Ohau Channel Diversion

Environment Bay of Plenty has been granted resource consent to build a wall structure in Lake Rotoiti. The structure is designed to stop water flowing from Lake Rotorua into the main body of Lake Rotoiti via the Ohau Channel. Construction is now underway and HEB Smithbridge, a local Tauranga firm has been awarded the contract to build the wall.

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

**See photos of the wall's progress - click on the link**

• Ohau Channel Diversion Wall photo gallery

The diversion wall will start at the entrance to the Ohau Channel which links Lake Rotorua and Lake Rotoiti. It will run about 75 metres offshore parallel to state highway 33 and extend to Te Akau Point. The wall structure will be driven into Rotoiti’s lake bed and rise to just above lake level. It will be made of large, precast interlinking concrete and steel wall segments designed to last for at least 50 years. There will be a cap on top of the wall to discourage people from walking on it, for safety reasons.

The wall is expected to cost up to $11 million to build. This is being funded by Environment Bay of Plenty and central Government.
 
The final design of the structure has been based on feedback from the public at various hui and public meetings, as well as comments received on the 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

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

The diversion will prevent 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 per cent of the nutrients entering the lake come through the Ohau Channel. It is not expected to have any significant impact on Kaituna River quality.

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. 

Scientists say the diversion wall will have very little impact on Kaituna River quality. Environment Bay of Plenty has done studies that found 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, Environment Bay of Plenty 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 proposed diversion. The Strategy will identify 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.

The Rotoiti Delta boat ramp near the Ohau Channel will need to be closed for approximately 14 months while the structure is built. This is to ensure the construction is not affected by people launching boats and other activities. Alternative boat ramps are available at Otaramarae, Hinehopu and Gisborne Point as shown on the map displayed.

Click on the image on the above right to enlarge the map.

The Okere Arm access lane has been removed and the Okere Arm is restricted to 5 knots from the 5 knot/200 metre buoys to the south of Namaste Point.
 
An exclusion zone surrounds the construction area and the Delta launching facility. This exclusion zone must be strictly observed by all vessels not associated with HEB Smithbridge, unless in the event of an emergency. Yellow can shaped special marks (buoys) show the boundary of the exclusion zone. The exclusion zone extends from these buoys to the shoreline.

Click on the image on the above right to enlarge the map.

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. Recognising that there are concerns over the two lake level regimes, Environment Bay of Plenty notes the need to undertake detailed consultation in the lead-up to the consent renewals in 2010.

Sediment
Modelling shows that the proposed diversion structure would have 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. Some baseline monitoring of fish and waterbirds has already begun.

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
Environment Bay of Plenty will monitor the impact on fish, on water quality downstream (including in shellfish in the Maketu Estuary), and on birds around the proposed structure. This monitoring is proposed to be part of the conditions of consent for the wall.

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

Environment Bay of Plenty
Telephone: 0800 ENV BOP (368 267)
Facsimile: 0800 ENV FAX (368 329)
Address: 5 Quay Street P O Box 364, Whakatane

Rotorua District Council
Telephone: 07 348 4199
Facsimile: 07 346 3143
Address: 1061 Haupapa Street Private Bag 3029, Rotorua

Te Arawa Maori Trust Board
Telephone: 0508 TE ARAWA (832 729)
Facsimile: 07 347 7349
Address: First Floor, 1194 Haupapa Street, P O Box 128, Rotorua