Mataura River stop banks
In August 2022, an evaluation of the Mataura River stop banks by specialist engineers was received that identified concerns about their capacity.
The evaluation was part of the climate resilience works being undertaken, which included investigations by flood protection experts. The investigations highlighted that further work is required, urgently, to better understand the capacity and integrity of the stop banks.
Most of our stop banks are at least 30 years old and were designed to address flooding issues at the time. Challenges such as increased and more extreme weather events as a result of climate change have put them under pressure.
There is now good information on the likely impacts of climate change and we also know that we are in a cycle of increased flood events and high river flows as a result of the Interdecadal Pacific Oscillation, an oceanographic/meteorological phenomenon.
The stop banks help up well during the February 2020 floods on the Mataura River, but there were areas where some remedial work was carried out.
Our catchment team is gathering the necessary information, including geotechnical data and hydraulic modelling, to help us establish the best options to protect Gore, Mataura and Wyndham. We are already working to a plan for the Gore stop banks, but the Mataura situation is a bit more complicated for a number of reasons, including the proximity of the stop bank to the road. Following the evaluation we sought more data so that we could be clear about the appropriate remediation options.
Read the complete Mataura flood protection flyer here.
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.
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: firstname.lastname@example.org or phone 0800 76 88 45.
Community safety – evacuation
Because the frequency of flooding events is likely to increase, we are working closely with Emergency Management Southland to take a precautionary approach to keeping the communities of Gore, Mataura and Wyndham safe.
This means that people living in these communities may need to be evacuated sooner and at a lower river level/flow than they have previously, while we work to better understand the current capacity of the stop banks.
Residents across all communities, as part of being prepared for emergencies, are encouraged to prepare for evacuation. Have a plan, gather supplies and get connected – go to the Emergency Management Southland website for more information. If any evacuation does need to happen, being prepared will help to make it easier and less stressful.
In the event of a flood event, Emergency Management Southland (EMS) will make the decisions about when and if to evacuate and will lead any evacuation. EMS can provide advice to people to evacuate and once a state of emergency is declared by the mayor, EMS can make evacuation mandatory. Each event will be assessed on a case by case basis, as it is not possible to predict conditions for every situation.
We do have some factors working in our favour, including being able to predict how quickly flood waters travel. Our hydrological team will know up to 12 hours ahead of time, what the situation is likely to be around the catchment. We also know from experience during the 2020 floods that we can quickly and effectively evacuate these communities within a couple of hours.
Read the Gore District Prepare to Evacuate flyer here.
Floods caused severe damage across Southland.
Environment Southland identified possible issues with the flood protection measures on the Mataura River (north east side). We invested in completing some strengthening work in this stop bank.
Initial investigation by a Chartered Professional Engineer including Gore and Mataura specifically, highlighted concerns with the stop banks’ integrity, especially flood waters that remain high over a period of time. Recommended investigations to determine severity of issues.
Gore District and Southland District Council mayors advised.
Media release issued highlighting need for community to be prepared for flooding and potential early evacuation. Drop-in sessions promoted
Quote for geotechnical investigations received to 9.2km of Mataura stop banks
19 AUGUST – 1 OCTOBER
Drop-in sessions were held in Gore, Mataura and Wyndham to share information with community.
Potential flood warning monitoring site scoped on the Tomogalak Stream that will give improved flood forecasting for Gore. This work is progressing.
Geo-technical ground work underway. This information will help us to better understand the integrity of our stop banks and determine the best design solutions.
Quote received for 2D hydraulic model covering Gore, Mataura and Waimumu.
Additional assessments of likely impacts underway.
SEPTEMBER and ongoing
Evaluation underway of erosion risks on the true right of the Mataura stop banks behind Alliance to determine river level trigger points for emergency activation.
|DECEMBER||First geophysical report received. Confirms no obvious
structural damage to Mataura stop banks from the 2020 floods. River engineers raise concerns about erosion risk on the
true left bank in the Mataura township, upstream of the old paper mill.
Community reminded to prepare for evacuation at lower river levels than in the past. Immediate procurement of 27,000 tonne of rock to remediate erosion risk. Work scheduled to begin in January 2023.
Work underway by Emergency Management Southland to prepare Mataura Flood Response Plan.
Community drop-in sessions
Further community sessions will be held in February-March 2023. Check back or follow our Facebook page for more information.
We have held a number of drop-in sessions for people to come along and ask questions to help them better understand the current situation. These were held in Mataura, Gore and Wyndham.
Mataura Senior Citizens Centre
- Friday 19 August 2022 11am-1pm
- Thursday 1 September 2022 4-6 pm
Gore District Council
- Monday 22 August 2022 11am-1pm
- Friday 2 September 2022 4-6pm
Wyndham Community Centre
- Friday 26 August 2022 4-6pm
- Tuesday 4 October 2022 6-8pm
To stay up to date with the work in the Mataura catchment, subscribe to the Flood protection and Climate Change e-newsletter – www.es.govt.nz/subscribe