The Waihopai concrete arch dam rises 33 metres (m) above the riverbed on the Waihopai River and created head for the power station built between 1925 and 1927 to supply electric power to the Marlborough region.
The dam is 4.3m thick at the base, reducing 10m below the crest to a width of 1.2m. The consulting engineers, Vickerman and Lancaster, recognised the lake created by the dam would have a short life due to silt and gravel infill. Therefore, their design incorporated a siphon to maintain a clearwater area at the power station intake when sediment filled the lake at the, correctly, estimated life of ten years. The 0.6m diameter siphon was found to be insufficient to cope with the gravel and was supplemented with a 5 tonne crane and grab, mounted on the crest of the dam. This was later replaced with a sluice cut into the dam.
The principle contractor for the dam was Williamson of Christchurch. During construction the river was diverted through a tunnel, approximately 92m long, with dimensions of 2.8m by 2.2m. Although the rocky gorge in which the dam was built seemed ideal for the purpose, it transpired that an old river channel behind the right abutment where the spillway was located required additional grouting and protection to prevent erosion of the spillway.
The installation is an interesting example of a hydrowork that has been able to keep operating as a run-of-the-river station despite the depletion of the lake storage.
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25 kilometres south of Renwick on the Waihopai River, Marlborough.
The dam can be viewed from Waihopai Valley Road.
Joy Stephens, 'Power to the People,' The Prow, last modified 2008.