Engineering Work (eg road, bridge, sawmill, dam)
The Waihopai concrete arch dam rises 33 metres above the riverbed on the Waihopai river to create head for the power station built in 1925-27 to supply electric power to the Marlborough region. The dam is 4.3 metres thick at the base decreasing in thickness to 1.2 metres 10 metres below the crest. The principle contractor for the dam was Williamson of Christchurch. During construction the river was diverted through a tunnel 2.8m by 2.2m with a length of approx 92 metres. Althought the rocky gorge in which the dam was built seemed ideal for the purpose it transpired that an old river channel behind the right abutmentwhere the spillway was located required additional grouting and protection to prevent erosion of the spillway. The consulting engineers, Vickerman and Lancaster, recognised the that the lake created by the dam would have a short life due to silt and gravelinfill. 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 0.6 metre 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 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.
25 kilometres south of Renwick on the Waihopai River
Drive south up Waihopai Valley road 25 km from Renwick
Nature of Engineering
Electrical Power Generation and Distribution