What other option was considered? 

Another option was considered in detail but has been set aside based on affordability. This option involved the main flood scheme and Carters Beach being built to the same 1.5% AEP (Annual Exceedance Probability) protection level as Option One and Option Two, but with additional stopbank height to address forecast changes in climate over the next 30 years. 

This option would involve stopbanks approximately 0.3 metres higher than the other options but because it cost about $8.8m more than Option One and $7.7m more than Option Two, it was considered too high an additional cost for the community to meet and was set aside as an Other Option considered. 

The original scheme proposed to provide 1 in 100-year level of flood protection.

The level of freeboard (extra height allowance for uncertainty) was 600mm but this has been reduced due to enhanced computer modelling.

This scheme would now cost around $70m to deliver.

RCP6 was factored into the original design.

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.

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.

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.

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.

Freeboard is additional stopbank height to address uncertainties. It is extra height built on top of the design flood level as a safety margin. 

It is usually measured in millimetres and the higher the level of freeboard the greater contingency for ‘over design’ events (events that exceed the design flood level).  

The original Westport scheme included a freeboard provision of 600mm. That will now be 400mm. 

The technical 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.

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.

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.

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.