$10 million diversion wall could improve Lake Rotoiti quality within a few years

For immediate release: Tuesday 22 February 2005 Lake Rotoiti could have fewer algal blooms within three to five years if a $10 million wall is built to channel water from Lake Rotorua directly down the Kaituna River.

Water quality modelling by Professor David Hamilton of Waikato University has shown the proposed diversion structure would reduce the likelihood of blue-green algal blooms within quite a short period.

More than three quarters of the nitrogen and phosphorus entering Lake Rotoiti comes from Lake Rotorua via the Ohau Channel. Most of the enriched water mixes with Lake Rotoiti’s water before it eventually flows out through the Kaituna River. The proposed wall would divert more of Lake Rotorua’s water directly down the Kaituna River, improving the quality of Lake Rotoiti.

Detailed hydraulic modelling by the National Institute of Water and Atmospheric Research (NIWA) shows it would need to be about 700m long and rise above the lake surface to work properly. It would start at the Ohau Channel exit, which carries nutrient-rich water from Lake Rotorua, and run across the narrow Okere Arm.

Boats would still be able to navigate around it to gain access to the main body of the lake, explains Environment Bay of Plenty group manager regulation and resource management, Paul Dell. “But it looks like the wall would effectively block a large amount of the flow of water from Lake Rotorua, turning it away from the main body of Lake Rotoiti and directly down the Kaituna River and out to sea,” he says. “We consider it the most effective immediate action available to reduce the amount of nitrogen and phosphorus entering Lake Rotoiti.”

Mr Dell says the diversion wall will redirect the Ohau Channel flow to the Kaituna River without allowing it to dilute in Lake Rotoiti first. He says this will cause a small increase in the level of nutrients entering the Kaituna River. However, the flow of water and turbulence in a river would make it difficult for algae to grow.

Environment Bay of Plenty is looking at different options for the placement and design of the structure and will be seeking feedback from the public on its shape, length and location.

Mr Dell says staff will be holding public information days in April. He hopes people will attend the public meetings or call the regional council to talk about the project.

In June 2004, the Government pledged up to $4 million towards urgent emergency works in Lake Rotoiti. Environment Bay of Plenty will fund the remainder of the cost. The project is expected to cost about $10 million in total.

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