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Ohau Channel diversion wall

Bay of Plenty Regional Council was granted resource consent to build the diversion wall structure in Lake Rotoiti. The diversion wall is designed to stop water flowing from Lake Rotorua into the main body of Lake Rotoiti, instead diverting it down the Kaituna River via the Ōhau Channel.

Most of Lake Rotoiti's problems are caused by nutrients flowing into it from Lake Rotorua, so the diversion wall will, over time, significantly improve the lake's water quality. Scientists say it will have little impact on Kaituna River quality.

See also the Diversion Consent page and questions about the Diversion Structure.

The Ohau Diversion Wall information sheet (1.29MB, pdf) has information about the wall.

 

The wall

The diversion wall starts just south of the entrance to the Ohau Channel which links Lake Rotorua and Lake Rotoiti. It runs 75 metres offshore parallel to State Highway 33 and extends to Te Akau Point (1275 metres). The diversion wall is driven into Rotoiti's lake bed and rises to just above lake level. It is made of large, precast interlinking concrete and steel wall segments designed to last for at least 50 years. There is a cap on top of the wall to discourage people from walking on it, for safety reasons.

The diversion wall cost approximately $10 million to build, funded by Bay of Plenty Regional Council and central Government.

The final design of the structure was based on feedback from the public at various hui and public meetings, as well as comments received on questionnaires, and a number of technical studies. These studies include:

  • Water quality modelling
  • Sediment transport modelling
  • Hydrodynamic modelling of the lake
  • Geotechnical investigations
  • Assessment of effects on downstream water quality in the Kaituna River and Maketu Estuary
  • Landscape and visual assessment
  • Cultural assessment
  • Ecological assessment, including effects on fish and water birds.

 

What the wall achieves

The wall diverts water currently flowing through the channel from Lake Rotorua into Lake Rotoiti, directly down the Kaituna River. Previously, about 40 percent of this water flows into the Kaituna River, mostly in summer. The rest of the year it flowed mostly into Lake Rotoiti.

The diversion wall prevents 180 tonnes of nitrogen and 15 tonnes of phosphorus entering the main body of Lake Rotoiti from Lake Rotorua each year through the Ohau Channel. The diversion is expected to improve Lake Rotoiti's water quality within five years, as research has shown that 70 percent of the nutrients entering the lake come through the Ohau Channel. It is not expected to have any significant impact on Kaituna River quality.

What else is being done?

The diversion is enough on its own for long-term improvement, but it will be supported by sewerage reticulation in some lakeside communities, upgrades to septic tank systems and restoration work around lake and stream margins.

These actions will help to avoid more nutrients entering the lake from the Rotoiti catchment in the future. It is extremely important that we improve Lake Rotoiti's water quality and a lot of work is being done to achieve that, including a multi-million dollar sewerage reticulation programme.

Lake Rotorua's problems are more complex and, if urgent actions cannot be implemented, it will be many years before we see any improvement. This is because the catchment's groundwater is already laden with nutrients that will seep into the lake for decades to come.

Kaituna River and Maketu Estuary

Scientists say the diversion wall will have very little impact on Kaituna River quality. Bay of Plenty Regional Council has done studies that found that while the wall will increase the amount of nutrients that enter the Kaituna River, this will not adversely affect fish, water birds and downstream water quality. This is because algae growth in the Kaituna River is limited by the speed of the flowing water, not nutrients.

However, to make sure the river stays protected, Bay of Plenty Regional Council is developing a Kaituna River Management Strategy with Tauranga City Council, Western Bay of Plenty District Council, local hapu and affected groups. The strategy is separate from the diversion. The Strategy identifies the issues and priorities for the river system. A computer-based water quality model for the entire river will account for any minor downstream effects of the Ohau Channel diversion.

Delta Boat Launching Ramp

The Delta Boat Launching Ramp is open for public use. Boat Launching ramps are also available at Otaramarae, Hinehopu and Gisborne Point.

Ōkere Arm

The Okere Arm is restricted to 5 knots from the yellow conical shaped 5 knot/200 metre buoys to the south of Namaste Point.

Channel and Exclusion Zone

A 5 knot/50 metre wide channel on the western side of the diversion wall allows boat access to/from Lake Rotoiti and the Ohau Channel. This channel is between the diversion wall and the red conical buoys.

An exclusion zone between the red conical buoys and the lake edge protects birdlife.

Issues raised by the community

Generally, feedback on the proposed diversion structure has been quite positive. However residents have identified some concerns and issues about the effects of the structure.

These include: the potential adverse effects on the Kaituna River and Maketu Estuary; the more localised effects on the Ohau Channel and Okere Arm environment, particularly relating to fish being able to swim around the structure; the potential effects of existing lake water level control structures on lake water quality; the cumulative effects of other works on both Lake Rotoiti and the Kaituna River, including proposed hydroelectric schemes and the proposed Tikitere geothermal fluid diversion; the visual and safety effects of the proposed structure; and the need for monitoring of potential effects both in Lake Rotoiti and downstream in the Kaituna River and Maketu Estuary.

Lake water level controls

Modelling shows that a lower water level would have no impact on the declining water quality in the lake.

Sediment

Modelling shows that the proposed diversion structure has little effect on the rate of sedimentation in the Ohau Channel, or on the water levels in Lake Rotorua.

Fish and waterbirds

With this type of proposal, it is difficult to predict the ecological effects with any accuracy, therefore an extensive monitoring programme has been developed.

Bay of Plenty Regional Council commissioned Wildland Consultants to undertake monthly bird surveys at Lake Rotoiti and to report on these results to meet the requirements of these resource consent conditions.

Watercress - Kaituna River

Watercress that grows along the Kaituna River is unlikely to be affected by the proposed diversion structure. There will be an increase in nutrient levels in the Kaituna River as a result of the proposed diversion structure, but this will still be much lower than dairy farm drains or canals (such as the Pukehina Canal where watercress is abundant).

Monitoring

Bay of Plenty Regional Council is monitoring the impact on fish, on water quality downstream (including in shellfish in the Maketū Estuary), and on birds around the structure. This monitoring is part of the conditions of consent for the wall.

Reports

For the latest scientific, research and monitoring reports, refer to the Rotorua Te Arawa website

 

For more information

If you would like more information on the proposal, please contact us: