Areas covered by flood modelling in Southland

Environment Southland prioritises flood modelling in areas with the greatest flood risk, aligning with where the flood infrastructure network is concentrated across the region. Our models show how flooding might occur, assess potential impacts, and inform decisions about infrastructure works, land use and emergency planning.

Current focus areas

• Mataura River – Athol to Wyndham
• Ōreti River – from above Mossburn to Kōreti New River Estuary, including the Makarewa
• Invercargill – includes Waihōpai, Ōtepuni and Kingswell
• Aparima River – Avondale to coast (starting with Ōtautau to Fairfax)
• Waiau River – focusing on Tuatapere

Bathymetric LiDAR and historic event data collection are underway across these areas.

Mataura River flood modelling project

The Mataura project improves our understanding of flood hazards in Gore, Mataura, Waimumu and Wyndham. These towns rely on flood banks and other flood-infrastructure assets to reduce the effects of flooding from the Mataura River and its tributaries.

Environment Southland commissioned Land River Sea Consulting (LRS) to develop a detailed flood model of the Mataura River. The model enables us to:

• Assess current levels of service provided by flood infrastructure assets
• Identify flood-bank breach risks
• Evaluate potential infrastructure improvements
• Explore nature-based solutions and land-use changes to reduce flood impacts

📌 We recommend reading the Executive Summary as a starting point, as it provides a clear overview of key findings, flood risk insights, and model limitations.

For a detailed understanding of the Mataura River Flood Modelling Project, you can access the full report in the sections below:

Key report sections:
📄 Mataura River Flood Modelling (2024) – Executive Summary
📄 Mataura River Flood Modelling (2024) – Report (without Appendices)
📄 Mataura River Flood Modelling (2024) – Appendix A, B, C
📄 Mataura River Flood Modelling (2024) - Maps

Peer review:
📄 Mataura River Flood Modelling (2024) – Peer Review by Philip Wallace, River Edge Consulting Limited

Wyndham flood modelling:

📄 Wyndham Flood Modelling Report (2024)

Why model the Mataura River?

• The Mataura catchment is Southland’s second largest, covering 5,360 km², with the river flowing ~240 km from source to sea.
• Severe flooding has historically affected Gore, Mataura, Wyndham and surrounding rural communities, including significant events in 1978 and 2020.
• While flood banks help reduce the effects of flooding, extreme rainfall can still cause widespread inundation.
• Climate modes such as the Interdecadal Pacific Oscillation (IPO) and Southern Annular Mode (SAM) influence flood frequency, and current trends point to an increased likelihood of large flood events.

How the model works

• The 2D hydraulic flood model was built using MIKE 21 FM software, incorporating high-resolution LiDAR data from 2020 and 2022.
• It is split into two sections —upstream Gore and downstream Mataura —allowing for a detailed analysis of flood behaviour.
• Surveyed flood bank elevations and recent flood infrastructure upgrades were integrated into the model, including:
  • The new section of the flood bank in southeast Gore
  • The raised Boundary Creek flood bank

Both of these upgrades were constructed post-February 2020.

• The model uses flow data from Environment Southland’s hydrology monitoring sites, though recent high-flow measurements in the Gore reach are limited.

What this model tells us

• Current flood bank performance in Gore and Mataura under various flood scenarios.
• Potential impacts of flood bank breaches and where floodwaters would spread.
• Flood depth, speed, and hazard mapping for 1-in-50-year (2% AEP) and 1-in-100-year (1% AEP) flood events.
• Insights to inform future flood infrastructure planning and investment.

Limitations of the model

• February 2020 flood calibration is subject to uncertainty due to a lack of recent high-flow gauging and changes in river morphology from willow removal and sediment buildup.
• The Mataura Gorge presents challenges due to its steep terrain, high velocities and turbulence, leading to greater uncertainty in flood predictions for this reach.
• The model is a fixed-bed model and does not simulate sediment transport, which may influence real-world flood behaviour.
• The model was peer-reviewed in May 2024 by Philip Wallace of River Edge Consulting Limited. Several recommendations for improvements were implemented in consultation with the reviewer. However, due to technical limitations in software and data uncertainties, further refinements to the model may not be feasible at this time.

Next steps

• NIWA is currently reviewing hydrology estimates for the Mataura River, which may lead to refined design flow rates.
• The model will support future decision-making on flood risk management and infrastructure improvements in the Mataura catchment.
• Community engagement and public access to flood mapping data will be available via the Southland Natural Hazards Portal.

Future modelling

As technology advances and funding allows, Environment Southland will continue expanding flood modelling across the region, prioritising areas with the highest flood risk. We will also refine and improve our models by enhancing inputs such as climate change projections and maximum credible events.

A key part of flood modelling is gathering a broad range of data and insights into historic flooding. If you have photos, videos, or observations of past flood events, we encourage you to share them with us.

Murihiku Southland’s flood infrastructure network is more than 30 years old and is under increasing pressure from climate change. To keep it performing well into the future, Environment Southland continues to invest in flood modelling, infrastructure upgrades, and long-term planning to reduce the effects of flooding and support community safety.