Waihī Estuary is one of Aotearoa New Zealand’s most degraded wetlands thanks to decades of drainage, land use change, contaminated runoff, and river channelisation across its 35,000ha catchment.
New wetlands are being constructed adjacent to the estuary to improve water quality, biodiversity, and climate change resilience via carbon storage, and enhance the cultural and recreational value of the area.
By the end of 2025, a 27ha coastal treatment wetland and a 3ha tidal coastal wetland will replace the existing 30ha dairy farm at Cutwater Road, Pukehina.
The new wetland is expected to treat around 2.75 million m3 of contaminated catchment water annually.
The $2m project is a partnership between Bay of Plenty Regional Council (BOPRC), iwi collective Te Wahapū o Waihī, Waihī District Drainage Society, and local farmers, who broke ground in late November 2024.
Awa Environmental served as Chartered Professional Engineering Lead, combining water modelling and engineering design to deliver a hydraulically feasible design for the new freshwater wetland.
Our innovative pump station-driven design overcame the hydraulic challenges of the site’s completely flat topography.
The project was recognised nationally, becoming a finalist in Social Impact category of the Building Nations – 2025 Impact Awards.
Challenges
Designing a new wetland of this scale presented many hydraulic and engineering challenges, such as:
Modelling and understanding highly complex water flow patterns
Understanding climate and weather impacts, including rainfall, temperature, evaporation, and winds
Assessing potential design elements – such as pumps, culverts, baffles, bunds, and sediment ponds – and their layouts. Surmounting the significant hydraulic challenge posed by flat topography.
The design had to deliver broader outcomes, including improved water quality, climate change resilience, and enhanced cultural and ecological value.
It also needed to be delivered in close collaboration with, and with the approval of, diverse stakeholders: Awa Environmental working with our client BOPRC and Waihī District Drainage Society, and BOPRC working with iwi collective Te Wahapū o Waihī and local farmers.
Solutions
Awa Environmental solved the flat landscape challenge with an innovative solution: a pump station-driven wetland design.
It uses the existing pump station from the Waihī District Drainage Society to draw water out of the wetland, while a new pump station introduces water to the system.
It also features inflow and outflow culverts, which can be opened and closed to manage water levels depending on seasonal requirements.
Complex, 3D water modelling provided an analytical understanding of the impacts of pumping approaches specifically, as well as wetland layouts and structures in general.
The modelling involved an enormous amount of data, which our experts navigated to cost-effectively extract the most value for our client.
We used TUFLOW FV’s 3D modelling software to assess the wetland hydrological feasibility, understand the flow patterns during wet and dry seasons, and optimise the design of pump stations and culverts.
More than 100 simulations were run, using three years of climate data to understand long-term wetland behaviours and validate the solution.
Key insights included:
Dry period modelling showed the need for an additional inflow culvert to introduce brackish water when pumping rates were low
Wet period modelling showed the importance of the outflow culvert to relieve pressure on the outflow pump station.
Other design details investigated were sediment pond design and configuration,3D tracer showing flushing rates, temperature fluctuations, velocity behaviour, impacts of loss to groundwater, and impacts of tides on inflow and outflow.
This analytical understanding was combined with our engineering design expertise to create an innovative and practical design, that accounted for key factors such as cost and constructability.
Importantly, throughout the process, Awa Environmental worked closely with key stakeholders to secure their buy-in, ongoing support, and approval of the high-level design.
We also met project timelines and requests for design details during construction.
Benefits
Awa Environmental’s integration of water modelling and engineering design was crucial to overcoming the project’s challenges and ensuring it will provide the desired outcomes, including improved water quality and habitats for flora and fauna, and greater value to iwi and others in the community.
Among other things, our extensive modelling enabled us to prove the feasibility of the two-pump solution and overall design, giving stakeholders confidence to continue and reducing the risk on a complex and high-profile wetland project that involves the commitment of substantial public and private resources.
Our water modelling and engineering design expertise also meant we could review and provide Quality Assurance on the other project partner’s work, further reducing project risk and improving the likelihood of its long-term success.
