Outfall data collected from buoys

North Shore City Council has retrieved a series of buoys, which have been moored in the Rangitoto Channel, to obtain data needed to help it prepare design details for a new ocean outfall for the Rosedale wastewater treatment plant.

Information on water temperature, salinity and ocean currents recorded by data sensing equipment on the buoys, will now be analysed both here in New Zealand and in the United States.

The proposed new outfall, planned for completion by 2010, will take high quality treated effluent and discharge it 2.6 kilometres out into the Rangitoto Channel.

The water measurements are part of the council's wide-ranging assessment of the entire route, including the on-shore underground section of the outfall.

At the same time as the buoys were retrieved, two further surveys were carried out - one a seabed survey to measure sand levels, and the other a geophysical survey to investigate the composition of the seafloor.

The council is working in association with environmental consultants Maunsell Ltd, and the National Institute of Water and Atmospheric Research (NIWA) to gather and analyse the information.

Council planning and projects group manager Adrian Vosloo, says the buoys obtained a comprehensive set of data, covering the entire six month period they were in the water.

The data will provide valuable information on how the water discharge plumes from the new outfall will dilute and disperse into the Hauraki Gulf, and whether water will form thermal layers during the spring and summer.

Results from the buoys and the two surveys are expected in about six months time.

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