Ohau C power project : geological report on investigations

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Macfarlane, D.F. 1978 Ohau C power project : geological report on investigations. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 307 89 p.

Abstract: The Ohau C Power Project, located along the right bank of the lower part of the Ohau River, consists of a 7.3 km canal carrying water to a powerhouse located on the edge of Lake Benmore. The geology of the project area is dominated by the Mt John Outwash Gravel which has a thin surface veneer of reworked gravel. The outwash unit forms almost the whole foundation for the project and is not expected to settle significantly under applied loads. Large alluvial fans spread from the major gullies along the northern edge of the Benmore Range but do not encroach on the project area. Investigations work attempted to locate buried fans in the loaded foundation zone beneath the canal but did not demonstrate their presence. Gravels for embankment construction are available from the powerhouse excavation and from the cut section of the canal. Both areas have an upper silty zone which will necessitate careful handling. Lining material investigations have previously proven adequate volumes to be available from alluvial fans. No investigations into filter or armour sources have been carried out. The groundwater surface slopes towards the Ohau River. Available evidence suggests that recharge is largely derived from the large gullies along the edge of the Benmore Range where surface water flows disappear underground. Construction work below the water table will be subjected to groundwater inflow. It is expected that this will be primarily from discrete open or clean layers with the problem reducing over a period of time. Seismic risk to the Ohau C project is the same as for Ohau A but there is less chance of damage to structures. (auth)