30th August 2024

Two new studies investigating risk and floodplain intervention options will help inform decision-making for the Head of Lake Whakatipu Natural Hazards Adaptation programme.

Otago Regional Council Manager Natural Hazards Dr Jean-Luc Payan says this area, which includes the townships of Glenorchy and Kinloch and the Dart and Rees river floodplains, is susceptible to natural hazard including flooding from the lake or rivers, and liquefaction and land movement from an earthquake.

“These studies are an important piece of the puzzle for the natural hazard adaptation programme. They show us what some of the risks could be to people and existing property in a large flood or earthquake and this helps to determine some options for what could be done to help mitigate these risks in the future,” Dr Payan says.

The information builds on work from the past four years, which to date has already had positive outcomes for the community, including improved flood monitoring, forecasting, and warning.

ORC has been working with the community, Queenstown Lakes District Council, iwi, and Emergency Management Otago toward a more resilient future for the area.

“The risk analysis study was requested by members of the community to see a comparison between the Glenorchy-Kinloch area and other places in the country. It gives a better understanding of the natural hazard risks for Glenorchy and Kinloch, including how likely and severe these risks could be,” says Dr Payan. “It builds on all the previous work we have done to understand the hazards.”

“It is reassuring to see that the risk for life is categorised as ‘Acceptable’ for Glenorchy and Kinloch. Approaches for managing the property risk will be considered in the natural hazards adaptation strategy being developed currently”.

Two investigations were completed by consultants: a Glenorchy and Kinloch natural hazards risk analysis and an assessment of floodplain intervention options for the Dart, Upper Rees, and Lower Rees rivers. The latter was partially funded by the Ministry for the Environment through the ‘Nature-based Solutions for Resilience Planning’ programme.

Sustainably managing natural hazard risks and adapting to climate change are a challenge facing many parts of New Zealand. ORC is working on range of natural hazard adaptation programmes in Otago, including Glenorchy and in South Dunedin.

Dr Payan says the Dart and Rees floodplain intervention assessment offers feasible engineering and nature-based options that can help mitigate flooding and erosion risks. Nature-based solutions include concepts such as ‘room for the river’, wetlands and planting. The options identified will inform the natural hazards adaptation strategy being developed currently. The strategy is also considering other approaches to reduce the risk and assist future adaptation.

“Both studies build on findings from previous natural hazard investigations and will be used to help the community and councils make decisions around what to do now and in the future. They will also help us understand what actions we may need to take to manage these hazards and how best to prioritise them.”

All report findings will be presented to the community on 10-11 September.

7pm, Tuesday 10 September: in-person community presentation at Glenorchy Hall and online via livestream. The presentation will be available to view after the livestream.

2-7pm, Wednesday, 11 September: drop-in session at Glenorchy Hall.

The community event details are here: www.orc.govt.nz/holwsep24

The reports are available here: https://www.orc.govt.nz/get-involved/projects-in-your-area/head-of-lake-whakatipu/investigations-reports-and-presentations/

Findings from the Glenorchy and Kinloch natural hazards risk analysis

Natural hazard risks were analysed using the methods set out in the proposed Otago Regional Policy Statement 2021 (RPS, https://www.orc.govt.nz/your-council/plans-and-strategies/otago-regional-policy-statements). The proposed RPS categorises risks to health and safety and built environment into three categories, “Acceptable”, “Tolerable” and “Significant”. It further requires local authorities to undertake a consultation process with communities, stakeholders, and partners regarding risk level thresholds.

The risk analysis presents a snapshot of present-day risk levels and considered the risks from a range of possible hazard scenarios from smaller, moderate and large events.

Flooding risks

For the Glenorchy township area, analysis has shown that the life safety risks from Rees River and Lake Whakatipu flooding are considered ‘Acceptable’, but that the risks of property damage are relatively high in some parts of the township.

This is because floodwater characteristics (depth and velocity) in larger-magnitude flooding events would be expected to damage some residential buildings such that their functionality is compromised, and cause and disruption to road access where roads are inundated.

In Glenorchy a large proportion of the land area most prone to flooding and categorised as ‘Significant’ property risk is used for community recreation, including recreation reserves and the golf course.

Seismic risks

Risks from seismic shaking at Glenorchy and Kinloch are assessed as being ’Tolerable’, but the risks for properties from liquefaction and lateral spreading at Glenorchy are categorised as ‘Significant’. Risk to life is relatively low.

Damage to properties and infrastructure associated with liquefaction and lateral spreading is expected to be substantial, with the most significant damage being focused along the lake margins (due to lateral spreading).

Life risks for Lake Whakatipu flooding, and liquefaction and lateral spreading are also assessed as ‘Acceptable’. This is because lake flooding is a relatively slow process, which allows for a reasonable warning period, and reported injuries and deaths from liquefaction and lateral spreading are rare.

“It is reassuring to see that the risk for life is categorised as ‘Acceptable’ for Glenorchy and Kinloch. The property risk will be considered in the natural hazards adaptation strategy being developed currently”.

Findings from the assessment of floodplain intervention options for the Dart, Upper Rees, and Lower Rees Rivers.

This report offers potential adaptation options that could help to address natural hazard risks. Mitigation options considered in this assessment include:

Dart River: Creation of river-margin buffers using vegetation and rock-armouring to mitigate erosion of Kinloch Road.

Lower Rees River: Raising or modifying the existing Glenorchy floodbank to help mitigate flooding hazards at the Glenorchy township.

Upper Rees River: An approach to improve flood capacity in the area around the Rees River bridge through mitigation of existing flood hazards and providing ‘room for the river’ on the floodplain.

Next steps

These reports are the last pieces of technical information that will help to develop a natural hazards adaptation strategy for the Head of Lake Whakatipu area. The strategy will form a strong basis for decision making and guide us to work together effectively on adaptation responses.

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