ORC has recently completed two studies aiming to help better understand potential approaches for adapting to or managing natural hazards including flooding, liquefaction and lateral spreading.

These reports do not give decisions or recommendations for a specific action but are information resources to help inform both the councils and the community as we together work to decide how to respond to these natural hazard challenges.

We welcome your feedback, please email headofthelake@orc.govt.nz

Please let us know;

  • If you think the reports have missed anything?
  • Do you have any questions, or is any information unclear?
  • Do you agree with the expert assessments – if not, why not?
  • Which of the approaches discussed in the report do you think should be investigated further – and why?

Liquefaction and lateral spreading hazard management assessment

A new assessment by Tonkin + Taylor Ltd, completed in February 2023, focusses on assessment of possible engineered hazard mitigation or management approaches for liquefaction and lateral spreading in Glenorchy township.

It identifies a range of engineering mitigation techniques for liquefaction and lateral spreading that could be considered for use on land, buildings and infrastructure. These techniques considered range from very robust options through to “do nothing” or smaller-scale interventions.

The report then shows how these techniques could be applied across the township, and provides a preliminary high-level assessment of how effective these mitigation works could be in reducing damage, and an indicative relative cost comparison.

Tonkin + Taylor report: Engineering Approaches for Managing Liquefaction-Related Risk

Floodplain hazard management assessment

The report by Damwatch Engineering Ltd focuses on management of three areas where floodplain hazards - flooding or erosion – may impact on the community or infrastructure in the head of Lake Wakatipu area;

  1. The lower Rees River and Glenorchy township
  2. The Dart floodplain and Kinloch access
  3. The Rees floodplain and the Rees bridge

Floodplain hazard management report

The report identifies a range of potential hazard management interventions, and reviews the key benefits and constraints of those approaches.

These report does not give decisions or recommendations for a specific action. It is an information resource to help inform both the councils and the community as we together work to decide how to respond to these natural hazard challenges.

For each of the focus areas, the report includes tables discussing the engineering or river management interventions identified as possible approaches for management of floodplain hazards.

These assessments also included reviewing river management and engineered suggestions by community members as possible approaches for managing the flood hazard in the Rees River, Dart River and Glenorchy area.

They key report findings are presented as Tables 4.1, 5.1 and 6.1 of the Damwatch report. Each of these tables provides a summary of the potential intervention approach, and lists the key constraints which may be challenges in implementing that type of intervention.

Here is just one example.

 Intervention  Comments  Key Challenges for Inclusion in Adaptation Pathways Approach
River control structures or plantings (e.g. groynes to mitigate channel / stopbank erosion)

Intervention only aims to bolster the security of the existing stopbank by mitigating the erosion hazard to it. OLDC has recently applied some rock armouring to the most vulnerable part of the existing stopbank. Short stub groynes constructed of rock material are an alternative form of bank protection to a rock revetment type of protection (they push high flow velocities away from the bank being protected).

This intervention only provides increased erosion security to the existing stop bank, not enhanced protection from flood inundation. Other sections of the existing stop bank remain unprotected and vulnerable to attack by high flow volumes under flood conditions and as channel braids in the river shin over time. As with the existing stopbank, ongoing bed aggradation will eventually subsume groynes or other river control structures, thereby gradually reducing the level of service over time. Intervention does nothing to address the increasing flood hazard from the Rees River due to ongoing riverbed aggradation, flood breakout into the Glenorchy Lagoon and future climate change impacts.

Example extract from Table 4.1 of the Damwatch report to show how assessment findings are laid out.

 

Page last updated 21 June 2024.