Aniwhenua Hydropower Scheme Headpond Remediation

Innovation + value engineering

Working with Nova Energy, Tonkin + Taylor developed an innovative remedial works design, including substantial temporary works, and prepared building and resource consent applications to enable construction of the works. Liaison with technical and regulatory reviewers was key to facilitating the fast processing and obtaining of consents within the project’s short turnaround time. Substantial diversion works were required due to the upstream headrace canal and stream flowing into the headpond.

Design of the diversion works had to balance the safety, constructability, construction timeframe, cost, and decommissioning. Through close coordination between the design team and the operator, reviewers, and contractor, all these aspects were successfully incorporated. As part of the process to develop an appropriate repair solution, the risks associated with the uncertain existing embankment conditions and potential causes of the void formation were assessed. This required excavation into the dam to inspect and replace damaged materials during construction. Ultimately, the headpond embankment seepage control material was reinstated in the damaged areas during construction as a secondary seepage control system. A geosynthetic liner system was adopted as a retrofit seepage control measure to enable the works to be completed without substantial redesign due to encountered embankment conditions.

Nova Energy elected to adopt a liner over the entire headpond area to reduce the potential for future operational disruption resulting from potential future excessive leakage though existing headpond earth liner, which was of unknown condition at the time of design. The liner system included a 2mm thick white HDPE liner over a geonet with French drain type underdrainage. This drainage system had two compartments to enable the performance of the headpond and dam liners to be monitored independently for improved management of the dam. Minor structural remedial works were completed in the existing forebay to facilitate installation of the new liner system. The temporary diversion works included two 5 – 6m high EPDM faced rockfill coffer dams, a 1.8m diameter surface mounted FRP diversion pipeline, and a temporary timber forebay wall. These cost-effective diversion works arrangements were developed to reduce the construction timeframe and facilitate rapid removal upon completion of the works, while meeting the safety requirements.