Mataura River stop banks

What is the Interdecadal Pacific Oscillation?

The Interdecadal Pacific Oscillation, or IPO, is a large-scale, long period oscillation that influences climate variability over the Pacific. The IPO has a cycle of 15 to 30 years. When this is in the positive phase there are increased El Niño events which in Southland and the West Coast means more flood events. In Southland, El Niño events are associated with increased rainfall in the Summer months.

Risks to stop banks

Stop banks must be strong in a number of ways and there are a range of methods that can make them as resilient as possible.

Stop banks are put under increased stress when large amounts of water are held against them, and particularly when this water level remains high for an extended period.

Risks to our stop banks include overtopping, internal erosion, structural impacts, and tree damage. The work being undertaken now aims to improve the resilience of the stop banks to reduce these risks.

The surface of the stop bank is very important for erosion protection. Grass cover provides the best protection. Areas where bare soil develops underneath shrubs and trees on or near stop banks can be of significant concern, as this will erode very quickly.

Gravel removal

During the recent drop-in sessions, concerns were raised about erosion being caused by the gravel build-up below the Mataura bridge. We are working with partners Hokonui Rūnanga on a solution that will see gravel extracted and utilised as bulk fill in the stop bank upgrades happening across the catchment.

Trees and fences

Preliminary works will include identifying and removing fencing and trees located on the stop banks. Both trees and fences compromise the structural integrity of stop banks; our stop banks and floodways must be clear of barriers and obstacles to ensure that the infrastructure works as intended when needed.

We’ve engaged an arborist to provide an assessment of trees on or next to the stop banks and advise on a tree management plan. Wherever possible, we’ll carry out replacement planting in more suitable locations.

Assessment of stop banks

We commissioned the development of a 2D hydraulic model. The geophysical groundwork got underway in September.

We received the latest geophysical report on the Mataura stop banks, and it confirms there is no obvious structural damage to the Mataura township stop banks. However, our specialist river engineers have raised concerns about a 600m section of riverbank in Mataura that is at risk of erosion. High river levels/flows could create erosion leading to failure of the stop bank and flooding of the township on the true left bank.

We are prioritising works for this stop bank upstream of the old Mataura paper mill, however the true right riverbank will also need monitoring and protection works in future.

We are working with Emergency Management Southland (EMS) on the possibility of evacuations for Mataura residents at lower river levels/flows than in the past.

We have initiated an active monitoring programme of the riverbank. Catchment operations staff are monitoring the riverbanks weekly, and will move to daily monitoring if river levels and weather forecasts change.

River levels are monitored 24/7 by the Environment Southland hydrology team, who notify EMS of rising river levels.

Environment Southland is immediately procuring 27,000 tonne of rock for a permanent solution for the true left bank at the Mataura township. The work will begin as soon as possible in 2023.

Report in your observations

If you’ve observed anything unusual with the stop banks during past floods we’re keen to hear from you. You can help us identify areas of our stop banks that might need further investigation and work.

This picture shows ‘blistering’ of the Wyndham golf course turf during the February 2020 flood, indicating that floodwater has ‘piped’ underneath the stop bank, the first stage of a potential stop bank breach. We’d like to know about issues residents may have observed in the past so that they can be investigated and specifically addressed as part of the upgrade. If you have anything you’d like to share with our catchment team, please email: service@es.govt.nz or phone 0800 76 88 45.