flooding to properties located in this area. We’re now assessing other drainage options downstream of this area. 

We’re progressing flood mitigation investigations in Bellambi around Gladstone Street. We’ll soon go out and talk with the community in this area about this project.

How does Council manage flood risk?

Each year, Council spends millions of dollars on stormwater and floodplain management. Our team of flood experts prepare flood studies and floodplain risk management studies that help us understand the flood behaviour for a particular catchment and see if there are any ways of reducing flooding risk in an area. 

Floodplain risk management studies include a plan of potential solutions aimed at reducing the existing and future flood risk. Examples of these solutions include: 
  • Emergency response plans based on detailed understanding of flood behaviour
  • Building new structures that collect and carry stormwater into drains or creeks, such as detention basins and swales, or improving existing ones to better manage stormwater and floods 
  • Land zoning that says what can and can’t be built on flood-prone land
  • Voluntary purchase of houses built in high flood risk areas 

How do you predict flood levels?

There’s a chance that floods of any size will occur in future. As the size of a flood increases, the chances of it occurring becomes smaller. Some rare types of floods may not have occurred for over 100 years, so we have to predict the height of future floods using computer models. These models produce different flood levels and velocities (speeds) for a variety of different-sized floods. To predict flood levels, Council works with experts to establish and operate the computer models. Council also gets valuable community input on historical floods so we can adjust the model and make sure it copies what’s happened in the past. 

What is a catchment?

An area where water is collected by the natural landscape, usually surrounded by mountains or hills. In a catchment, rainwater run-off eventually flows to a creek, river, dam, lake or ocean.

How can I join the conversation?

Drop-in Information Sessions
  • Bulli Senior Citizens Centre, Saturday 22 June 9:30–11am 
  • Bulli Surf Club, Tuesday 25 June 4 – 5:30pm
Write Locked Bag 8821, Wollongong DC 2500 
Phone (02) 4227 7111

You can also share your feedback with us by completing the online feedback form.

How will my feedback be used?

At the conclusion of the engagement period, all feedback is read and considered. A report will be produced and provided to Councillors, and they will consider whether to adopt the Flood Study.