Roxburgh Dam

In the post WW2 era of New Zealand, one of the marvels of the age was electricity for the masses. It was gradually reaching into the lives of ordinary people, providing convenience in a fledgling consumer society where household appliances were making life easier. The Roxburgh dam was New Zealand's first large dam.

The site on the Clutha River at Coal Creek was chosen in 1947. Few people lived in the area, and it would be fair to say that New Zealanders at the time had little in the way of an environmental conscience. Besides, the Clutha River was regarded as a southern resource that belonged to the people of New Zealand, a mind-set that has lingered into the 21st century.

Electricity was a passport to prosperity, and the wonders of the Roxburgh Gorge were expendable. In hindsight we can understand why it was built, but we can also lament the loss of 'New Zealand's Grand Canyon' and this country's largest rapids, notably the Golden Falls at Island Basin, and the Molynuex Falls.

To Māori the Roxburgh Gorge was known as Kā Moana Haehae (the division of the waters). After the 1998 Ngāi Tahu settlement this name was applied to the bed of the Roxburgh reservoir.

Unlike the Cromwell Gorge, no one fought for the Roxburgh Gorge, so we cannot say that it was stolen. The Roxburgh dam, from the outset, provided the region with benefits that improved the lives of the population, and it has continued to contribute well into old age.

When the Roxburgh dam was built, technology was beyond question, and there was a belief that nature could be tamed by engineers. The lordly State Hydro-Electric Department (SHD), later renamed in 1958 as the New Zealand Electricity Department (NZED), ruled in all matters relating to power supply, and appeared to operate with impunity. Planning knowledge was accorded only to the privileged few, and any consultative process was undertaken with an air of disdain and irritation.

Cost overruns drew public criticism, but the cause of the mounting costs - inadequate investigation work and a ‘design as you go’ mentality, escaped public scrutiny. The final cost of the project was put at NZ£17,000,000, but it was probably much higher.

Commissioned in 1956, the Roxburgh dam, reportedly, had a design life of 50 years. That time has elapsed, so the obvious question is "What now?" This is not a popular question, but it cannot be ignored forever. Decommissioning is inevitable.

That issue aside, old dams and old reservoirs become increasingly problematic with age. The reservoir's sedimentation issue is well-known, but what is not widely known is that, geo-technically, the Roxburgh Gorge is similar to the Cromwell Gorge, with its own faultline and landslide areas. However, when the dam was built, these issues were not properly investigated, and little if any mitigation was carried out. The adage 'ignorance is bliss' comes to mind.

Also not well-known is the leak that occurred in 1963, the seismic cracking, and the vibration problem during high spillage. Given that the Roxburgh dam is close to a faultline at Coal Creek, and that a major quake along the Great Alpine Fault is expected in the next 1-20 years, this aging dam might be described as a ticking time-bomb. Most people believe that it is less able to withstand a major earthquake than the Clyde dam.

But it is the sediment build-up or aggradation issue that is the immediate problem. Proponents say that since the sediment has not yet filled the last section of the reservoir near the dam, that there is more life in the dam. However, this is because the movement of sediment is being restricted by the 'Gates of the Gorge' just below Alexandra, where the gorge is reduced to 39 metres in width. This is followed by a long section known as the 'Narrows' above Island Basin. As a result, much of the sediment burden is building up around Alexandra and in the Manuherikia confluence instead of escaping down the gorge. This, of course, raises the level of the river bed, and has already caused serious flooding at Alexandra, in 1994, 1995 and 1999. The obvious problem, therefore, is at the head of the reservoir.

In their Annual Environmental Report (February 2002) Contact Energy stated: 'The most significant environmental effect associated with the company's hydro operations on the Clutha River/Mata-au is considered to be the exacerbation of flooding at Alexandra caused by sediment build-up in Lake Roxburgh.'

Repeated attempts have been made to 'flush' the gorge by drawing down the reservoir ahead of high flows, to get the sediment moving through the bottleneck at the 'Gates.' Although this has achieved some movement of sediment down the reservoir, it is a temporary mitigation measure that is likely to become less and less effective. Some work has also been done to physically remove sediment that has accumulated at the Manuherikia confluence adjacent Alexandra, but again this is a short-term measure.

Physically removing the sediment from the constricted area of the gorge would be extremely costly, and is therefore not an option. The only long-term solution is to remove the dam. Decommissioning the Roxburgh dam will not come onto the agenda, however, until this issue worsens. Few people will take notice until there is another flood, perhaps higher than the Alexandra flood wall.

In 2007, Contact Energy was granted renewed consents to operate its Clutha hydroelectric dams for an additional term of 35 years. It is likely that further sedimentation and flooding issues with arise before this term expires.

The decommissioning of the Roxburgh dam, whenever it occurs, will take some years to complete. Perhaps, one day in the distant future, Alexandra will become the whitewater capital of New Zealand. Those magnificent rapids were not physically destroyed. They were simply flooded and gradually mothballed in silt. Another treasure of the Clutha, waiting to be re-discovered.