FLOOD PROTECTION
Flood Protection centres on building floodwalls and stopbanks with the aim of protecting Westport in the event of significant future flooding events.
This is the largest project being undertaken by Resilient Westport and is being led by West Coast Regional Council (WCRC).
The overall aim is to deliver structural mitigation works to prevent direct threats from both the Buller and Orowaiti rivers. These are caused by flooding generated upstream in the Buller catchment and from coastal inundation, particularly via the Orowaiti Lagoon.
The project is funded via a contribution of $15.6m from central government. The West Coast Regional Council’s contribution is estimated at $8.37 million – taking the projected floodwall project budget to $23.97m.
The current plan involves a combination of earth stopbanks, ‘planter-box’ stopbanks, concrete floodwalls, wooden floodwalls, and portable flood barriers. The total length will be around 16km. The heights of the flood protection structures will generally be around 2 metres, with a maximum height of 3.6 metres.
The work will be designed generally to provide 1% Annual Exceedance Probability with climate change under RCP6.0 (based on NIWA advice). A 1% AEP flood has a 1 in 100 chance of occurring in any given year.
It is also designed with a 600mm freeboard allowance in relation to the floodwalls. Freeboard is an engineering provision for estimating accuracy and other factors that vary from time to time, including the impacts of debris blockage, waves, wind and riverbed aggradation.
The scheme is divided into five main stopbanks:
Lower Orowaiti, Upper Orowaiti, North End, Wharf to Buller Bridge and Buller South (SH67 from Buller Bridge to Nine Mile Road).
Within that, there are approximately 20 sub-sections of flood protection including smaller banks like Cats Creek and a portable barrier, for the Floating Lagoon area (North End).
Stage One Projects
The Stage One projects include stopbanks at:
Floating Lagoon (vegetation clearance complete)
Avery’s (scheduled for early 2025)
Cats Creek (complete)
McKennas (underway)
The Floating Lagoon is a length of land near Westport where a stopbank and 200-metre-long portable flood barriers are planned. It is part of the North end stopbank. Vegetation clearance to allow remediation of the existing stopbank at Floating Lagoon is complete.
A stopbank on the Avery’s stretch of the Orowaiti River is estimated to provide protection from a one-in-100-year flood for the next 30 years of climate change. Avery’s is part of the Lower Orowaiti stopbank.
The Abattoir Drain project is a small earthen bund which will redirect floodwaters in Cats Creek to the Abattoir Drain. The bund will assist in reducing inundation of properties downstream of Cats Creek. Following the completion of geotechnical and planning work , physical work was carried out in September/October 2024.
The McKenna Road project is an earthen stopbank with an initial length of 780m, to assist with preventing overflow from the Orowaiti River. In September, tenders were requested. Consent was granted and the tender awarded to Rosco Contractors Ltd in November. Work started in late November/early December. Work is expected to take until March/April 2025 to complete. The stopbank will be around 18 metres wide at the base and around 2.2 metres high. A blessing ceremony was held on 18 December 2024 to mark the start of work. A Fact Sheet on the McKenna Stopbank provides information on how the project is being carried out. McKenna is the first of the larger stopbanks to get underway.
McKenna is part of the Upper Orowaiti stopbank.
West Coast Regional Council Chair Councillor Peter Haddock, along with Buller Councillor Mark McIntyre, and Jo Dooley, on behalf of her late husband Frank Dooley, do the honours celebrating the start of work on McKenna Stopbank.
Stage TWO
Stage two is the substantive work on Westport’s flood protection structures (not completed in Stage one).
Most of the Stage 2 works will be completed by June 2026, aiming for 90% completion by that date. While these initiatives will help deliver flood protection, they are not a guarantee of prevention of damage from future flooding. These measures are buying time to enable Westport to plan a more resilient future.
See the planned overall construction programme below for more on timings and locations.
The map shown below identifies the components of the substantive Stage Two flood protection programme.
Here’s a snapshot of what happened in 2024:
Work has begun on the McKenna stopbank, which marks a start of construction on one of the bigger stopbanks. The 780-metre stopbank will be complete in March/April 2025.
Floating Lagoon vegetation clearance is complete. This will allow the topping up of the existing stopbank and planning for the new stopbank (as part of the North End section)
A 50-metre bund at Cats Creek is complete.
Preparation is almost complete to lodge the consent application for the Avery’s stopbank in the New Year.
Geotechnical investigations have begun in the Buller South area. Geotech experts GHD were awarded the tender after a competitive process.
What’s in the pipeline?
It’s hoped construction will start on Avery’s (part of Lower Orowaiti stopbank) in February/ March 2025. The geotechnical work is complete along with final design. Construction will commence following consenting and tendering for the work in the New Year.
Floating Lagoon stopbank (part of North End) – final geotech work is underway and we expect to lodge consent application in first quarter of 2025.
Further work is required on assessing the structural competency of the bund near Talleys to see if it is suitable to support a portable flood barrier. Evidence of seepage has been found. This may be carried out in the New Year, along with a ground penetrating radar assessment of the bund and underlying ground competency. If necessary, the bund will need replacing.
Geotechnical work will continue in the Buller South area (SH67 from Buller Bridge to Nine Mile Road). Consenting will be on hold until a solution for flood mitigation for Carters Beach can be found.
An initial report into possible flood mitigations for Carters Beach has been commissioned. It contains options that require further investigation. We intend to share more with Carters beach residents, and the wider community, next year. Whilst work on solutions for Carters Beach continues, a consent application for the SH67 Buller Bridge to Nine Mile Road section of wall won’t be lodged.
Possible flood mitigations for Snodgrass Road are also being considered and will be discussed with residents early next year. Protection for Snodgrass Road or Carters Beach is not included in the Government funding envelope.
It’s all about the planning:
Sometimes we are aksed wy this all takes so long. Here’s a bit of a snapshot of what goes on long before any spade hits the ground. There’s:
Initial concept design
Flood modelling
Land owner negotiations for access
Geotechnical and groundwater studies (what is the land like essentially)
Assessment of environmental effects
Geotechnical design
Production of tender documents and drawings
Lodging and gaining consent
Tendering
And finally construction.
Possible flood mitigation options for Snodgrass Road, Westport
The West Coast Regional Council (WCRC) has commissioned a report into possible flood mitigation options for Snodgrass Road.
You can find the report along with the presentation shown to affected residents on 30 January 2025 here:
Snodgrass Road Flood Mitigation Report (complete)
Snodgrass Road Flood Mitigation Report appendices
Organs Island
Work to revegetate Organs Island, upriver from Westport, falls under the protect function of the Resilient Westport work and is led by West Coast Regional Council.
Orowaiti Overflow/Organs Island is the Buller River’s first overflow location. In the July 2021 flooding event, it overflowed into the Orowaiti Estuary and from there into parts of Westport.
The Orowaiti overflow is the original path of the Buller River and is where it naturally wants to go. It is called the ‘relic river channel’ and is the natural fall of the land.
When it was originally diverted in the early 1890s the engineer wrote detailed notes that Organs Island area should be heavily vegetated. He suggested using blackberries and willows because they have strong root density and grow into dense vegetation. Vegetation traps water and sediment/silt and slows flood water. Over time, the land rises and self-seeding occurs. This is a clever, natural flood mitigation technique.
However, the land was gradually cleared and used for farming, weakening the original intent for Organs Island to work as a natural flood mitigant. Design work is underway for the area be replanted to create a forested floodplain environment.
Modelling suggests revegetation would reduce water flowing down to the Orowaiti estuary by 10% in a large flood. It will also require less maintenance overtime, reducing costs for West Coast Regional Council.
A rock-training wall was rebuilt at Organs Island after the floods.
This land is currently administered by Land Information NZ and leased for grazing. Arrangements are being made to transfer ownership to Department of Conservation.
Your Questions Answered
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ARI stands for annual recurrence interval which is sometimes also known as ‘return period’. It is the average time period between floods of a certain size. For example, a 50 year ARI event would on average happen every 50 years.
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AEP stands for annual exceedance probability (AEP) which is the probability of an event occurring in any given year.
So a 1% AEP means there is a 1% chance in of the event occurring in any given year of the event occurring. This means that on average 1 event of this size will occur every 100 years. Therefore, 1% AEP is equal to a 100 year ARI, a 2% AEP is a 50 year ARI, and a 10% AEP is a 10 year ARI. Both ARI and AEP values are based on statistical calculations and represent the average timing of events.
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RCP stands for Representative Concentration Pathways (RCP). These are climate change scenarios to project future greenhouse gas concentrations.
These pathways (or trajectories) describe future greenhouse gas concentrations (not emissions) and have been formally adopted by the Intergovernmental Panel on Climate Change (IPCC).
The pathways describe different climate change scenarios, all of which were considered possible depending on the amount of greenhouse gases (GHG) emitted in the years to come.
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RCP 4.5 is described by IPCC as a moderate scenario in which emissions peak around 2040 and then decline. RCP 8.5 is the highest baseline emissions scenario (essentially business as usual) in which emissions continue to rise throughout the twenty-first century. In the Westport context, RCP 6 is applied in terms of climate modelling, which reflects some stabilisation of climate emissions. This is based on NIWA advice.
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Freeboard is engineering provision for estimate accuracy and other factors not included (waves, aggradation, debris etc). Freeboard is a safety feature that provides storage capacity and prevents spills.
The definition of freeboard is presented in Section 1.2.2 Definitions of New Zealand Standard NZS 4404:2010 Land Development and Subdivision Infrastructure. That states:
A provision for flood level design estimate imprecision, construction tolerance, and natural phenomenon (such as waves, debris, aggradations, channel transition, and bend effects) not explicitly in the calculations.
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The structural make-up of the floodwalls will be shared when the final design is complete. The current plan for the whole flood mitigation project involves a combination of earth stopbanks, ‘planter-box’ stopbanks, concrete floodwalls, wooden floodwalls, and portable flood barriers.
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Westport's stormwater and wastewater network, like those in many towns and cities throughout New Zealand, suffers from aging and under-invested infrastructure. Significant upgrades may be required to ensure functionality and resilience in the future.
Westport’s small population and rating base is a significant challenge for further network improvements and work is widely dependant on external funding.
Buller District Council (BDC) is working on ways to improve this and integrate improvements with the planned new floodwalls.
BDC is currently completing stormwater network modelling, which together with an improved understanding of the current stormwater infrastructure will help identify the scope of work required. The integrated stormwater will provide a range of design options and costings, including affordability considerations. The concept study will be completed mid-August 2024, which will then guide and inform next steps, including application for further government funding support due to the affordability challenges.
Alongside stormwater network upgrades another key long-term objective is network separation (separating stormwater from wastewater) and wastewater modelling to establish how to optimise the performance of the stormwater and wastewater systems.
A report by Worley's presented to Council in late 2024 provides options for creating a flexible and cost-effective stormwater management system to reduce flooding in Westport during both routine and extreme weather events. This includes using high-capacity and mobile pumps, as well as nature based solutions such as rain gardens and detention basins.
Further modelling is required before decisions can be made about actioning the suggestions made in the report. A Regional Infrastructure Fund applicaiton has been made meanwhile to secure possible funding.
Buller Flood Recovery and subsequent work
Efforts to improve the stormwater system have been undertaken in the immediate wake of the floods.
As part of the post-flood essential infrastructure repairs, National Emergency Management Agency (NEMA) funded the lifting of four wastewater pumpstation electrical cabinets to above 1 in 100 year flood height. These were: North Beach pumpstation, Derby Street pumpstation, Rintoul Street and Roebuck Street pumpstation.
It is hoped that more pumpstations can be lifted in the future. This work will be prioritised based on flood risk and in conjunction with long term pumpstation renewal schedules.
Nineteen WaStop® valves have also been installed at key stormwater outfalls throughout the Westport township. The valves provide more network resilience and are better able to protect against floodwater infiltration (backflow) at many key stormwater outfalls.
Crown Infrastructure partners Better Off funding covered the cost of supplying and installing 14 of these WaStop® devices. Another 5 were installed with funding from National Emergency Management Agency (NEMA) as part of the post-flood essential infrastructure repairs.
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The wave buoy located off the coast of Westport will be used to collect data in real time, including maximums, means and minimums for wave period, height and direction; wind speed and direction; sea temperature, and barometric pressure.
This information, collected over time, will form a valuable dataset able to be used to update the flood model that the National Institute of Water and Atmospheric Research (NIWA) has developed for Westport, in partnership with the WCRC.
These datasets will also help those developing the flood models to set more appropriate downstream boundary conditions for the flood inundation maps that are created from the flood model forecasts.
Unfortunately, the wave buoy was badly damaged in a storm in 2024 when it came free of its mooring, and it’s hoped a replacement will be in place in early 2025.