New approach to address flooding, erosion, pollution

North Shore City Council will this week consider a comprehensive approach to managing stormwater, which seeks to minimise the environmental impact of development in the city.

The council's works and environment committee chairperson, Joel Cayford, says a major change in the way the city manages stormwater is urgently needed.

"Last year we introduced new policies on stormwater outfalls and kerb outlets. We now need to encourage all property owners to take responsibility for the stormwater that runs off their property," Councillor Cayford says.

Hydrological neutrality is a concept that means stormwater is managed at source, in line with the Auckland Regional Council's Air, Land and Water Plan. This means runoff does not increase when new buildings and hard surfaces such as roofs, driveways and solid paving are constructed. Because most of North Shore City has Waitemata clay soils which can be almost as impervious as concrete, good urban design and stormwater management techniques are needed to reduce runoff.

Joel Cayford says adopting a policy of hydrological neutrality could limit stormwater discharges from new developments, to those of the pre-developed bush catchment in areas where protecting the streams is a priority. The priority areas are being addressed by the Project CARE Kokopu Connection, North Shore City's network consents for the ongoing operation of the council's wastewater and stormwater systems.

Low impact stormwater design practices that contribute to hydrological neutrality include the use of onsite stormwater management (OSM) devices, such as rain gardens, permeable paving and rainwater tanks on private property, and stormwater ponds, swales and detention tanks shared between a number of private properties.

"Council needs to practice what it preaches and we need to address problems with flooding, erosion and stormwater quality from runoff that flows from roads and other public places," says Councillor Cayford.

"We're already installing water quality devices such as sand filters, gross pollutant traps and catch pit inserts throughout the city. We may also need to fit ponds, swales and detention tanks, to better manage and treat runoff from roads."

North Shore City's water services general manager, Geoff Mason, says the issues involved are complex and major changes in the way council manages stormwater would likely affect council activities and every property in the city, over time.

"Changes in policy to meet the city's environmental objectives, outlined in our City Blueprint, would also mean significant changes in how we manage stormwater," Mr Mason says.

"The kerb and channel style of road, management of sediment and erosion during development, responsibility for flood prone areas, stream management and how capital works and maintenance are carried out, could all be up for review."

"We would need to carry out comprehensive investigations and cost-benefit analysis during which the issues of affordability and practicality would be addressed," he says.

Other issues that would need to be decided include the ownership of onsite stormwater management devices on private property and who pays for the device, installation, maintenance and renewal in future years. Devices would also need to be monitored to ensure that they were working effectively.

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