Buoys to seek data at outfall site

North Shore City Council will use a series of moored buoys to obtain reliable data on water temperature, salinity and ocean currents to help it prepare design details for a new ocean outfall for the Rosedale wastewater treatment plant.

The proposed new outfall, planned for completion by 2010, will take high quality treated effluent from the plant and discharge it 2.5 kilometres out into the Rangitoto Channel. The tests are a part of the council's wide-ranging assessment of the entire route, including the underground section of the outfall route and the construction timetable.

In conjunction with environmental consultants Maunsell Ltd and the National Institute of Water and Atmospheric Research (NIWA), the council has placed data sensing equipment on buoys at two sites to the east of Tatarata Point, which is at the northern point of Murrays Bay.

The data will provide valuable information on how plumes from the new outfall will dilute and disperse into the Hauraki Gulf. And most importantly, it will indicate whether water from the outfall will form thermal layers during the spring and summer, says council planning and projects group manager Adrian Vosloo.

Chairman of the council's works and environment committee, Joel Cayford, says the increased efficiency and capacity of the treatment plant, especially with the longer-term growth in the northern part of the city, means that the outfall will be designed and positioned to do its job with the least impact on the environment.

The council will ensure that the new outfall meets stringent consent requirements, and information from this study will greatly assist the design process, Councillor Cayford says.

The first measuring site is 2500 metres directly east (true) of Tatarata Point, and the second 1200 metres east (true) of the Murrays Bay beach jetty.

The first site is at the proposed end point of the new outfall, where the sensing equipment is placed on three buoys moored in approximately 11.5 metres of water. Two of the buoys sit three metres or more below the surface, with the third buoy, a large cylindrical yellow marker beacon lit at night, sited between the two.

Mounted on the mooring cables are a series of water current meters and thermometers, and a water level recorder is placed on the seabed.

The second site duplicates the first, and is used to verify conditions closer inshore. Both sets of buoys have been placed by divers from NIWA's survey vessel Rangatahi II. The first set of buoys will be in position for six months, and the second set for about six weeks.

The crew of the Rangatahi II will take additional salinity (salt content) and other water tests and check the instruments and data every two weeks.

Note: Site one will be at co-ordinates 174 47.06'E and 36 43.80'S; and site two at 174 46.30'E and 36 44.00'S.

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