Mataura flood protection upgrade
Environment Southland is undertaking riverbank stabilisation work and upgrading the stop banks that protect the town of Mataura (population 1500+). The work includes upgrades to the Boundary Creek and Waimumu Stream stop banks and remedial flood protection measures within the township.
The Boundary Creek and Waimumu Stream stop bank upgrades are Resilient River Communities projects. Resilient River Communities is a joint initiative between Kānoa - the Regional Economic Development & Investment Unit, regional councils, and local authorities focused on developing and upgrading vital river management and flood protection schemes in Aotearoa New Zealand.
By having resilient river communities, we hope to minimise environmental, economic, and social damage caused by flooding – making New Zealand safer for everyone.
The Mataura flood protection scheme is a comprehensive river control scheme providing flood protection through stop banks and channel edge stability through rock rip-rap revetments and willow trees. The network contains floodwater to the river corridor and the smaller tributary creeks and streams.
Extensive flooding in Southland in February 2020 saw stop banks in Mataura tested by floodwaters to near capacity, with the largest volume of water to pass under the Gore and Mataura bridges in recorded history. Emergency management and council staff, alongside Southland emergency services, evacuated 1500 residents in Mataura alone and 4500 + throughout the Mataura catchment.
Environment Southland did some remedial work on Mataura's stop banks soon after the February 2020 floods. Specialist engineers then evaluated the stop banks as part of our climate resilience programme. This work identified a need for detailed geotechnical investigations and 2D hydraulic modelling to better understand the capacity and integrity of these stop banks and determine the best design solution for the town.
We received the preliminary geophysical report in November 2022 and, on that basis, confirmed that there is no apparent structural damage to the stop banks on the Mataura River as a result of the 2020 flood event. However, river engineers did raise concerns about a 600 m of river bank along the Mataura township at risk of erosion. High river flows could create erosion, leading to failure of the stop bank and flooding of the township on the true left bank– upstream of the old Mataura paper mill. In mid-January 2023, councillors and staff from Environment Southland and the Gore District Council, as well as members of the Mataura Community Board and Hokonui Rūnanga, all visited the site and were updated about the strengthening work being undertaken.
The work
The work to upgrade Mataura's flood protection scheme includes:
- Strengthening, raising and reshaping the Boundary Creek stop bank, including an extension of the stop bank by 380 m to the west
- 4000 tonnes of rock strengthening on the left bank of the Waimumu Stream, mostly between Glendhu Road and McBride Road, and the removal of high-risk trees along the stop bank
- Rock protection for 750 m of river bank on the true left of the Mataura River and 710 m on the true right within Mataura township (Mataura Gorge)
- Gravel extraction from the gravel island in Mataura township
Benefits
These upgrades will:
- Provide greater protection for properties, homes, and businesses in Mataura by increasing the resilience of the current flood protection scheme
- Enhance our ability to adapt to a changing climate and plan future asset upgrades
Funding from central government has enabled this work to be brought forward in the Long Term Plan and delivered at a much lower cost to ratepayers.
Project update as of 15 December 2023
- Boundary Creek stop bank upgrade and extension completed April 2022
- Bulk earthworks and high-risk tree removal for the Waimumu stop bank upgrade completed May 2023
- Sourced 27,000 tonnes of rock armouring, contracted, and completed strengthening 750 m of river bank (true left) alongside Mataura township (Mataura Gorge)
- Removed 40,000 tonnes of gravel from the gravel island in Mataura township to construct a new stop bank in south-east Gore, completed October 2023
- Sourced 32,600 tonnes of rock armouring and rubble, contracted and completed strengthening the first 210 m of river bank along the true right of the Mataura Gorge by late August 2023
- Completed rock armouring 710 m of river bank on the true right of the Mataura Gorge late October 2023
- Final earthworks, culvert replacements and compacting of upgraded stop bank on the true right of the Mataura Gorge and re-grassing completed in November 2023
Next steps
- Completion of 2D hydraulic modelling for the Mataura, including the townships of Mataura, Gore and Wyndham
- Further community engagement
- Use 2D hydraulic modelling to better understand the capacity and integrity of Mataura town stop banks and determine the best design solution for the next phase of climate resilience work
