The Nadarivatu Renewable Energy Hydro Power project takes water from the upper reaches of the Sigatoka River headwaters and diverts it to discharge into the Ba River, utilising the 350m head difference. The scheme includes a dam with a storage lake, a conveyance system involving a tunnel and penstock, and a powerhouse with pair of Pelton wheel turbines. The principle water retaining structures include:
- 40m high dam retaining reservoir of one million cubic metres stored volume at full supply level
- 2km tunnel through the range dividing the river catchments
- 90m deep surge shaft near the outlet portal
- 1.4km long steel penstock 2.25m diameter
The dam is founded on basalt lava flows, pillow lava and flow breccia. Some key details of the design include:
- 36,000m³ reinforced concrete using aggregate from site quarry.
- Extensive grout curtaining beneath dam to create a seepage cutoff.
- Construction of three radial gates to discharge floods.
- Construction of two sluice gates beneath dam to remove gravel build up.
- Protective rock bolting and shotcreting of exposed faces above dam site.
The tunnel was constructed from both ends simultaneously and utilised drill and blast techniques. The requirements for the initial lining were determined by the rock conditions encountered using the Q system. Support was given by a combination of rock bolting, lattice girders and shotcreting. A smooth concrete lining will complete the tunnel.
The powerhouse is in the base of a deep gorge with difficult access. Two Pelton wheel multi-jet turbines working under a 350m head provide 40MW out put. Power from the scheme is transmitted at 132kV to a switching station adjacent to the main cross island transmission line 5km away.
Temporary diversion pipes and coffer dams were constructed upstream and down stream of the dam during construction of the base. The dam is situated in cyclone susceptible area which added some challenges to the construction of the project during the cyclone season.