BoPRC Ohau Diversion Wall Update

BoPRC Ohau Diversion Wall Update

The Lake Water Quality Society hosted a community meeting for Rotoiti residents over  Matariki Weekend to provide an update on the Ōhau diversion wall. The meeting was prompted by concerns over the development of holes near the lake surface in the wall, which have allowed some Ōhau Channel flow to leak into Lake Rotoiti.

Initial signs of accelerated corrosion on the wall were noted in 2014 during a dive inspection. Since then, Regional Council has conducted detailed underwater inspections every three years to assess necessary maintenance for ensuring the walls functionality for at least 50 years. The latest inspection in 2023 revealed a reduced rate of corrosion on the sheet piles but identified numerous holes within the splash zone (within 1m of the surface). The king piles remain structurally sound in critical areas but may require maintenance in 15 to 20 years.

In 2019/20, Regional Council installed a strengthening waler to enhance the wall's structural integrity, which the engineering report confirms it is extending the lifespan as intended. Regional Council is evaluating four main options for repairing the walls holes. Trials are underway to ensure the final option will be fit-for-purpose and durable in the challenging lake environment. The preferred repair approach is expected to be finalised by early 2025, with repairs scheduled to commence by mid-2025.

Regional Council has allocated a budget of $1.6M for the repairs, pending further testing and analysis to refine this estimate. Additional funding may be required, pending Regional Council deliberation. The Lake Water Quality Society and a Technical Panel, including Geoff Dainty, John Gifford, Grant Wallace and Ken Kennedy are collaborating to keep the community updated on the investigation and repair progress. For further information, please contact Andy.Bruere@boprc.govt.nz.