At JN, we believe that responsible development and considered engineering design can mitigate the risk of flooding. With heavier-than-usual rain events and larger storm events becoming common in Australia, engineers must create responsible designs to reduce risk of flooding.

There is no way of preventing flooding entirely. But risk management strategies can help limit exposure.

Here, we explore flood risk, how it’s assessed, and measures that can be taken to mitigate its impact.

Key Flood Terminology

• • Flood Impact Assessment (FIA): Assesses a proposed development against council flood policies. Required for DAs in flood risk areas. 

• • Annual Exceedance Probability (AEP): The likelihood of a flood of a given size occurring in any year. A 1% AEP means a 1 in 100 chance annually, equivalent to the older 1 in 100 year ARI measure.

• • Probable Maximum Flood (PMF): The theoretical worst-case flood under extreme conditions. Used to design dams, bridges and other critical infrastructure. 

• • Flood Modelling (TUFLOW & HEC-RAS): Software that simulates flood behaviour using survey and terrain data. HEC-RAS models flows within a single channel; TUFLOW models flows across roads, properties and open land. Results feed into Flood Planning Levels and Evacuation Management Plans.  

• • Flood Planning Levels (FPL): The safe minimum floor level for buildings in flood prone areas, set above the flood level plus a council-specified ‘freeboard’ margin. 

• • Flood Evacuation Management Plans (FEMP): Site-specific plans covering safety measures, evacuation routes and emergency contacts for flood-affected buildings.

How is flood risk assessed in Australia?

Flood risk combines the likelihood of flooding in a given area with the severity likely to occur. Common causes include heavy rainfall, storm events, tidal surges and dam or levee failure. Planners assess flood risk by weighing both the probability of a flood and its potential damage.

To find out your site’s flood risk, apply for a flood information certificate from your local council. If no data is available, JN can conduct flood modelling to determine a site-specific flood level for your property.

How to mitigate flooding impacts

Several measures can be taken to mitigate the risk of flooding:

• • Land use planning shapes where and how development happens, such as avoiding flood prone areas or requiring buildings to sit above flood level.

• • Insurance transfers the financial risk of flooding to an insurer, covering repairs, replacement and temporary housing costs. Standard home insurance often excludes flood cover, so check with your insurer. Insurance won’t reduce the disruption or emotional toll of a flood, but it protects you financially.

• • Physical mitigation includes large-scale infrastructure like levees, dams and widened river channels, as well as site-specific measures tailored to a property’s terrain and flood risk.

• • Crisis planning covers what to do before, during and after a flood: an emergency kit, evacuation routes, and a safe destination, including provisions for pets.

JN’s flood mitigation engineering services

JN’s flood mitigation services provides site-specific solutions using expert engineering and the latest modelling technology.

We’re highly experienced in flood modelling to assess flood affectation for development sites, using industry-leading software TUFLOW and HEC-RAS.

We determine Flood Planning Levels (FPL) for developments in flood zones, calculating freeboard from council flood data or our own modelling, then setting the FPL for your design.

We also develop, review and update Flood Evacuation Management Plans (FEMPs), ensuring they stay current and compliant with regulations.

Before a new build or renovation, contact your local council to confirm whether your site is flood affected. If it is, get in touch with our civil engineers .

Learn about JN’s capabilities in assessing and minimising flood risk:

Flood Modelling | Galbraith Street, Quakers Hill
Client: Belle Property Strathfield

JN conducted a flood and stormwater assessment for a development site, and assessed existing flood conditions, stormwater systems and catchment flows, then modelled existing and proposed flood extents using TUFLOW. Findings supported the client’s Council submission.

Flood Study | Industrial Units – 318 Horsley Road, Milperra
Client: The Walsh Group
JN assessed flood and stormwater conditions for an industrial site in Milperra, an area regularly affected by flooding, overland flow and tailwater from the Georges River and Chipping Norton Lake. We modelled existing and development flood extents using TUFLOW to support the Council submission.

Civil, Structural and Flood Study | Carnes Hill Recreation and Community Precinct
Client: Liverpool City Council Architect: DWP Suters

JN delivered civil and structural documentation for the $30 million Carnes Hill Recreation and Community Precinct, spanning 70,000 square metres. Our scope covered roads, car parking, pavement design, stormwater drainage, WSUD swales, earthworks and a full flood study. Habitable buildings were set with 0.5m freeboard above the 1% AEP level, with access roads and car parking set at or above the 5% AEP level. The precinct now houses a gym, library, skate park, community centre and other public facilities.

Flood Modelling | The Oaks Airfield
Client: Stephen Bennett

JN developed a flood model to support the proposed redevelopment of The Oaks Aerodrome, a site bounding Werriberri Creek. With much of the site below the 1% AEP flood level, JN engineers identified key risk areas and proposed offset storage solutions, allowing the site to be raised without impacting neighbouring properties.

Flood Modelling and Project Management | Aged Care
Client: RSL LifeCare

After flood waters reached 0.5m inside an aged care facility in March 2022, requiring evacuation of residents, JN carried out TUFLOW modelling of the worst-case 1% AEP event, finding potential flood depths over 1.0m. We’re now consulting with flood barrier specialists on temporary protection measures, alongside Council’s planned drainage upgrades for the suburb.