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Norman Hardie (picutred on the Douglas River), civil and structural engineer, mountaineer, and environmentalist makes a compelling case for hydro-electric power development on the West Coast with canal and tunnel hydro schemes.
Yet the West Coast imports most of its electricity over two vulnerable passes from the Waitaki lakes in dry Canterbury. The line losses over such great distances to get to Karamea and Fox Glacier are approaching 50 percent. During 1982 an assessment of annual rainfalls was done in a line extending from Hokitika to the Rakaia headwaters and Mount Hutt. The published results indciated very high precipitation – frequently about eight metres a year and sometimes 12 metres. Very few places in the world have so much useful rain, especially as it spreads approximately over a year, not concentrated for four months as it does in monsoon climates. Most of this rain falls west of the main divide and there is a considerable rain shadow in the Rakaia basin and in the Canterbury plains. Since the decision in 1983 to build the Clyde high dam, the country has suffered from hydro-phobia and there have been no large hydro installations built, only minor upgrading of existing schemes. The permanent flooding of the arable flats for 18 kilometres upstream of Cromwell towards Tarras gave hydro schemes a bad odour. It also contributed to the departure of the Muldoon government in the following year and the subsequent timid attitude to any structures on our rivers. In Europe some hydro construction has continued and not one major valley has been flooded in the last 50 years – through the thoughtful use of tunnels and canals dozens of rivers have been diverted to places where they can discharge into penstocks for power and still be used downstream for irrigation or town supply. In some circumstances a canal can be built along ancient glacier terraces and then be dropped down in steps with each step containing a generating station. In the Waitaki this has been done successfully at the Ohau canals. Tunnel hydro potentialIn the daily pattern of domestic use of electricity there are two major demand peaks. One is at the beginning of the day and the other is each evening for a bigger and longer loads. These peaks are larger in winter when more heat and lights are required. Electricity supplied from wind, coal, geothermal, tides and some gas sources is unsuitable to meet these short term peaks and it is only hydro-electricity and some gas turbines that can be turned on for an almost instantaneous boost. There is now much activity in the construction and further planning of wind farms and one hears of objectives for this source going up to 25 percent of our total capacity. For this to happen there has to be large sources of controllable water to manage the daily peaks and for the longer periods of calm weather. Opening and shutting gates for additional hydro turbines to operate is not wholly practical in a canal system as they cannot tolerate much change in levels of water without overflowing. Ideally a canal system should have a small reservoir, one square kilometre or so, upstream of each penstock, to give the flexibility to meet short term peak demands. In the South Island several large lakes, Ohau, Wanaka and Wakatipu have no gates at their outlets so their outflows should be regarded as ‘run of the river’. Lake Manapouri does have gates, smaller than originally intended, but most of its output proceeds to the Bluff smelter. The three gated lakes available for the main grid are Tekapo, Pukaki and Hawea and are only ones where outlets can be closed to build up water during periods when the weather is calm and wind generation is down. Most West Coast rivers fall very steeply from the main divide. However, there are tributaries of two large rivers that are high enough to be able to add considerably to the storable assets of Lake Pukaki. These rivers are the little-known Landsborough and Douglas. A tunnel seventeen kilometres long could be bored from the east and have a ten kilometre branch to intercept the Landsborough. The best interception sites are at about 900 metres above sea level. The water would join the Tasman River just downstream of the Mount Cook airfield and flow on to the lake that has a storage level of 532 metres. A tunnel to take water eastward would be drilled from Canterbury so the spoil is carted to that side, thus protecting the pristine environment of the west. Only water would be taken and no tracks roads or power pylons would be left on the west. All the construction plant would travel through the tunnel and then be withdrawn to leave a tunnel entrance and a sand and gravel trap. This is needed to intercept material which would erode the tunnel floor and severely wear the turbine blades on the Canterbury side. There are already in Westland good examples of river intakes and settling ponds for the small 40-year-old Arahura-Kumara scheme. I have been involved with many construction projects in Westland, regrettably not hydro-electric. I have also enjoyed mountaineering, rafting and kayaking in that wonderful district. As one drives the 450 kilomtres from Karamea to Jackson’s Bay, a river is crossed every twenty or so kilometres. Many of these have very large falls which could be harnessed by tunnel diversions to be large power sources. The existing small scheme at Kumara is a good example that uses enlarged canals that date back to gold miners’ sluicing enterprises. Regrettably they are not now large enough to supply just one of the three large towns, let alone the 280 dairy farms and the milk powder factory that have appeared since the power scheme was built. In the last twelve months three small Westland schemes have received their initial approvals but they are subject to appeals. One of them uses existing tunnels from gold mining activities. Tunnels for diverting water were frequently built by miners. In the early 1960s tributaries of the Whanganui river were tunnelled through to Lake Taupo, generating power on the way. More recently a tunnel boring machine was used for the successful drilling of the second Manapouri tailrace. There has been great progress in the design of these boring machines and, with their use, many more hydro schemes could be built in Westland without massive flooding of scenic valleys. The Westland district would benefit from more employment, local power charges would be reduced and the surplus exported to Canterbury.
Energy NZ Vol.4 No.5 September-October 2010 |
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