How flood models work

What is flood modelling?

Flood modelling uses scientific data and computer simulations to estimate where, when and how flooding may occur. Environment Southland uses these models to understand flood risk, plan flood infrastructure works, and test the effects of climate change.

Flood models help with:

• Identifying flood-hazard areas
• Designing, upgrading and sequencing flood-infrastructure works (e.g., flood banks and pump stations)
• Informing minimum floor levels for new buildings (set by district/city councils)
• Supporting the location and design of critical infrastructure (roads, utilities, telecommunications)
• Investigating nature-based solutions to reduce the effects of flooding on communities and the environment

The two main types of flood modelling

There are two key types of flood models used to assess Murihiku Southland’s flood risks:

Hydrological Modelling: Understanding How Much Water is Flowing

Hydrological models assess how much water enters rivers and streams after rainfall. They consider factors such as:

• Rainfall amounts and intensity
• Land use and vegetation cover
• Soil type and how much water it can absorb
• Runoff into rivers and streams

This helps predict how much water will flow in our rivers during a flood event.

Hydraulic Modelling: Understanding Where the Water Will Go

Hydraulic modelling simulates how water moves through river channels, floodplains and infrastructure such as bridges and culverts. It helps predict:

• The level of service provided now and under future scenarios (i.e., what size events the flood infrastructure network is designed to manage)
• Which areas are likely to flood under different-sized events

By combining hydrological and hydraulic modelling, we build a more accurate picture of flood risk in Murihiku Southland.

What data is used in flood modelling?

To create accurate flood models, Environment Southland collects detailed environmental data about our rivers, land and climate. Key data inputs include:

• Topographical information – High-resolution LiDAR mapping and riverbed surveys provide detailed 3D maps of rivers and floodplains.
• Bathymetric data – measures the depth and shape of the riverbed, mapping the underwater topography, including contours and key features, by recording water depth at various points along the river channel.
• Rainfall data – Collected from rain gauges and climate modelling tools like HIRDS (NIWA’s High-Intensity Rainfall Design System).
• River flow data – Measured through Environment Southland’s hydrometric network of river monitoring stations.
• Flood scheme assets – For example, flood banks, dams, ring drains and pumps.
• Historical flood data, insights and images – We use this information to validate the flood model accuracy and ensure that the flood characteristics throughout the simulated flood match the data and understanding of how a flood occurred (e.g. the 1984 and 2020 floods)
• Catchment characteristics – Understanding land use, soil types and vegetation to predict how water will move.
• Climate change scenarios – Models incorporate future projections of more intense rainfall and rising sea levels.

Because data collection and modelling are complex and resource-intensive, we focus on rivers and streams with the highest flood risk or existing flood infrastructure. Our flood modelling work currently focuses on the following river systems and locations:

• 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 – from Avondale to coast, starting with Ōtautau to Fairfax
• Waiau River – focusing on Tuatapere

How accurate are flood models?

Flood models provide powerful insights into flood risks, but they are only as accurate as the data on which they are based. Advancements in technology, bathymetric LiDAR mapping, hydrological understanding and climate projections continue to improve modelling accuracy. However, there are some limitations:

⚠️ Models are a snapshot in time – They reflect the conditions when they were created but may not capture land use changes, development or shifting river courses over time.

⚠️ Obstructions aren’t always included – Fences, trees, and debris can alter flood patterns but may not be included in models.

⚠️ Not all models are the same, but all have value – While some models offer higher precision than others, each is designed for a specific purpose and contributes important insights for flood risk management and planning.

Planning for the future

As climate change drives more extreme rainfall and rising sea levels, flood modelling is essential to reduce the impacts of flooding on Murihiku Southland’s communities and environment. It helps prioritise and future-proof flood infrastructure works, support adaptation planning, and inform decisions on water and food security as flood risk evolves.

We are adopting the New Zealand Vertical Datum 2016 (NZVD2016) for all new flood models to improve consistency across councils and agencies. As models are developed and improved, Environment Southland will engage with communities so our work reflects local knowledge and supports planning for Murihiku Southland’s changing climate.