Flood modelling in Southland

Why flood modelling matters

Flooding is Murihiku Southland’s most common natural hazard, affecting people, property, businesses and infrastructure. With four major river systems — Mataura, Ōreti, Aparima and Waiau — our region is particularly vulnerable to river flooding, which can have significant environmental, economic and social impacts.

To better understand flood risk, Environment Southland uses flood modelling to assess how river systems respond to rainfall, land-use change and climate change. These models inform decisions that help reduce the effects of flooding on communities, critical infrastructure, and the regional economy.

What is flood modelling?

Flood modelling uses computer models, mathematical equations and observed data to simulate how rivers, streams and floodplains behave during different rainfall and flood events. It brings together hydrology (how catchments generate flow) and hydraulics (how water moves across land and channels) to assess:

• Where floodwater flows and ponds
• How deep and how fast water will move
• How infrastructure such as flood banks, roads and bridges may be affected
• How climate change may alter future flood behaviour

Understanding these factors supports emergency response and recovery, guides flood-risk reduction measures, and helps explore long-term solutions for more resilient catchments.

How flood modelling supports Murihiku Southland’s resilience

Environment Southland manages 500+ km of flood banks, more than 1,000 km of drainage networks, spillways, dams, tidal gates and pump stations to help reduce flood impacts. Our Flood Intelligence Programme uses modelling and monitoring to:

• Plan, design and maintain the flood infrastructure network (e.g., flood banks and pump stations)
• Advise councils (Southland District, Gore District and Invercargill City) on minimum floor levels, flood-prone areas, and inputs to planning, consenting and LIMs
• Support critical infrastructure operators (transport corridors, bridges, three waters, telecommunications, airport) with risk information
• Prepare for a changing climate, including more intense rainfall and rising sea levels, by testing scenarios and identifying priority works