North Shore City Council awarded McConnell Dowell a design and construction contract for the new Rosedale Wastewater Treatment Plant Outfall Project in New Zealand. The new outfall will discharge high quality treated effluent from the plant 2.8 km out to sea into the Rangitoto Channel.

MacDow's scope of work consists of the design and construction of a 3 km long (2.4 km onshore and 0.6 km offshore) TBM driven, segment lined tunnel with an internal diameter of 2.8 m. This connects to a 1.6 m diameter, 2 km long HDPE submarine pipeline laid in a dredged trench. The marine pipeline also includes a 300 m long diffuser section.

MacDow's scope also includes inlet works and a shaft at the treatment plant. Working with our designers, a detailed sedimentation balance, model and risk analysis was prepared to design control measures. The transition riser was designed using CFD-modelling to mitigate siltation in the lowest point of the outfall.

Suspended sediment testing, WTP water quality monitoring, and anticipated outfall flow-rates were used to estimate a sediment mass balance. A sedimentation model was used to anticipate sedimentation patterns in the tunnel outfall.

The transition riser is a critical part and was designed to improve flushing and to facilitate maintenance operations. Extensive consultation with the North Shore City Council and other stakeholders was carried out prior to construction to fully explain the potential impacts of the project. Through this initial engagement a special committee was formed including representation by the different stakeholders to facilitate continuous and open communication throughout the project.