Mataura flood protection upgrade

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 December 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 13 September 2023

• Sourced 27,000 tonnes of rock armouring, contracted, and completed strengthening 750 m of river bank (true left) alongside Mataura township (Mataura Gorge)
• Bulk earthworks and high-risk tree removal completed for the Waimumu stop bank upgrade (Contracted to Downer NZ Ltd.)
• Commenced gravel extraction from the gravel island in Mataura township to construct a new stop bank in south-east Gore
• Sourced 32,600 tonnes of rock armouring and rubble, contracted and completed strengthening the first 200 m of river bank along the true right of the Mataura Gorge

Next steps

• Re-grassing and re-fencing the upgraded Waimumu stop bank as weather and ground conditions permit
• Rock armouring a further 510 of river bank on the true right of the Mataura Gorge (710 m total)
• 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.