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 in the form of 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 carried out some remedial work on Mataura’s stop banks soon after the February 2020 floods. Specialist engineers then undertook an evaluation of 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 December 2022 and on that basis were able to confirm that there is no obvious structural damage to the stop banks on the Mataura River as a result of the 2020 flood event. However, river engineers have raised concerns about a 600 m section of riverbank along the Mataura township that is 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
- Strengthening 4 km of stop bank on the left bank of the Waimumu Stream
- Rock protection for 750 m of riverbank on the true left within Mataura township (Mataura Gorge)
- Gravel extraction from the gravel island in Mataura township
Benefits
- Greater protection for homes, businesses, and communities in Mataura township
- Safeguarding industries and infrastructure
- Flood protection upgrade projects completed much sooner and at much lower cost to ratepayers
Project status (1 March 2023)
- The Boundary Creek stop bank extension and upgrade was completed in April 2022
- Survey of the Waimumu stop bank, detailed design and construction drawings (completed)
- Consents and approvals for the Waimumu stop bank work obtained and tendering (completed)
- Community drop-in sessions in Mataura, Wyndham and Gore held August to October where Environment Southland technical staff shared information about the status of Mataura’s stop banks and answered questions
- Detailed geophysical investigations of the Mataura Gorge (completed)
- Preliminary report detailing the geophysical investigations (received in December 2022)
- 2D hydraulic modelling of the Mataura Gorge (underway)
- Downer NZ Ltd. contracted for Waimumu construction contract and commencement of earthworks in December 2022, with expected completion in autumn 2023
- Procurement of 27,000 tonne of rock for strengthening the Mataura Gorge riverbank (underway December 2022)
- Site visit by councillors, ES and GDC staff, members of the Mataura Community Board and Hokonui Rūnanga
- Consents and approvals for rock-strengthening work within the Mataura Gorge obtained and work underway
- Consents and approvals granted for gravel extraction from the gravel island, Mataura township
- Further public drop-in sessions in Mataura held in late February to update the community on the flood protection work
Next steps
- Completion of 2D hydraulic modelling for Mataura Gorge
- Completion of rock-strengthening work within the Mataura Gorge by mid-March
- Completion of stop bank improvements on the left bank of the Waimumu Stream by May 2023
Photos of the works at Boundary Creek.
