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Feature

Hydro Mountain High

New Zealand's wet mountain ranges could yield hydropower resources with minimal environmental impact.

Earl Bardsley

Hydro power development in New Zealand has been based on giving preference to those projects which have the best yield for the least construction cost, leading to major rivers being dammed or diverted with power generated from a large discharge passing down a small vertical fall. The inevitable consequence is our large number of submerged valleys, near-dry river beds, and modified lake levels.

One alternative suggests moving away from hydro power and developing higher-cost generating systems such as solar and wind energy. However there are more expensive options for hydro power too which will also have little environmental impact. This despite the fact that history might suggest it would now be impossible to construct any large hydro scheme in New Zealand without major environmental disruption.

One suggestion, using underground aquifers to store water, has been discussed previously [Underground Hydro Lakes, April 1994], but the topographic and geological constraints probably mean that there will not be many sites in New Zealand where new power schemes could be constructed using this principle. Another subsurface approach is to have a stream interception system similar to the Tongariro scheme, but with underground storage in a "lake" of interlinking tunnels penetrating through the mountain range at a high collection elevation. Power would then be generated by passing a relatively small discharge through a large vertical fall. In principle, this has wider potential application as we have an abundance of wet mountain ranges.

Storage carved from mountain interiors could prove viable given the specification of water collection at altitude. That is, only the topmost headwaters of mountain streams would be diverted to underground storage for subsequent power generation utilising the high head. The volume of subsurface rock excavated to create the water storage space need not be impossibly large because the higher the elevation, the smaller the volume of water needed to store a given quantity of energy. At the same time, there is less environmental impact from diverting water at higher elevations because less discharge is abstracted.

Despite the desirability of greater elevation, there are limits to how high a water storage/collection system can be located within a given mountain range. Although higher elevations yield greater generating heads, there is also less catchment area to provide runoff. In addition, natural topographic drainage networks tend to be less well formed at altitude so some slope runoff may bypass diversion locations in adjacent stream channels.

Given that a sufficiently elevated storage/collection systems can be constructed, it is difficult to think of any alternative storage-based hydro schemes that would have less environmental impact. High-altitude schemes also come close to the concept of true sustainability. Any accumulation of fine sediment in the storage tunnels could be flushed back out to the mountain streams from time to time. The storage volume of the underground lake could thus be maintained in perpetuity. In contrast, many existing hydro lakes have a finite lifetime due to sediment infilling.

Weighing against high-altitude environmental advantages are the costs involved in creating storage networks within a mountain range at the design elevation. In general, it would be desirable to have a storage volume corresponding to at least three months of water throughput, as there might be negligible winter runoff due to precipitation falling as snow above the collection elevation. There is also the need for sufficient storage to be available to hold runoff from intense rainfall events.

A typical high-altitude hydro system in the Paparoa Range south of Westport would collect water by diverting numerous mountain streams to underground storage at the 700-metre contour. Power could be generated from some suitable sea-level powerhouse operating with a 700-metre head.

A simple calculation reveals the magnitude of the rock excavation required. Stream runoff on the Paparoa Range can be collected at the 700-metre contour from a total upland area of about 250 km2. Given annual mean intercepted discharge equivalent to 6 metres of rainfall and allowing 3 months of storage (1.5 metres of collected rainfall), the required volume of rock to be excavated is 3.75 x 108m3. This represents an amount of solid granite thirty times the volume of the fill used to construct the Benmore dam!

One positive aspect of the hypothetical scheme's massive excavation requirement is that the resulting volume of rubble might be used to construct port facilities near Westport. Interestingly, the excavated rock itself has potential energy by virtue of its elevation. If that volume of granite was removed to sea level over a five-year period, then this would produce a mean energy flow of 44 MW over that time. However, it seems unlikely that this "rock power" could be readily converted into electricity.

If the economics of the rock excavation process could be overcome, then high-altitude projects like the Paparoa scheme could provide useful power to the national grid. Given the runoff and elevation values mentioned earlier, the mean power output of the Paparoa scheme would be around 300 MW -- roughly twice the mean output of the Tongariro scheme. At the same time, the Paparoa scheme would have more storage capacity (measured as potential energy), and a high peaking ability could be built in.

Just as important as power production, completed high-altitude schemes would be almost environmentally invisible. If the power stations were located underground, then the only surface evidence of the schemes would be small diversion structures set into stream beds at the design elevation. There would be some flow reduction, but all discharge originating from precipitation falling below the (high) design elevation would continue down the natural stream channels as before. It could happen that some channels are intercepted at such a high level that they only contain flowing water during periods of heavy rain or snowmelt floods.

Sadly, the magnitude of underground excavation is likely to ensure that high-altitude schemes in the Paparoa Range and elsewhere remain in the hypothetical category for some time to come. It is nonetheless useful to add this option to other environment-friendly energy technologies to be considered in the future.

Dr Earl Bardsley is senior lecturer in hydrology at Waikato University.