Diversion may be the saving of Lake Rotoiti

Tuesday 7 December 2004

A structure to channel Lake Rotorua’s water directly into the Kaituna River should have a dramatic impact on Lake Rotoiti’s water quality.

Rotorua lakes project coordinator Paul Dell says the proposed diversion will make all the difference to the troubled lake, especially when supported by sewerage reticulation in lakeside communities.

The project, which could begin late next year, is expected to cost $10 to $15 million in total. In June, the Government pledged $4 million towards it.

Lake Rotorua is the main cause of Lake Rotoiti’s water quality problems. The water that flows from Lake Rotorua, through the Ohau Channel and into Lake Rotoiti carries large amounts of nitrogen and phosphorus with it. Most of this water then mixes with Rotoiti’s water, degrading its quality, before exiting via the Kaituna River.

Engineers and scientists are modelling designs for a wall-like structure in Lake Rotoiti that would divert the water flowing through the Ohau Channel so it bypasses the main body of Lake Rotoiti. It would need to protrude above the surface to work properly, Mr Dell says.

To help with investigations, the National Institute of Water and Atmospheric Research has built a three-dimensional computer model of the Ohau Channel and Lake Rotoiti dynamics. The model can predict the effects of different structures on surface and sub-surface water flows and sedimentation.

Professor David Hamilton, who holds the Environment Bay of Plenty Chair in Lakes Management and Restoration at Waikato University, will feed the NIWA results into his water quality model, which investigates the impacts on Lake Rotoiti’s water quality.

Mr Dell says the results from the computer model scenarios and community feedback will help select the most effective option for the money spent and one that minimises the environmental impact. A barge will carry construction materials and equipment to avoid harming the ecologically significant wetland and Maori values around the Ohau Channel outlet and delta.

A consent application will be publicly notified so people can make submissions supporting or opposing it. Mr Dell expects consents to be lodged in April 2005.

ENDS

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