4. Description of Present Operations and Potential Changes to Operations
4.1 Waikato Hydro System
4.1.1 Present Operations
The main operational features of the Waikato hydro system are Lake Taupo and the eight hydro-lakes and their associated power stations along the Waikato River. The Waikato River and its Catchment is described in Appendix II. In assessing the environmental effects of a change in operations an important consideration is the available storage of water and the way in which this water is routed through the cascade of stations.
Lake Taupo forms the main storage reservoir, with a capacity of up to 855 million cubic metres. This is 14 times the combined storage available in the downstream Waikato lakes due to their small surface areas and operational ranges of less than two metres.
Figure 1: A. Operational Ranges (in Metres) of the Waikato Hydro Lakes.
B. The Percentage of the Operational Range of Each Lake Used (Showing the Median and the 5th and 95th Percentile)
Figure 2. The Waikato Hydro Scheme Showing the Taupo Control Gates and the Hydro Stations
The legal operational ranges of the hydro-lakes are shown in Figure 1 together with an analysis of the extent to which the existing hydro-lake range is actually used. In Lake Aratiatia virtually the full 1.4 m range is used, whereas in Lake Karapiro water levels are mainly kept within the top 35% of the 1.9 m range. Factors which constrain the present use of the full range available include:
- Operational and commercial conservatism;
- Operational limitations;
- A desire to avoid perceived lake access problems for the public; and
- The Karapiro ramping rate policy.
4.1.2 Operating Regime
The way in which the Waikato hydro chain may be operated could include:
- More peak loading;
- A tendency to be less operationally conservative within the permitted range; and
- Operating over more of the available level range.
It should be noted that ECNZ could operate the system in this manner today without restriction.
Commercial, practical and legal constraints and limitations include:
- Management of the system within legally-binding lake level limits which limit the scope for any significant operational changes;
- Restriction on the ramping rate and minimum flow at Karapiro;
- Minimum flow requirement from Lake Taupo;
- Bottle-necks in the system caused by flow limitations at certain stations which physically limit the rate at which water can be passed through the hydro chain;
- Reliance on the water stored in Taupo as Waikato SOEs only fuel source imposes a requirement to manage this efficiently to maintain long term fuel supplies; and
- Requirements to meet contractual commitments to the electricity market and avoid penalties for breaches.
It is clear that from an environmental effects perspective there are strict operational limitations within the present system which will limit operational alternatives available to Waikato SOE. Based on existing scientific knowledge, environmental effects are not expected to be significant. Specific operational changes, which may be possible in future, are:
Lake Taupo
Presently Taupo is operated in a way so that changes in lake levels follow a pattern basically similar to a natural regime. In future it is possible that variations in operating regimes could result in some seasonal changes to patterns. The limited permitted maximum range of 1.4 meters (winter) or 1.25 meters (summer) means that significant changes are not possible.
Lakes Whakamaru and Maraetai
These lakes are presently operated by ECNZ within the top half of their operational ranges. This seems to have been, at least in part, because of the perception that lower levels would cause access difficulties for other lake users. Future operation could see more of the available range being used. The ecology of the lake and its margins are such that this is unlikely to cause any significant environmental effects. Potential conflicts with other lake users resulting from possible difficulties with access and lake weed would need to be identified and suitable mitigation measures put in place should this occur.
Lake Arapuni
In future Arapuni may be operated so that levels fluctuate more on a weekly basis. However, the need to accommodate potentially high tributary inflows in this reach of the river system will limit this.
Lake Karapiro
As with Whakamaru and Maraetai, Karapiro has been operated in the past by ECNZ over a very narrow range and increased fluctuations within the permitted range could be expected. Karapiro is also a high use recreation area and appropriate mitigation to address problems should they occur could be required.
4.2 Marsden A & B Power Stations
Marsden B plant is mothballed and non-operational and Marsden A operates in a voltage support role only. Marsden operates under existing authorities and is subject to the provisions of a number of local authority plans and policies
4.3 Mokai and Wayang Windu Power Developments
There will be no changes in operations and subsequent environmental effects resulting from the split of ECNZ.
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