Stopbanks – repairing and enhancing community infrastructure

Clients: Various

Community resilience is a key focus of many of Taylors’ projects. Over the past year, we have been involved in several stopbank projects in Tasman, Marlborough, and the lower North Island. Our efforts have centred on repairing and enhancing infrastructure in response to recent flood events and proactively constructing stopbanks to protect people and property from future flooding.

Motueka Stopbanks, Tasman

Client: Tasman District Council

Taylors worked to refurbish and improve the resilience of sections of the Motueka River stopbanks. The stopbanks were at 10 separate sites across a five-kilometre section on the river. The project has been underway for the past three years and was completed in 2024. The work supports flood protection for approximately 1400 hectares of surrounding land, including northern Motueka. The refurbishment improves the integrity of the stopbanks and provides safer access for ongoing maintenance.

The third and final phase of the stopbank project was undertaken in 2023-2024 and involved an extension of work at the end of Whakarewa Street, additional work at Hurley Bank and Brooklyn Stream, adjacent to the Plant and Food Research Centre, and Peach Island. The back channel of the Motueka River at Peach Island was also cleared of debris deposited during Cyclone Gita.

Key achievements:

• Repairs involved adding compacted clay to the front and top of the existing stopbank. The aim was to resolve two types of issues:
• Raising the crest of the stopbank in areas where it is no longer high enough to provide protection from a 1-in-50-year flood event.
• Repairing the stopbank in areas where it might have weaknesses that could cause it to fail in a large flood event.

The total value of the project is $10 million, of which central government funded $7.5 million via the COVID-19 Response and Recovery Fund.

Mills Street Stopbank

Client: Greater Wellington Regional Council

Key collaborators: Fletcher Construction, Aecom, Winstones, Heron Construction

This flood resilience project aims to upgrade the stopbank from a current 1:74 to a 1:400 flood event level. The scope includes site vegetation clearance, haul road construction, installation and removal of erosion and sediment control measures, extraction of 55,000m3 of river gravels, excavation of a new alignment and pools in the Hutt River, and screening of the river gravel to generate the filter layer product.

The project required the winning, transport, and placement of low permeability fill material from a nearby Winstone quarry, as well as the supply, transport, and stockpiling of Rip Rap rock and construction of the new stopbank.

Key challenges: Working in a river environment, ecological and consent requirements, managing the logistics of barging rocks between the South Island and North Island and through ports, and quarrying activities.

Key achievements: No injury to staff to date, and no environmental compliance breaches.

Facts and figures:

• 22,000 tonnes of Rip Rap extracted, transported, barged, delivered to site, and stockpiled
• 55,000m3 of river gravel extraction
• 2000m3 of filter product screened
• 8000m3 of low permeability fill excavated, transported, and placed
• 18000m3 of pre-load river gravels excavated and placed in the new stopbank.

Fish Salvage and Relocation Activities Summary

Gravel extraction and river realignment works within Te Awa Kairangi / Hutt River was undertaken in four main stages with fish salvage and relocation activities required by a team of ecologists (Stantec / Aecom) to take place prior to the commencement of construction works within each stage.

A combined approach was applied using electric fishing methods as well as overnight deployment of Fyke nets and Gee Minnow Traps to ensure that fish abundance within the original flowing river channel was reduced prior to construction impacts within each affected reach.

In one of the four impacted reaches of original habitat, a total of 1073 fish and 11 fish species were relocated, including the ‘Threatened’ lamprey and five ‘At Risk’ species including longfin eel, giant kōkopu, inanga, giant bully, and bluegill bully.

More than 4000 fish and 14 species were successfully caught and relocated to suitable areas outside the impacted work area during these works.

The abundance and diversity of fish discovered during these works is indicative of high quality fish habitat within this section of Te Awa Kairangi.

Marlborough Stopbank Realignment

Upper Condors

Client: Marlborough District Council

This contract involved the construction of the Stage 2 (West section) of realigned stopbank of approx. 760 lineal metres. Client feedback was extremely positive, with the team being congratulated for their workmanship, professionalism, and collaborative attitude.

The team prepared the new stopbank footprint by clearing the topsoil strip and an unsuitable undercut. The river gravel was then crushed and placed to form the stopbank core (30,000m3). Existing stopbank material was relocated or reused.

Key challenges:

• Key in and merge new stopbank to existing stopbank ends
• Stopbank batter Filter Medium placement/compaction on floodway side
• Stopbank batter Facing Material placement/compaction on floodway side
• Topsoil re-spread on stopbank surface and landward side batters.

Problem solving and innovative approaches during the project included using machine-controlled GPS to meet design tolerance requirements and resolving design issues in-house using survey design model analysis. Taylors’ specialised excavator attachments were used for compaction of filter material on the batter slope, with compaction methodologies produced to meet design specification requirements.

The team achieved an excellent health and safety record during their work on the project.