Draft Overland Flow Flood Study

March 2024

Public exhibition of 2023 Draft Sutherland Shire Overland Flood Study ends

April to August 2024

Reviewed 2,180 community submissions and began regular community updates.

August to December 2024

Carried out site inspections at over 60 key areas identified by community feedback and updated the model using new rainfall guidance published in 2024.

January to July 2025

Updated the flood model using the improved data from site inspections, regularly updating the Flood Risk Management Committee.

August to November 2025

Completed further site inspections, refined the study further and began developing an improved communication and engagement approach.

December 2025 to February 2026

Updated the model based on second round of inspections, started the independent expert review.

March to May 2026

Finalised the technical updates based on the expert review and conducted final round of inspections.

June 2026

Revised draft report including expert review presented to Flood Risk Management Committee which recommended it for public exhibition.

In NSW, local councils are responsible for stormwater drainage and managing flood risks in their communities. The NSW Government supports councils with funding and technical advice through its Floodplain Management Program.

The aim of this program is to better understand flooding and reduce its impacts on people, homes and businesses.

As part of this program, councils prepare flood risk management plans. These plans explain how flood risks will be managed in the area and can include actions like improving stormwater drainage or other flood mitigation measures.

Flooding in the Sutherland Shire can result from rivers, creeks or the coast, however, the most common flooding which residents experience is overland flow flooding.

Overland flow typically occurs when rainwater flows over a surface, such as lawns, roads, roofs, car parks and natural areas, on its way to the nearest drain, creek or river. Overland flow flooding can occur during heavy rainfall when stormwater systems temporarily cannot capture all runoff, causing water to flow across:

• Natural low points in the land
• Open channels and drains
• Roads and footpaths
• Public reserves
• Private property

Floodplain Management Australia (FMA) is the national peak body for flood risk professionals. It promotes responsible development on floodplains and raises awareness of flood risks to help reduce impacts on people and property.

FMA has also developed an Overland Flow Factsheet to support clear communication with Councillors, Council staff, ratepayers and the wider community.

Flooding in the Sutherland Shire can result from rivers, creeks or the coast, however, the most common flooding which residents experience is overland flow flooding.

Overland flow flooding can cause damage to property and create safety risks. Importantly, it can affect areas not typically considered at risk, such as properties on hills or far from creeks or rivers. These risks are considered in planning for future development and in the design of homes and infrastructure, including improved drainage and higher building levels to reduce the likelihood of flood damage.

The NSW Government provides the NSW Flood Risk Management Manual and Guidelines to help councils and experts carry out flood studies, for all kinds of floods including riverine, coastal and overland flow.

Council has engaged BMT, a specialist flood consultant, to prepare this Flood Study. BMT has completed similar studies for other councils across NSW using these guidelines.

The study is overseen by the Flood Risk Management Committee. This group includes community representatives, Sutherland Shire Councillors and technical experts from the NSW Government, the SES and Council.

The first step in managing flood risk is to identify where the risks currently exist. Flood modelling was carried out to understand how water flows during different types of storms. This included checking the model against data from past flood events to improve accuracy. it looked at a range of rainfall events, from smaller, more frequent storms through to rare and extreme events. It also considers potential changes in climate to understand how flood risk may look in the future.

Once finalised, the Flood Study will be used to develop flood risk management plans. These plans help Council identify ways to reduce flood risk, such as drainage improvements or other mitigation works. Completing this in line with the NSW Flood Risk Management Process may also help secure government funding for these projects.

Property values are influenced by a broad range of factors. There is no clear evidence that the adoption of overland flow information on its own would result in a sustained impact on property values.

Each insurance provider has its own risk assessment process used to set premium prices. The Insurance Council of Australia advises that insurers already hold extensive flood risk data. The industry has developed the National Flood Insurance Database (NFID), which insurers use to assess flood risk. In determining insurance premiums, insurers consider NFID data alongside other factors, including building type, location, and claims history.

We cannot rule out that Council's revised draft Overland Flow - Flood Study could impact premiums, however, it is likely it provides more accurate detail than studies conducted by an insurance company.

The standard insurers' definition of flood in Australia is defined in the Insurance Council of Australia's General Code of Practice 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.

Overland flow does not fit into this definition and is sometimes considered as ‘stormwater’ by insurers. However, it is recommended that you speak with your insurer for a full understanding of policy coverage and other types of flood risk at your property.

Further information on flood insurance is available from the Insurance Council of Australia:

Flood insurance explained - Insurance Council of Australia

Floodplain Management Australia partnered with the Insurance Council of Australia in developing three factsheets on flood insurance:

The study is not expected to have any impact on your rates. We collect rates from residents and businesses to help fund local infrastructure and services based on the unimproved property values set by the NSW Valuer General.

Council’s stormwater system includes drains, pits, pipes and open channels. Like all underground drainage, it is designed to handle smaller, more frequent rain events so people aren’t dealing with puddles every time it rains. During larger storms, the system cannot capture all the water. When this happens, excess water flows over the ground, this is 'overland flow'. Roads and valleys are often the main paths that this water follows, and open channels can often manage larger flows, but these cannot be placed everywhere.

It is not always possible or cost-effective to upgrade the drainage system to handle every storm. However, flood studies help identify problem areas where improvements may be needed. Follow up Flood Risk Management Studies & Plans can then assess whether upgrades to drainage could reduce flooding and improve safety.

Ideally, excess water flows along roads, through parks or along drainage easements. However, water will always follow the lowest points in the landscape. These low areas are often where natural creeks or drainage paths existed before development.