NPDC has cleared away a disused dam that was causing erosion damage and damaging the river corridor environment downstream from Lake Mangamahoe.

An aerial view of the existing dam site after its removal

The dam was built in 1917 but never performed as expected and was replaced in 1931 by the structure that ultimately formed the lake.

“The low-head dam remained in place, and it deteriorated over the years, eventually causing streamside erosion,” says NPDC Infrastructure Project Manager James Harrop.

“Removing it was a complex job because you’re working in a wet environment and you need to look after the river and land around it during the demolition work, so this took a lot of planning with our partners to get it right.”

NPDC worked alongside various stakeholders including Taranaki Regional Council, Manawa Energy, Fish & Game NZ, NP Mountain Bike Riders Club, Ngāti Tawhirikura and Ngāti Te Whiti hapū, and Te Atiawa, Ngā Mahanga, Ngāti Tairi and Taranaki iwi.

The project required a 100-tonne crane to lift out five tonne blocks of concrete during emergency repairs after heavy rainfall caused the dam to fail in 2022, followed by upstream works of the dam to reinstate the river profile for safe fish passage and erosion control.

This was followed by the removal of the dam's structures and reinstatement of the river's channel and surrounding area, which will encourage the natural return of native aquatic species and vegetation to the area.

Hāpu took the opportunity to collect data before and during the works, which will help with long-term goals of improving water quality and mahinga kai (food resources and ecosystems).

It took about two months to remove the concrete dam entirely, which was about six metres high and 32m wide. The area around the dam site has been levelled out and seeded, and future planting is being planned with hapū.

Fast facts

• New Plymouth District’s main source of water is Lake Mangamahoe, which is supplied by the Waiwhakaiho River.
• The lake holds 10 days’ supply of water.
• Its water is piped to the New Plymouth Water Treatment Plant for treatment and supply to 26,000 homes and businesses from Omata to Urenui.

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