Diversion may save Lake Rotoiti

Diverting Lake Rotorua's outflow directly into the Kaituna River may be the saving of neighbouring Lake Rotoiti, experts say.

Planning is under way for a structure to deflect the water, which is carrying large amounts of nitrogen and phosphorus from Lake Rotorua through the Ohau Channel into Rotoiti, degrading the second lake before leaving through the Kaituna.

Rotorua was the main cause of Lake Rotoiti's water quality problems, Rotorua lakes project coordinator Paul Dell said yesterday.

Without the Rotorua mix there should be a dramatic impact on Rotoiti's standard of health.

The proposed diversion would make all the difference to the troubled lake, especially when supported by sewerage reticulation in lakeside communities, he said.

Two new reticulation schemes, at Okawa Bay and Mourea, were already under way.

Mr Dell said the diversion project could begin late next year and was expected to cost $10 million to $15 million.

In June, the Government pledged $4 million towards urgent works and the strategic partners - Environment Bay of Plenty, Rotorua District Council and Te Arawa Trust Board - would discuss further cost sharing options.

Engineers and scientists were modelling designs for a wall-like structure in Rotoiti, which could be about 700m long and protrude just above the lake surface. It would divert water flowing through the Ohau Channel so it bypassed the main body of Lake Rotoiti, Mr Dell said.

To help with investigations, the National Institute of Water and Atmospheric Research had built a three-dimensional computer model of the dynamics. The model could predict the effect 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, would feed the Niwa results into his water quality model which investigated the impacts on Lake Rotoiti.

Results from the computer model and community feedback would help select the most effective option and the one that minimised the environmental impact.

Mr Dell said he expected a consent application to be lodged in April. It would be publicly notified so people could make submissions.

Once consent was obtained, tenders would be called for the construction. A barge would carry materials and equipment to avoid harming the ecologically significant wetland and Maori values around the Ohau Channel outlet and delta.