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Journal of the Nelson and Marlborough Historical Societies, Volume 2, Issue 6, 1995

The Cobb – The History of The Cobb River Hydro-Electric Power Scheme

page 49

The Cobb – The History of The Cobb River Hydro-Electric Power Scheme

Athol Blair's history of the Cobb must be the most comprehensive book ever written about a New Zealand power scheme. It is also the history of an important facet of Nelson's development. Considering the Cobb's small size, a mere 32 MW, when compared to the schemes more recently completed in New Zealand it is at first glance surprising that nearly 500 pages could be written about it. What makes the Cobb scheme so intriguing is its complexity, taking 20 years to build, and the reasons why it was constructed in the first place.

The scheme was built because of the presence of asbestos in the maze of mountains west of Mt Arthur. Combining materials and the energy that could be harnessed from the abundant rivers draining the mountains was Walter Hume, a remarkably inventive Australian. Hume, the inventor of the spun concrete pipe, conceived the idea of using Cobb power, the asbestos deposits in the Upper Takaka River that were being promoted by another remarkable man, Henry Chaffey, and cement from Tarakohe. Not envisaging that he would need all of the power generated from the Cobb River, Hume proposed to sell the surplus to consumers in the Nelson region, who were then being serviced by small local power stations.

In 1935 the government granted a license to the Hume Pipe Company (Australia) Ltd and the Cobb scheme was born. Its growth to maturity was to be long and difficult. One condition of the licence to Humes was that power be supplied to consumers in the Golden Bay and Waimea electric power boards within two years. This soon proved to be a highly optimistic and hopelessly unachievable objective. It was 1944 before Cobb began producing power and it took another eleven years after that for the scheme to reach its maximum output, on completion of a dam in the headwaters of the river. It is this long unrelenting struggle to bring power to the consumers of Nelson which makes the Cobb story so intriguing and which is so graphically portrayed by Athol Blair.

Firstly there were the sheer physical obstacles to overcome. Most of the year the upper Cobb River meanders placidly through a wide open U-shaped valley, carved by a long disappeared glacier. However on breeching the moraine, which temporarily held back a natural lake following the melting of the ice, the river descends through a narrow tortuous gorge to join the Takaka River, dropping almost 600m vertically in the process. The Cobb scheme was based on controlling this drop and converting the huge amount of energy involved into electricity.

The scheme envisaged a concrete dam at the start of the gorge and a tunnel through the mountains to a penstock that plunged down the precipitous slopes to a powerhouse at the junction of the Cobb and Takaka rivers. With the nearest access route, the Upper Takaka-Tableland park track, several kilometres to the east, the power builders had first to gain access up the rugged Takaka River to the power station site, which proved to be a major undertaking in its own right.

From here the difficulties multiplied at an alarming rate. When Humes were not able to complete their goal, the New Zealand government took over the scheme in 1940. As the war situation worsened, labour and materials became increasingly hard to obtain. Although Kiwi ingenuity helped offset some of these setbacks, the scheme lingered and power supplies in Nelson and Marlborough were becoming critical.

page 50

Nevertheless work continued, frequently halted by heavy rainfall and subsequent floods, snow and the intense cold that permeated the sunless valleys in winter. Disappointments continued, the dam site was belatedly found to be unsuitable for a concrete dam (a lesson forgotten years later at Clyde) and an earth dam was constructed. This was then the largest of its type in New Zealand and the engineers were at the cutting edge in determining the parameters for its design. The tunnel and penstocks were also to provide many difficulties and anxieties for both engineers and workers.

Although prior to commissioning of the dam the Cobb was able to generate some power, it was insufficient to meet peak loads. The race to get the dam finished in time to meet predicted demand is an exciting one that becomes a common theme throughout the book. To boost output a number of small glacial lakes were dammed and the water released at critical times. These efforts seem puny until one realises that, with the high head of the scheme, an apparently infinitesimal amount of water could be converted into a worthwhile quantity of power. The commissioning of the dam in 1954, and the rebuilding of the powerhouse soon after, meant that the scheme could generate its planned 32 MW.

This book is not just about engineering, for it gives a full account of the men and women who lived in the Cobb. There were, over the years, a large number of people involved and the excellent index is a boon for readers. It is amazing how many familiar names keep appearing throughout the book. There is also a generous use of photographs which graphically show the scheme in all its stages, along with details of the camps and the hardships faced by all who worked on the scheme. There is one spectacular sequence of photographs of a flood in 1954 which nearly overtopped the partly built earth dam.

It is a book well worth reading and readers will probably find that once they start turning the pages they will find it difficult to put down. The Cobb scheme marked the change from shovel and wheelbarrow to bulldozer, and it had the last of the primitive construction camps that gave way to the townships now associated with major civil projects. Considering how environmentally sensitive the Cobb Valley is, it is perhaps as well that bulldozers were then not as common as they are today. The Cobb scheme, with its powerhouse nestled deep in the valley and the reinstated glacial lake behind its earth dam, blends into the northwest Nelson mountains. Athol Blair is to be congratulated for documenting how it came about.

Mike Johnston