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Geotechnical

Water Resources

Deep Stream Hydro Project

 CLIENT 

Trustpower Ltd

 LOCATION 

Dunedin

 RILEY TEAM 

 THE BRIEF 

This project uses the 310m elevation difference between an existing pipeline diverting water from Deep Stream and Lake Mahinerangi (the principal storage for the Waipori power scheme near Dunedin). 

 

The Deep Stream Hydro power scheme includes two power stations, a storage lake and conveyance systems involving canals, penstocks and associated structures. Principal water retaining structures include:

  • main dam, 25m high, retaining a reservoir of just under 2 million cubic metres stored volume at full supply level

  • saddle dam, 9m high

  • 8km of canal, incorporating a third dam about 8m high.

 

The main dam provides supplementary water storage for Dunedin City in drought conditions. 

 

Both dams are founded on schist bedrock; with the main dam, 2.5m to 3m of soft organic soils were removed in the stream valley. Key design features include: 

  • dam cross section incorporates a silt core (colluvial/residual soil) with schist rockfill shoulders

  • vertical chimney drain

  • foundation cutoff into moderately weathered rock

  • three-layer blanket drain in the valley section, with a series of strip drains on the abutments

  • toe cutoff drain

  • batter slopes 1:2 downstream, 1:2.75 upstream.

 THE OUTCOME 

The catchment to the dam is modest (70ha). A combined service and emergency spillway is provided on the true left of the main dam. The gated outlet structure to the canal during everyday operation will tend to minimise use of the spillway. 

A 5m-high cofferdam upstream of the main dam, along with temporary diversion pipes, provided flood protection during construction, and proved effective in practice.