The scheme’s 33km2 catchment sits 380m above the coastal strip, rising to 900m above mean sea level (amsl). The recorded average annual rainfall is 5.55m but this is likely to increase due to climate change as demonstrated by the moving rainfall average peaking over 8m in the last two years. When fully developed as consented, historic rainfall records predict the scheme will collect over 90% of runoff and generate 240 GWh. The hydro resource will pass through two power stations: 12MW Weka PS located on the Plateau at 380m amsl with ~180m operating head, and 48MW Granity PS located on the coastal strip 12m amsl with ~368m operating head. The Granity PS will discharge all the runoff from Stockton Plateau to an ocean outfall (excluding spills from reservoirs during extreme events).
The scheme has been designed to maximise the collection of runoff (the hydro resource) AND to maximise collection of acid mine drainage from the coal mines on the Plateau. This would otherwise drain to the Ngakawau River. Acid Mine Drainage (AMD) is an environmental legacy of 120 years of government owned coal mining. From 2006, the worst effects of AMD have been reduced by the miner diverting drainage from workings within their licence areas to sumps for treatment by dosing with lime-based reagents prior to diversion back to the tributaries (lime-dosing). The costs of lime-dosing infrastructure and reagent are shared by the miner and the government. The government has accepted liability for treating acid mine drainage generated from workings prior to 2017 when the current miner, BT Mining, took over the mining licence.
Mangatini Falls in 2008, before the miner was incentivised to improve water quality. Runoff from Stockton Plateau's western catchments enters the Ngakawau River at these falls. A poorly managed lime-dosing program after the miner's exit would see a return to these conditions.
AMD with acidity above pre-mining background levels is expected to persist for c.100 years after cessation of mining, creating a long-term environmental liability for the government. By diverting all runoff from the wider plateau to an ocean outfall (and not just from the current coal mining licence area) the scheme would largely avoid long-term treatment of AMD. The scheme provides the government with a lower risk and lower cost alternative to lime-dosing. This should attract government support for the construction and operation of the scheme.
The scheme is described in SPHS Resource Consent Application & Assessment of Environmental Effects and SPHS Consented Scheme Plans. Interconnecting pressure tunnels will divert flows between the reservoirs and through turbines to an ocean outfall. SPHS Consents are held for the two reservoirs (up to 3.5 and 7 million cubic metres respectively), two power stations, 9.1km of pressure tunnels and the ocean outfall. The Commissioners’ SPHS Decision & Conditions of Consent provide for a highly constructable staged development. In 2020, lapsing of the consents was extended to 2026 as work to promote the scheme continues. Access to the Conservation Estate, where the scheme is located, is via existing public and government roads currently providing access to Stockton Mine (Millerton Road, Burma Road, Fly Creek Mine Road).
The consents anticipate that the scheme would be built in stages to support the planning and needs of the developer, miner, government and generator. In 2011, Solid Energy entered into an agreement with HDL to build the first reservoir (Mt William Reservoir aka Lower St Pats Reservoir). The reservoir was to be incorporated into the full scheme of works as these were progressed. Final details to exercise the SPHS land exchange for the reservoirs and SPHS concession for the tunnels and access roads were agreed between Solid Energy and the Department of Conservation. The concession required detailed boundaries and was left in the final draft until final design was completed. An alliance was established with Downer to build the reservoir and geotechnical drilling and design of the reservoir was advanced. In 2012, Solid Energy had to shelve the project as it moved to avoid insolvency.
