FLOOD PROTECTION

Read more FAQs on the Westport Flood Protection Scheme

The stages of a construction project

Concept Design

A line on the map showing the proposed alignment, an initial cross-section, and early cost estimates – this was what was prepared for the business case.

Preliminary Design

Geotechnical investigations and detailed site investigations are done with bore holes drilled, a 3D civil model developed, survey works completed, and preliminary volumes calculated. A preliminary set of drawings is prepared. These drawings require a peer review before moving to the next phase. Documentation of the existing environment and key values is completed. Safety in Design is considered, and options are assessed.

Detailed Design

Early Contractor Involvement (ECI) is used to help understand temporary works requirements

Geotechnical analysis completed, including seepage. Full design of all elements, including a full set of drawings, a schedule of quantities, and specifications covering all services such as utilities.

A detailed design report is prepared, including the final assessment of effects.

Feedback is gathered from affected parties, including iwi, ecology specialists, and landowners.

Peer reviews are completed for seepage analysis and stability, and civil engineering is peer reviewed by WCRC.

Safety in Design is confirmed. A contamination remediation action plan or long-term site management plan is developed if required.

Final Design

Feedback from the consenting process and engineering approvals is incorporated. Changes from the detailed design stage are typically minimal. Final drawings, schedules, specifications, and contract documentation are prepared to support construction.

It’s all about the planning!

Sometimes we are asked why 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.

Most of the structures will be earthen stopbanks, constructed by compacting layer upon layer of gravel, then covering the stopbank in topsoil and planting grass or vegetation. This may vary and will be shared when the final design is complete for each section.

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.

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 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.