The details of this activity are listed below and copies of consent documents attached at the end of this page.
1. Wall construction
- Interlocking steel sheets through the sediment and embedded in rock.
- Top of the wall will be 500 mm (1/2 metre) above the normal lake level.
- The top of the wall will be capped with a wooden strut.
- The wall will extend 1250 metres from land south of the Ohau Channel delta to a high point in the lakebed opposite Te Akau Reserve, about 75 metres off the Lake Rotoiti shoreline.
- Between the wall and the shoreline, buoys will mark a 50 metre wide channel for boat access.
- Land surrounding the wall will be restored to its current condition.
- The wall will have navigation lights and other safety provisions.
- Fish passes may be retrofitted or included as part of the construction.
- 12 month construction period.
- The boat ramp off State Highway 33 at Mourea will be used as a construction site. However people will still be able to use the boat ramp during the holidays.
2. The predicted effects of the diversion over the first 5 years
- Lake Rotoiti's dissolved nutrient concentrations and dissolved oxygen concentrations should stay about the same.
- cyanobacteria numbers in Lake Rotoiti should decrease 40% in three to five years.
- This could be sufficient enough that blooms would no longer occur.
3. In the future
In the future Environment Bay of Plenty may consider other actions for Lake Rotoiti, like physical oxygenation or nutrient flocculation, to further improve the lake water quality.
4. Other effects
- The discharge to the Kaituna River will reflect Lake Rotorua quality rather than Lake Rotoiti quality. This will increase the biological solids (dead and live algae) in the water. But the amount of blue-green algae will decrease because they came from Lake Rotoiti water in recent years.
- Algae growth in the river is limited by the fast water flow and water retention time (1.5 days). The Kaituna River and Maketu Estuary will still meet their relevant Water Quality Standard.
- Effects on kaimoana and kaiawa will be less than minor.
- A slight (0.1 - 0.2 m/sec) increase in water speed down the portion of the Okere Arm enclosed by the wall.
- No effect in water speed or sedimentation in the Ohau Channel.
- Perceptual landscape effects will be moderate, because of the navigation lights and timber walers, but not necessarily unpleasant.
- No effect on birdlife, except the birds may appreciate the new roosting site on the wall.
5. Fish effects
- It is impossible, at this stage, to predict effects on fisheries with certainty. Trout, smelt, and other migratory species may have no problem navigating around the diversion, or there may be:
- Less wild trout in Rotoiti because their migratory cues are diverted down the Kaituna.
- Less smelt in both lakes, if they cannot move around the diversion wall and the wall diverts the passive downstream migration away from Lake Rotoiti.
- Larval smelt and juvenile trout, journeying down the Ohau Channel, heading straight to the Kaituna River and not entering Lake Rotoiti.
- The above effects may not happen if fish can find the new Lake Rotoiti entrance by detecting differences in the current between the diversion wall end and Te Akau Point, and following it upstream into Lake Rotoiti. Or, if enough spawning takes place in Lake Rotoiti to maintain the current populations.
- Larval smelt numbers in Lake Rotoiti should benefit from improved water quality after the diversion is in place.
- Extensive fish monitoring will be ongoing to work out how the fish respond to the diversion. Once this is known, a fish pass may be built if necessary.
6. Kaituna River/Maketu Estuary Management Strategy
Environment Bay of Plenty is beginning a Kaituna River/Maketu Estuary Management Strategy to discuss the Kaituna's environmental issues holistically with local communities. To help with this, consultants are modelling the effects of different management scenarios for Lakes Rotorua, Rotoiti and the Kaituna River.
7. Term of consent: 12 years
This allows five years for construction and changes to water quality, and seven years of further operation to determine the diversion wall's effectiveness. If the wall works well with no other problems, Environment Bay of Plenty can apply for a renewal of consent. If it does not work, it may need to be removed.
8. The key consent conditions (in brief)
- Notification of commencement dates, contact persons, and complaints process.
- Erection of boat ramp signs.
- Site plan, security and lighting measures on the boat ramp.
- Erosion and sediment control plan to stop dust nuisance, soil, sediment, stormwater runoff and debris leaving the site.
- Noise management plan by an acoustic specialist.
- Traffic management plan approved by Transit New Zealand.
- Spill management plan for petrol and oil to the barge and machinery.
- Navigational measures to minimise safety risks.
- Archaeological measures, should anything archaeological be uncovered or disturbed.
- Complete rehabilitation, restoration and maintenance programme for the boat ramp.
- Complete rehabilitation of the Ohau Channel wetland area disturbed by construction.
- Specific conditions for the contractors yard.
- Location and construction of diversion wall as per #1 on this webpage.
- Wall must be able to withstand the lateral force of a 50-year wave event.
- The visible part of the wall shall be the natural wood colour of the walers - nothing brighter attached to it except the navigation and safety markers.
- Navigation lighting must be modified to reduce visual impact from the State Highway and Te Akau Point without compromising navigational safety.
- Maintenance of the diversion wall so it is kept safe and structurally sound.
- Construction works only allowed between 7 am and 6 pm, and not on Sundays or public holidays.
- Pile driving can only occur between 8 am and 6 pm on weekdays.
- Construction must be complete by the end of October 2007.
- Minimum annual inspections of the diversion wall.
- Water quality monitoring at a number of sites of (at least): dissolved reactive phosphorus, total phosphorus, ammoniacal nitrogen, nitrate nitrogen, total nitrogen, blue-green algae, and suspended sediment.
- Baseline monitoring of waterbird populations on Lake Rotoiti.
- A survey to check for any wave erosion along Lake Rotoiti's northern shoreline, especially effects on bird habitat.
- Monitoring of algae levels in shellfish if the Kaituna River exceeds health guideline values.
- Extensive fish monitoring, including:
- An independent expert fishery review panel, to review monitoring and give recommendations.
- Larval and juvenile smelt monitoring.
- Identifying origins and spawning areas of trout, smelt and koaro.
- Kakahi and koura monitoring. Click here to view 'An assessment of the koura and kakahi populations in the Okere Arm and Lake Rotoiti' (610KB, pdf) - Ian Kusabs and Associates Ltd
- Monitoring of diversion effects on fish migration.
- Fish pass design and installation checks.
- Setting "trigger values" where the trout would be considered "significantly affected".
- Reporting requirements.
Three tangata whenua representatives were nominated to advise on cultural aspects relating to the diversion.