North Shore City engineers have been carrying out further studies for the city’s proposed new $100 million wastewater outfall.

The outfall, due to be completed by 2010, will take highly treated effluent from the wastewater treatment plant at Rosedale and discharge it more than two kilometres out into the Rangitoto Channel.

The offshore section will run in a trench from Mairangi Bay, but first, engineers need to know more about the nature of the seabed along the route.

On June 10, a dredge scraped a three metre deep by 30 metre long trial trench in the seabed 1600 metres off the beach.

We were keen to see how easy it was to dig, how well the dredged mud stayed where it was placed, and how much sediment was stirred up. Divers will periodically check the excavation to see how quickly the trench fills in over time.

Findings from the dredging trials will identify the best methods to help control silt during actual construction.