$10m wall could cut algal blooms

A planned $10 million wall to channel water from Lake Rotorua directly down the Kaituna River could cut algal blooms in Lake Rotoiti.

Yesterday, a health warning in parts of Rotoiti was extended because of the latest cyanobacteria bloom.

Bay of Plenty and Lakes medical officer Phil Shoemack has asked Rotorua District Council to put up signs alerting people and advises against drinking or swimming in the water.

Waikato University professor David Hamilton has shown that a wall to deflect water from Lake Rotorua would reduce the likelihood of blue-green algal blooms in Rotoiti "within quite a short period".

More than three-quarters of the nitrogen and phosphorus entering Lake Rotoiti comes from Rotorua via the Ohau Channel. The degraded mix eventually flows out through the Kaituna River.

Scientists say the proposed wall would divert more of Lake Rotorua's water directly down the river, improving the quality of neighbouring Rotoiti.

Detailed hydraulic modelling by the National Institute of Water and Atmospheric Research shows the structure 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 the wall to get to the main body of the lake, said Environment Bay of Plenty regulation and resource manager Paul Dell.

"It looks like the wall would effectively block a large amount of the water from Lake Rotorua, turning it away from Rotoiti directly down the Kaituna River and out to sea," he said.

"We consider it the most effective immediate action available to reduce the amount of nitrogen and phosphorus entering Lake Rotoiti."

Despite a small increase in the level of nutrients entering the Kaituna, the flow and turbulence in the river would make it difficult for algae to grow there.

Mr Dell said Environment Bay of Plenty was looking at options for the placement and design of the diversion and would be seeking public feedback on the shape, length and location.

Last June, the Government pledged up to $4 million towards urgent emergency works in Lake Rotoiti. The regional council will fund the rest of the $10 million cost.