Swales to be given eco trial

North Shore City is introducing a new way of providing stormwater-friendly road edges and drive crossings in an urban environment. Called a "swale", it consists of a shallow mowable grassed ditch at the edges of the road, and piped vehicle crossings. Permeable paving can also be provided for roadside vehicle parking.

Joel Cayford, who chairs the council's works and environment committee, says the design helps prevent dirty stormwater from entering city waterways by using the vegetation in the shallow drain to help filter stormwater from the road. Councillor Cayford says local residents are being asked not to park on the soft grass within the swale, and preferably to keep the grass no shorter than 150mm to help it do its filtering and cleaning job.

North Shore City Council is currently rebuilding Roland Rd in Greenhithe at a cost of $700,000 to make use of this stormwater-friendly option. At this week's works and environment committee meeting, councillors approved a tender for the work to start, which is subsidised by a water quality improvement grant from Transfund. Joel Cayford says that while the Roland Rd swale is experimental in North Shore City, it is widely used in other parts of the world. The design can cost more than conventional kerb and channel to build, but saves downstream costs by reducing the amount of pollution that passes into city waterways.

"This one should serve as a good example for developers to adopt in future to provide a softer and greener road edge," Councillor Cayford says.

It is expected that the grassed swales will be built along the side of a road as an alternative to an unformed ditch or conventional kerb and channel.

North Shore City's stormwater operations manager, Roger Hawthorne, says the Roland Rd project is designed to reduce by 80 per cent the level of contaminants from roads reaching sensitive waterways and the Waitemata Harbour.

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