The Glenorchy area is exposed to a wide range of potential natural hazard risks such as flooding or alluvial fan activity, but other less frequent hazards such as earthquakes could also have a high impact. This area is only 40-50 km from the Alpine Fault and other active faults. Earthquakes could trigger other events such as liquefaction, landslides and rockfall, or even a tsunami following a large landslide into the lake. 

The flooding risks in the Glenorchy area are continually changing through time in response to natural processes and large-scale environmental changes. These include migration of the braided Dart and Rees rivers across their floodplains, growth of the Dart and Rees delta and landforms, and stream flooding activity across the many alluvial fans. 

As the Dart and Rees deltas continue to grow into the lake, riverbed levels will also rise with ongoing sediment deposition, and the flood risk for the Glenorchy and Kinloch areas will increase. In addition, climate change is expected to bring more frequent and heavier rainfalls, increasing the flood risks for the rivers and lake. The potential impacts of these on people living in these areas is expected to increase, particularly for low-lying lakefront areas and floodplains. 

The area encompasses the Head of Lake Whakatipu, including Glenorchy, Kinloch and the surrounding rural Dart and Rees Valleys, Paradise and Greenstone areas.

As well as Glenorchy, many of these other locations are exposed to several natural hazards such as flooding and erosion on the Dart and Rees floodplains, and flooding and debris flows from the tributary streams or hill slopes to the east and west.

An active fault – the West Whakatipu Fault – runs along the western side of Lake Whakatipu from Mount Nicholas towards the Routeburn Valley. 

A new study of liquefication hazards at Glenorchy has found that widespread ‘high to severe’ liquefaction damage is likely for all moderate to major earthquake scenarios, including an Alpine Fault rupture. Lateral spreading is also expected for some earthquake scenarios, which would cause severe ground deformation and damage in the western areas near the lakefront.

At Glenorchy township, geological investigations show all of the sediments underlying the near-surface Buckler Burn gravels are highly susceptible to liquefaction.

A major earthquake and Alpine Fault rupture would likely cause ‘High to Severe’ liquefaction damages, comparable even to the 2010-2011 Christchurch earthquakes. In addition, lateral spreading would also cause severe ground deformation and damages in the western areas nearer the lakefront.

The map below shows the liquefaction hazard categorisation developed by T+T for liquefaction and lateral spreading hazards at Glenorchy. The hazard categorisation is based on New Zealand guidance from MBIE and MfE and is shown in more detail below.

Liquefaction and lateral spreading can occur when strong ground shaking during an earthquake disturbs ground sediments, causing them to behave as a fluid. The ground surface above liquefied soil can tilt and sink, affecting buildings, roads, and underground infrastructure such as water supply and septic systems at varying degrees. Lateral spreading is when these liquefied soils move sideways, usually towards water or lower ground. When the soil underneath moves, cracks can also appear in the ground. 

Figure 1: An illustration of liquefaction and lateral spreading processes and their effects (IPENZ, 2012).

Modelling shows that in major flood events, such as an event with a 1% chance of occurring in any one year (a ‘100 year’ flood), there is widespread flooding in the northern parts of the Glenorchy township.

Floodwaters may be deep (into the range 1-2 metres) and fast flowing (into the range 1-2 metres per second).

Possible impacts of flooding have been assessed based on the modelled depth and velocity characteristics. This shows that in the areas of deepest and highest velocity floodwaters, it may be unsafe for both people and vehicles, and buildings may be vulnerable to structural damage.

The topography in the township is the main control of the flooding extents – as ground slopes up towards the south and southwest those slightly higher areas are likely unaffected by flooding from the Rees River or Lake Wakatipu.

(But note the current flood hazard modelling project does not include other potential flooding sources such as Bible Stream or the Buckler Burn. These flooding sources may affect areas outside the modelled flood extents of the current project).

The effects of climate change on river flows, or a breakout of the Rees River channel eastwards towards the Glenorchy lagoon, do not cause major increases in flooding hazard.

In a major flood event, there would also be widespread flooding of the Dart and Rees floodplains.

Model results for the wider Dart-Rees floodplain area show large sections of the Kinloch Road, as well as parts of the Glenorchy-Routeburn road at the foot of Mount Alfred, would be inundated in a major flood event.

These modelled events are similar, but even larger, than recent flooding events in this area such as March 2019 and February 2020 (as pictured) (Image by Luke Hunter).

Model results for a flooding scenario at Glenorchy, these are coloured by floodwater depth.

Landscape and climate changes are expected to cause increases in the severity and likelihood of natural hazard impacts to this area – particularly for rainfall-driven hazards such as flooding.

This environment is very dynamic, with a long history of large-scale environmental changes. These include migration of the braided Dart and Rees rivers across their floodplains, growth of the Dart and Rees deltas (landform created by sediment carried down the river to where it enters the lake), and common stream activity across the many alluvial fans. 

As the Dart and Rees deltas continue to grow into the lake, the flood risk for the Glenorchy and Kinloch areas increases, and riverbed levels rise with ongoing sediment deposits. In addition, climate change is expected to bring more frequent and heavier rainfalls, increasing the flood hazard from the rivers and lake. The impact of these on people living in these areas is expected to worsen, particularly for low-lying lakefront areas and floodplains. 

Model results for a flooding scenario at Glenorchy, these are coloured by floodwater depth.

A paper presented to the ORC Data and Information Committee (DAIC) in June 2022 presents a summary of recent findings and an update on other ORC work towards this adaptation project. This paper is available here.

The natural hazards in the head of Lake Wakatipu area present a difficult challenge – because of the number and complexity of the multiple natural hazard threats in the area, and the fact that future climate and landscape changes will increase the potential consequences. 

ORC has previously undertaken engineering works to address natural hazards issues such as flooding and river management. While engineering works provide temporary benefits, they are unlikely to be sustainable financially or environmentally in the long run. 

An adaptation approach will provide the ability to develop a more comprehensive strategic response – this will involve a range of hazard management interventions, and both short-term and longer-term responses. 

Our project objective summarises the goal of the adaptation project: “To provide a framework to actively manage risks associated with natural hazards for the resilience of the area located at the Head of Lake Wakatipu, including Glenorchy and Kinloch.” 

The ‘Adaptation Pathways’ approach has been developed by the Ministry for the Environment as a blueprint for community-led decision making in areas affected by natural events and climate change. The approach will help plan and adapt to situations where the future is uncertain. It allows for flexible and adaptive decision-making, and for planning under conditions of uncertainty on the rate, timeframes and magnitude of future changes. 

The Adaptation Pathways guidance is structured as a 10-step decision cycle, organised around five key questions (Figure below); 

• What is happening? 
• What matters most? 
• What can we do about it?  
• How can we implement the strategy?  
• How is it working? 

Figure 1: 10-step decision cycle, Coastal Hazards and Climate Change (MfE).

The project started in mid-2019, with preparation work to plan the project approach and scope. 

The following stages of the Adaptation Pathways process have involved an extensive programme of work over the past two years to build on our natural hazards understanding. The main work activities or the project to date are summarised in the figure below, showing where they fit within the Adaptation Pathways cycle. 

We have also held two community drop-in sessions to discuss the project and the natural hazards in this area. These have helped us receive local feedback to ensure community views and local hazards knowledge are taken into account. 

The new studies recently completed add to our knowledge and now provide a level of detail to inform adaptation decision-making by the community, local government and central government 

ORC’s next project steps (5 and 6 in the graphic) will identify and evaluate ‘pathways’ of adaptation actions through engagement with the local community and other key project partners such as QLDC.  

One of these studies is already underway - Damwatch Engineering Ltd are assessing possible strategies for mitigation of floodplain hazards at the Dart and Rees rivers and Glenorchy. 

Next steps are to work towards a preferred strategy of adaptation in engagement with the local community and other key project partners such as QLDC.  

Community input is central to the Adaptation Pathways approach, and it is our intention to involve and incorporate your feedback in all stages of this project. 

So far, and despite some disruptions due to Covid-19, these have included: 

• Attendance at several meeting of the Glenorchy Community association to provide updates on new natural hazards work and hear concerns about hazards issues in the area. 
  
  
• Public talk by a consultant – Professor James Brasington – to give his expert view as a river scientist of the natural hazards in this area. 
  
  
• Two drop-in sessions with the community (December 2020 and April 2021) to discuss the natural hazards and possible adaptation approaches. Adaptation interventions raised by the community being considered as part of a consultant project to assessing. 

Our regular (monthly) community newsletter is another way we are aiming to keep you updated on the project. Sign up or view previous newsletters here.  

The new natural hazards findings were presented in an online meeting on 2 June, and there will be further opportunities to discuss these in more detail over the next months. 

• All findings for these and any other studies will be made publicly available on our project webpage. This content will include presentations and technical reports. 

The upcoming Data and Information Committee (DAIC) meeting will be streamed on the ORC YouTube channel.​

If anyone has any questions or concerns or would like to contact the project team for any reason, feel free to contact the ORC Natural Hazards Manager, Jean-Luc Payan, on 0800 474 082 or email us for more information at headofthelake@orc.govt.nz.  This email will go to our Natural Hazards team, and we will provide responses to any emailed questions, and facilitate answers from our consultants if needed. 

Environmental monitoring of rainfall, river flows and lake levels provides a highly important dataset for understanding of flooding events and for flood warning. 

In the last two years, we have installed two new monitoring stations at Glenorchy and one in the Rees Valley. These are: 

• Glenorchy Lagoon water levels 
• Rees River flows, measured near Invincible 
• Lake Wakatipu levels, measured at the marina 

These three new sites were all proposed by the community following the February 2020 flooding as actions to improve awareness of flood hazard. 

Together with two older ORC monitoring sites in the Dart Valley, the current environmental monitoring network provides greatly improved monitoring coverage – this is summarised in the image below.  

All of these sites are telemetered and their near real-time data can be viewed on ORC’s WaterInfo webpage.

We are working closely with QLDC on this project as they are a partner with us. We share all natural hazards findings with them. 

Planned follow up in-person discussions in late June. 

And other opportunities for community input (as many as needed) with the community throughout this adaptation process.  

These will be opportunities to discuss findings and let us know your feedback – we will value your input to make sure any plans reflect community values. Our information is regularly updated and can be found here.

You can also subscribe to our monthly newsletter here or contact us on Headofthelake@orc.govt.nz. We will provide responses to any emailed questions and facilitate answers from our consultants if needed.  

ORC, with partners including QLDC, are leading the adaptation strategy for the head of Lake Wakatipu following a ten-step decision-making adaptation pathways cycle (pictured below), based on the Ministry for Environment’s national guidance. This is a strategic, holistic approach working together with community; community and stakeholder input and collaboration is central to the Adaptation Pathways approach.

Hazards investigations including these two new reports are steps to better understand natural hazards and their risks, and provide a solid basis for any decision-making.

The next steps (5 and 6 in the cycle shown above) will identify and evaluate ‘pathways’ of adaptation actions.

One of these actions includes a study is already underway by Damwatch Engineering Ltd to assess possible strategies for mitigation of floodplain hazards at the Dart and Rees rivers and Glenorchy.

We will have follow up sessions (as many as needed) with the community throughout this adaptation process. These will be opportunities to discuss findings and let us know your feedback – we value your input.

Feel free to contact the ORC Natural Hazards Manager, Jean-Luc Payan, on 0800 474 082 or email us for more information at headofthelake@orc.govt.nz. This email will go to our Natural Hazards team, and we will provide responses to any emailed questions, and facilitate answers from our consultants if needed.

You can also subscribe to our monthly newsletter here, which details news, activities, and upcoming community sessions.