The project is now complete

Council is carrying out a detailed flood study of the Lake Illawarra, Mt Warrigal and Oak Flats catchments to define the existing flood behaviour and better understand the flood risks within this area of the City.

This study will establish the basis for subsequent floodplain management activities and is being prepared to meet the objectives of the NSW State Government's Flood Policy.

Council will manage this project and work with BMT,(External link) an independent company specialising in flooding and floodplain risk management. BMT will undertake the study and the NSW Office of Environment and Heritage(External link) is providing financial and technical assistance.

The Study area

These catchment areas are all located on the southern side of Lake Illawarra and include the suburbs of Mt Warrigal, Warilla, and Oak Flats, along with a small portion of Blackbutt and Shellharbour City Centre. The study area is within the Shellharbour Local Government Area (LGA) and includes a number of smaller catchments, all less than 2 km2 in area.

Study area


What is the Probable Maximum Flood (PMF)?

The PMF is the largest flood that could conceivably occur. It is typically estimated from probable maximum precipitation coupled with the worst flood producing catchment conditions. While it is a rare and improbable occurrence, every property potentially affected by a PMF is considered to be on a floodplain and has some element of flood risk. Under the State Government's Floodplain Development Manual (2005), councils must consider the full range of risk when managing floodplains.

What is the 1%AEP flood event (100-year flood)?

A 1% AEP flood event (often referred to as the 100-year flood) is the flood that has a 1% probability of occurring in any given year. If you have experienced a 1% AEP flood event, it is still quite possible for you to experience another event of similar magnitude within your lifetime, as large flood events do occur randomly. Some parts of Australia have received two or three 1% AEP flood events within a few years of one another. On average, if you live to be 70 years old, you have a better than even chance of experiencing a 1% AEP flood event.

What does Australian Height Datum mean (AHD)?

Australian Height Datum is a reference of the elevation of any object or point above mean sea level, which is taken as 0m AHD. All flood levels, floor levels and ground levels are given in meters AHD.

How can I get detailed flood information for my property?

In 2005 the New South Wales Government put forward the updated Floodplain Development Manual which outlined the required method for councils to manage flood liable land. A key objective in this manual is to undertake flood studies for appropriate catchments to provide detailed information on flood behaviour.

Council has completed a number of Flood Studies and Flood Plain Risk Management Studies. The adopted studies are available on Council's website. If your property is flood affected and you require flood levels and velocities for your property, you can submit a request for flood level information to Council. Council will then provide you with property specific flood information letter that can be used to manage your risk and inform the design of your development proposal.

Why do flood levels and information need to be reviewed over time?

Flood behaviour, including flood levels, velocities and hazard is calculated using detailed computer models to simulate floods of varying magnitudes. These models may be reviewed periodically when:

  • new data becomes available from new flood events
  • flood mitigation works are undertaken
  • developments occur, and
  • more advanced computer models become available.

My property was never classified as ‘flood prone’ or ‘flood liable’ before. Why is it now classified as being flood affected?

Many parts of Shellharbour City have little or no flood data or records available at Council. Over time as new data is received and new flood studies are adopted by Council, more flood liable land will be identified and mapped accordingly. Also, under changes to the NSW Government's Floodplain Development Manual in 2005, flood liable land became inclusive of land that is affected by the Probable Maximum Flood (PMF). Before this only land inundated by the 100-year flood level was considered.

If your property is now classified as flood affected, the real world flood risks to your property is not likely to have changed; only that new information has been made available.

Will my property value be altered if my property is flood affected?

Councils floodplain management program has been ongoing now for more than 15 years. Council first adopted a flood study for Lake Illawarra in 2001. A flood study was adopted for Elliot Lake Little Lake in 2006 and again for Horsley Creek in 2011. Concerns over effects on property values were raised from residents in these catchments prior to adoption by Council, however no evidence has been presented to Council to suggest that there have been negative impacts on property values or development in these study areas.

Will I be able to get house and contents insurance if my property is flood affected?

In 2012, a standard definition of flood was agreed upon by the Insurance industry. This is now known as:

The covering of normally dry land by water that has escaped or been released from

the normal confines of:

• any lake, or any river, creek or other natural watercourse, whether or not altered or

modified; or

• any reservoir, canal, or dam.

However, Insurance companies each have their own distinctive ways in which they calculate risk and determine insurance policy premiums. Many insurance companies will offer house and contents insurance, with each individual insurance company determining their own policy and conditions.

Flood insurance premiums generally reflect the level of flood risk at a property and the cost of repairing or rebuilding the property. In practice, this can be broken down to three factors which would be assessed by all insurers when setting a flood premium for a property:

  • Likelihood of flooding;
  • Expected depth of flooding relative to the insured building; and
  • Expected cost of recovery.

Likelihood and depth of flooding are assessed at an individual address level, using results from computer flood modelling which simulates how water flows through a catchment. Expected cost of recovery includes repair, rebuild and replacement costs, temporary accommodation, and other factors such as the potential shortage of materials and labour after a flood event. Some insurers may also consider property-specific information such as number of storeys, floor levels, building materials used and construction type.

Insurance policies and conditions may change over time or between insurance companies, and you should confirm the specific details of your situation with your insurer.

Floodplain Management Australia has developed flood insurance fact sheets to assist property owners in understanding the information surrounding this process.

Can a flood affected property be developed?

Flood affected property may be developed if the proposed development meets the relevant standards and requirements. The applicable planning controls are largely determined by the proposed land use and the extent of flooding on your property.

If my property is identified as flood prone, what development controls are likely to apply?

A Local Environmental Plan (LEP) is a legal document which allows Council to regulate land use and development. They are prepared by Council and approved by the State Government.

A Development Control Plan (DCP) provides detailed planning and design guidelines to support the planning controls in the LEP. Sections of Council's DCP provide criteria for assessing applications for properties potentially affected by flooding. The above documents are available on Council's website.

How does Council maintain its stormwater drainage network?

Council owns and manages a significant number of stormwater pits, pipes, channels, culverts, basins and Gross Pollutant traps throughout the city and we are committed to maintaining this infrastructure within the limits that current funding and resources permit. Council has an annual budget for cleaning and maintaining this infrastructure, as well as a budget for Capital Works (upgrading and building new infrastructure). All works are scheduled on a priority basis, where those works that are most critical become highest priority.

Why doesn’t Council remove the vegetation that clogs up the creeks and waterways?

Although the impact of vegetation on flood behaviour is very important, vegetation has an essential function in managing erosion and providing habitat to wildlife. Unless identified within an adopted Floodplain Risk Management Study & Plan, the flood impacts associated with the removal of vegetation from creeks and waterways is relatively unknown. For example, carrying out such works may have little to no benefit in reducing flood risks. Alternatively, such works could reduce the flood risks in one area, whilst increase it in other areas. These issues are considered in the context of a Floodplain Risk Management Study.

Useful info

Flood studies are a comprehensive investigation of flood behavior in a specific area. The study provides information about the nature of flood risk including levels, depths, extent and distribution of floodwaters for a range of storm events, including a 100 year flood. When the flood study is complete it can help to manage potential risks of future floods to people, property and infrastructure.

Other studies

We have also developed flood studies for other parts of the City;