Deep Stream Water Supply

… The final example of an engineering achievement worthy of the title ‘built to last’ comes from the 1970s, the Deep Stream Water Supply. Water supply has been a major problem to successive City Corporations and Councils for most of the city’s life. We have a modest rainfall with lengthy dry periods every few years, and very limited space to store water to cover such dry periods. Run-of-river sources are a long way from the city so are very expensive to use. The City lived a hand-to-mouth existence for its water until well after the Second World War. In 1956 the City tapped the river gravels beside the Taieri River at Outram, and this gave us adequate water for the next twenty years. However by the late 1960s it was clear that another major supply would be needed to cope with the ever increasing demand for water. The search was on again for more water.

In the early 1970s the options had been narrowed to two. More pumped water from the Taeri River, or a gravity supply from Deep Stream in the Lammermoor Range. The Deep Stream scheme had been proposed by City Engineer J G Alexander in 1930, but it was rejected by the Corporation at that time as too costly. The more modest 1936 Deep Creek scheme was built instead. However, detailed survey plans of the 1930 Deep Stream scheme were held in the City’s records, and an Act of Parliament in 1930 authorised the taking of water from the Deep Stream. It comprised some 58 km of pipeline falling from an intake at 425m above sea level to Mt Grand at 300m above sea level. A treatment plant at Mt Grand would be able to supply water to almost the entire city.

The choice between the two options was made on the economics. Deep Stream was very high capital cost but low operating cost (no pumping needed). Taieri was relatively low capital cost but high operating cost (requiring electricity for pumping). In the event the Deep Stream option was approved by the Council in 1972 and it was completed in 1977. Just after the decision was made, the first oil shock in 1973 occurred and huge increases in energy costs resulted. This markedly increased the advantage of the low-energy Deep Stream option, and is reflected today in our relatively cheap water charges.

The Deep Stream Scheme was forecast to meet the City’s water needs till the early 1990s. In the event the Burnside Freezing Works, a heavy consumer of water, closed and population did not increase as much as expected so the water supply system, including the Deep Stream Supply, is still meeting the demand, indeed is supplying areas of the post-1989 amalgamated city not envisaged over 40 years ago in 1968 when the demand forecast was made. Deep Stream was certainly built to last! It cost some $6.2 million (($55.1 million today).

Extracted from: Otago Settlers News, March 2010, Issue 104, In the Pipeline. Engineering Feats Beneath Our Feet. Trevor J Williams, BE FIPENZ MICE MNZIS NZIM, former Dunedin City and Drainage Engineer.