The Inland Rail alignment crosses several watercourses and floodplains in Southern NSW (SNSW). We acknowledge that responsibly managing the potential risks of flooding in known flood prone regions is a fundamental expectation of communities along the alignment.
We’re committed to designing, building, and operating Inland Rail in a way that minimises disruption to flood behaviour and doesn’t harm the surrounding environment. Our approach to this is closely monitored and assessed by the NSW Department of Planning, Housing and Infrastructure (DPHI).
We also listen to feedback from landowners, councils, government agencies and hydrology experts, and use flooding data to inform our designs. This ensures that the infrastructure we build is resilient and balances the needs of the Inland Rail Program with community expectations.
Our flood modelling shows the impacts of various flood scenarios prior to construction of Inland Rail. We then overlay the model with engineering designs to identify any changes to hydrology or flooding caused by the new or upgraded railway. The design is progressively refined to reduce or where feasible, remove negative impacts.
Our approach to flood impact modelling
Inland Rail works with landowners, councils and government agencies to complete hydrological and geomorphological assessments that align with national guidelines and industry best practice.
We engaged leading engineering consultancies to develop hydraulic models for the Flood Design Verification Report (FDVR):
- Illabo to Stockinbingal: John Holland and BG&E.
A key input into our flood modelling is the Australian Rainfall and Runoff (ARR) Guideline. It gives us reliable tools and data—like rainfall records and flood prediction models, to estimate how floods might behave in different parts of Australia. This helps ensure our decisions are based on solid science and are consistent across regions.
Our flood models consider:
- water depth
- data from councils and government agencies
- landscape features and land use
- water flow rate and velocity
- landowner feedback, including imagery of historic events
- topography
- extent of flooding
- real-time data collection in periods of flood
- flooding hazard
- satellite imagery of previous flood events
- built environment
- flood duration
- rainfall data
- flood event modelling
- soil type
- hydraulic structures.
We also factor in the scope of nearby planned and current infrastructure projects to test cumulative (combined) impacts.
How we talk about flood risk
Flooding risk is reported as Annual Exceedance Probability (AEP), which measures the likelihood of a flood or rainfall event of a specific magnitude occurring or being exceeded in any given year. For example a 1% AEP has a 1% chance of occurring in a one-year period. 1% AEP is sometimes expressed as a 1 in 100-year event. This can be problematic as theoretically a 1% AEP event can and has occurred several years in a row in some communities in Australia.
Example of a typical culvert bank showing the relationship between 1% and 10% AEP flood peak level.
Flood impact limits
The NSW Department of Planning, Housing and Infrastructure (DPHI) has set limits on the maximum impact that Inland Rail can cause before mitigations are required, for scenarios up to and including the 1% AEP flood event.
These limits are known as Quantitative Design Limits (QDLs) and are set at varying levels, depending on the environment and flood affected object or area such as a building, road or farm.
QDLs cover changes in:
- peak water levels (by land use and receptors)
- water flow velocity (maintain existing where practical; manage discharge)
- inundation duration
- flood hazard (depth and velocity)
- erosion threshold velocity (ETV).
The process:
- If our modelling shows QDL exceedances (non-compliance with QDL), we design solutions to reduce potential flooding impacts.
- If design refinements can’t remove impacts, we consult with landowners on other mitigation measures — often over several meetings — to agree a solution.
- If agreement isn’t reached, the Flood Design Consultation Protocol outlines the steps for consultation, agreement and dispute resolution with affected parties.
Examples for peak levels:
Residential buildings – up to +10mm. Above this we must mitigate.
Agricultural land – up to +200mm. Above this we must mitigate.
View more flood models for Illabo to Stockinbingal showing a 1% AEP event (current scenario, without Inland Rail).
How design initiatives mitigate and minimise flood impacts
We work with impacted landowners and technical specialists to ensure any proposed hydrological solutions have minimal impact on properties and on-site infrastructure.
Design solutions we could implement to reduce flooding impacts include additional culverts, bridges and scour protection around culverts. For example, on the Illabo to Stockinbingal section, we refined our flood model and added about 200 culverts to balance flows and reduce impacts.
What happens once operations begin?
All Inland Rail sections are subject to strict environmental conditions of approval including several related to flood impact monitoring, reporting and management. These conditions are designed to monitor flood behaviour, assess that impacts occur as predicted by our model and address any non-compliant impacts.
The ultimate manager of Inland Rail infrastructure, the Australian Rail Track Corporation (ARTC), will oversee compliance monitoring for up to 15 years after Inland Rail becomes operational.
Two key requirements applicable to NSW projects are:
1. Operational Erosion Mitigation and Monitoring Program (OEMMP)
An OEMMP will be developed in consultation with landowners and DPHI to monitor any changes in erosion which may be caused by the railway. The program must:
- identify monitoring areas including the upstream and downstream ends of all drainage structures
- provide a baseline pre-construction survey of conditions and report annually and after rain events greater than a 20% AEP for a period of 15 years or as otherwise approved by the DPHI
- outline a process for adaptive management and mitigation measures if monitoring determines the project is impacting erosion and a process for addressing complaints received that relate to erosion.
2. Flood Review report after construction
Following the first of several defined flood events (i.e. greater than 1% AEP, 1-5% AEP, 5-10% AEP and 10-20% AEP) post construction, an experienced hydrologist must prepare a Flood Review Report that compares the observed flooding characteristics to what was predicted. The report must identify properties and infrastructure affected by flooding and document any adverse impacts caused by Inland Rail. Following consultation with affected parties, the report must propose mitigation measures to reduce future adverse impacts including timing and responsibilities for implementation.
You can read more about flooding impact assessment and conditions of approval by reviewing the Environment Impact Statements for Inland Rail’s Narromine to Narrabri, Narrabri to North Star and North Star to NSW/ Qld Border sections. Please visit the NSW Government’s website at pp.planningportal.nsw.gov.au/major-projects and search by section name.
Talk to the Inland Rail team
If you would like to contact Inland Rail’s NSW team, you can either call, send an email or visit one of our offices. We also hold regular events, and you’re welcome to subscribe to our enewsletter.
Alternatively, you can fill out the below form, and request a team member to contact you:
