By Shonagh Lindsay, Intercommunications Specialist, Healthy Waters and Flood Resilience, Te wāhanga mō ngā Wai Ora me te Manawaroa ā-Waipuke, Auckland Council

How the Alnwick Stormwater Pond was given a new life.

Nestled in the heart of Warkworth, the Alnwick Stormwater Pond is a quiet yet vital player in safeguarding the area’s waterways. Since 2007, it has managed stormwater for a 23.8-hectare catchment, protecting streams from erosion and filtering water before it flows into the Mahurangi Harbour. But by 2024, sediment build-up, undersized pipelines, erosion from overland flow, and inadequate forebay and outlet designs had left the pond struggling to do its job. 

Faced with a pond nearly 50 per cent clogged with sediment, a comprehensive renewal project aimed to not only restore its functionality, but to also embrace sustainability in every possible way. 

What began as a project to improve water quality quickly transformed into a mission to enhance local biodiversity. The restoration of 1340 square metres of pond area brought back better water flow and healthier aquatic habitats. Overrun by weeds and sediment, the pond was given new life – ready to support both water quality and the creatures that depend on it. 

Before work began, more than 105 eels and several blackbird nests were safely relocated, showing how environmental care took centrestage. Fish passages were also improved around manhole rises, helping aquatic life to move freely in and out of the pond. The future looks even brighter with 2650 native plants set to be added along the pond’s edges in May 2025, offering food and shelter to local wildlife. 

The project team, including Glasgow Contractors, also went to great lengths to minimise carbon emissions, embracing sustainable approaches at every step: 

• Nature-based design: Stabilising the pond banks with native plants instead of concrete reduced the need for energy-intensive construction and added resilience. Native vegetation was planted into biodegradable coir logs and coconut matting to both hold the banks, and to act as planting material until the plants had rooted into the ground. 
• Low-carbon materials: During the design stage, options to reuse the existing pipe or crush it for reuse were explored. Glasgow Contractors Project Manager Michael Cole says, ‘We explore all options to reuse material on our sites, as our waste management plans explore all avenues to assist in reducing our carbon footprint.’ The concrete stormwater pipes were constructed with EcoSure cement, which has a 27 per cent smaller carbon footprint and a high standard of durability and strength, and is suited to most environments throughout the country.
• Repurposing: Instead of trucking away more than 600 tonnes of dredged sediment to landfill, it was instead reused onsite, saving an estimated 4500 kilograms of carbon dioxide – comparable to taking a car off the road for a year. Of the sediment, 400 tonnes was repurposed as a planting medium and more than 200 tonnes was used to stabilise the pond’s embankments.
• Low-impact machinery: Glasgow Contractors utilised its unique amphibious Truxor machines to dredge and pump the sediment from the pond into Geotubes, where clean water is filtered out and returned to the pond, with the remaining sediment later mixed with mulched vegetation. 

‘The amphibious Truxor machine looks like a small army tank, capable of seamlessly operating between land and water,’ says Cole. ‘It floats and is highly manoeuvrable, allowing us to operate with surgical precision and access difficult-to-reach areas. Due to its unique smaller and lighter design compared to conventional excavators, the Truxor machine has a significantly lower impact on the environment – to both the local fauna and flora, as well as to the atmosphere and waterways, with lower emissions and less earthwork requirements.’ 

The mixture of sediment and mulch was spread onto the new maintenance track and surrounding areas of the reserve, acting as a rich layer of topsoil, which not only reduced landfill, but also saved on importing costly materials. 

Cole says that Glasgow Contractors also used some of the removed sediment as part of its sediment reuse study to divert more waste away from landfill. By using a ‘muck spreader’, the company evenly distributed the organic material in thin layers over a piece of land that could eventually be used for cropping. This was a small‑scale viability trial, but Glasgow Contractors is excited about the future potential opportunities for reducing and diverting waste away from landfill. 

In addition, Healthy Waters recycled approximately 200 tonnes of GAP65 aggregate from another project, which would have otherwise gone to landfill, cutting transport emissions by 590 kilograms of carbon dioxide. A local farmer also reached out to Glasgow Contractors to query what was going to happen with the temporary site haul road material, which Cole offered to the farmer upon project completion. It was then repurposed for local farm road maintenance. This saved a further 1415 kilograms of carbon dioxide in cartage by diverting material away from landfill, while also supporting the local community. 

Thirty tonnes of aquatic vegetation removed from the pond was sent to GreenCycle instead of being dumped in landfill. Every year, New Zealand sends more than 270,000 tonnes of green waste to landfill because most of it is considered too difficult to recycle. GreenCycle transforms all organic material – including flax, palms, bamboo, logs, and noxious weeds – into quality landscaping products used in gardens, parks, and large‑scale landscaping projects around Auckland.

• Ride-sharing contractors: Team members carpooled daily, says Cole. ‘We try to put our workers on jobs near to where they live, and for those we can’t, they arrange to meet at a central location (often our yard), and then travel together in company vehicles to the sites.’ By keeping two cars off the road between their yard and the project, this saved an estimated 269 kilograms of carbon-dioxide-equivalent each week. Cole says Glasgow Contractors uses EROAD, as the platform provides detailed fuel consumption reporting, fuel card management and idle reporting to help reduce fuel usage.

The project also supported eight local businesses, from drainage contractors to fencing and haulage services. By keeping the work close to home, it ensured both efficiency and a positive impact on the economy. 

The Alnwick Stormwater Pond renewal project is more than just an upgrade – it’s a testament to what sustainable infrastructure renewal can achieve. Cleaner water, thriving biodiversity, lower carbon emissions, and a greener community make this a success story worth celebrating. Next time you pass by, take a moment to appreciate the quiet heroism of a pond restored with care and creativity.