The Greater Wellington Regional Council, through Greater Wellington Water (GW Water), undertook an assessment of the future of the Lower Wainuiomata Dam, a historically significant structure built in the 1880s. Although the dam once played a vital role in Wellington’s water supply, it has since been decommissioned and no longer serves an operational purpose. A report, prepared in 2004, evaluates options for managing the dam in a way that meets modern safety, environmental, heritage, and community expectations.
The assessment focused on three primary options:
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Retaining the dam and creating a lake or wetland in the existing basin. This approach would preserve the dam’s heritage values and create a new recreational or ecological resource. However, it would be the most expensive option due to the need for ongoing maintenance, seismic strengthening, and environmental management.
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Lowering the spillway to create a smaller, controlled wetland. This option would maintain some of the historical structure while significantly reducing the dam’s safety risks and cost of upkeep. It would also support biodiversity goals and reduce the likelihood of damage during an earthquake. However, it would still require some structural work and consent processes.
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Cutting a channel through the dam to fully restore the Wainuiomata River’s natural flow. This would eliminate the dam as a physical structure and allow fish passage to be re-established along the stream. It would be the lowest cost option in the long term and would reduce safety risks to near zero. However, this would also result in a complete loss of the historic dam and its potential for reuse or public engagement.
An engineering review by DamWatch Services Ltd indicated that the dam poses low risk to downstream residents, but it does not meet modern dam safety guidelines and would require seismic strengthening and spillway modifications to comply. Even though the dam is currently stable, there is concern about its performance in the event of a major earthquake.
After weighing the pros and cons, the report recommended lowering the spillway as the preferred option. This approach achieves a practical balance between cost, safety, environmental outcomes, and heritage value. It also aligns with the Council’s wider biodiversity and ecological restoration goals by allowing for the development of wetland habitat, although some additional work would be needed to maximise its ecological potential.
Overall, the report highlights the importance of considering not just engineering and safety factors, but also cultural, historical, and ecological aspects when deciding the future of decommissioned water infrastructure like the Lower Wainuiomata Dam.
