Where are we up to?
The Horsley Creek Flood Risk Management Study & Plan was officially adopted at the 28 October 2025 Council Meeting.
Background
The Horsley Creek catchment is located on the southern side of Lake Illawarra and mainly includes the suburbs of Albion Park Rail and part of Oak Flats. The catchment has a combined area of 9.25km2, draining from the quarries in the south of the catchment into Koona Bay and Lake Illawarra at the Horsley Inlet. The northern half of the catchment is largely covered by residential and light industrial development. The buffer areas surrounding the quarries are currently used for cattle and horse grazing.
The updated Horsley Creek Floodplain Risk Management Study and Plan will include a program of flood management options and initiatives for Council and emergency services to implement in the study area.
FAQs
FAQs
The Flood Planning Area (FPA) is land at or below the Flood Planning Level (FPL), which equals the Defined Flood Event (DFE) plus 0.5m freeboard for safety. The DFE is the 1% AEP flood plus climate change allowances for 2090. These definitions ensure development is compatible with flood risk and safe evacuation.
The PMF is the largest conceivable flood, while the 1% AEP flood has a 1% chance of occurring in any year. Experiencing one does not prevent another; multiple events can occur within a lifetime.
The study follows NSW Government policy, involving data collection, flood modelling, risk assessment, and plan development. Stages include gathering data, calibrating models, simulating events, assessing management options, and creating an implementation plan.
Updates include new NSW guidelines, climate data, and catchment development. The study now uses the NSW Flood Risk Management Manual and improved rainfall/runoff data to reflect current conditions.
Flood models are calibrated and validated against historical events (2011, 2014, and 2024) to ensure accuracy and reliability.
Development is allowed if it meets planning controls in Council’s LEP and DCP. These documents provide criteria for safe and compliant development.
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.
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.
Within the Horsley Creek catchment, a large proportion of the creeks and waterways are also within private property, and as such Council is unable to maintain these areas.
About
Why this project is happening:
Council, with the assistance of NSW Department of Planning and Environment (DPE), adopted a flood risk management study and plan for this area in 2017. The previous study identified the behaviour of floodwaters within the catchment, gave Council detailed information on flood risk and identified actions that Council, SES and the community can take to reduce flood risk.
Since the completion of the previous study, a number of other significant developments have happened within the catchment. These include the construction of the Albion Park Rail Bypass which involved modification to the Green Meadows detention basin and creation of a basin at Croome Road. In addition, there has been key revisions to Australian Rainfall and Runoff (2019) and the NSW Flood Risk Management Manual (2023).
What has happened to date:
Consideration and planning around putting in place some of the flood mitigation measures recommended in the 2017 study are ongoing as part of Council’s Floodplain Management Program. We have done a number of things from this plan already, this includes:
- Flood-related development controls have been updated in Council’s Development Control Plan (DCP)
- NSW State Emergency Services has been provided with updated emergency response plans.
- Investigation and concept design of a proposed detention basin at Oak Flats interchange.
- Flood information on Council’s website has been updated.
- Development and implementation of a flood education and awareness program for the community.
- Stormwater network upgrade works.
- Design and planning work to implement a detention basin upstream of the Oak Flats Interchange (Oak Flats DB4).
Who is involved in this plan:
Council successfully applied for grant funding from the State Government to undertake this review of the Floodplain Risk Management Study and Plan. Council has revisited the proposed flood risk management measures in light of the changes mentioned above and is providing updated flood information for Council in accordance with current industry best practice.
The project is being managed by Council, with financial and technical assistance provided by DPE. The project is being steered by the Coastal and Flood Risk Management Advisory Committee of Council.
Draft Options
Flood Management Options
The draft Floodplain Risk Management Study and Plan recommends several flood management options for Council to implement that have the potential to reduce flood across the floodplain.
Benefit-Cost Ratio for each option (cost of the work versus the benefit they provide). The recommended options include:
Flood Emergency Response Management Measures
- All relevant data be transferred to the SES for incorporation into their flood intelligence database and update of the Local Flood Plan.
- Raising of Industrial Road at Casuarina Street to provide flood free access for ambulances based at Industrial Road and to the Council works depot (see Option 8 below).
- Community flood education and awareness program.
Property Modification and Flood Planning Measures
- Update Flood Planning Levels and the Flood Planning Area for the catchment.
- Update Flood Risk Precinct mapping for the catchment.
- Consider adoption of Flood Planning Constraints Category mapping as part of future updates to flood-related development controls.
- Adopt the Defined Flood Event as defined as the 1% AEP flood with allowance for 2090 climate change.
- Update Planning Certificates with latest flood information.
- Consider Voluntary House Purchase of two properties on the western side of Station Road.
Flood Modification Measures
- Vegetation Management along identified sections of creek.
- Pioneer Drive Diversion Channel and Bunds (see Option 11 below).
- Tongarra Road Detention Basin and Regrading of Croome Road (see Option 2C below).
The study also recommends further consideration of a number of street drainage upgrades for Maple Street in Albion Park Rail and Brigadoon Circuit, Theodore and Lang Streets in Oak Flats. However, these options only provide limited and isolated benefits, rather than significant catchment-wide improvements.
Document Library
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FACT SHEET - WORLEY CONSULTINGPDF (264.42 KB)Initial Assessment of Flood Management Options
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IMPACT ON 1% AEP FLOOD LEVELSPDF (7.36 MB)
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.
Not all options are shown given the impacts between some options are similar.
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INVESTIGATED FLOOD MITIGATION OPTIONS - Options 2 - 11PDF (10.57 MB)
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HORSLEY CREEK COUNCIL REPORT 28 OCTOBER 2025PDF (116.97 KB)
