Flushing Flows Enhance Recreation And Ecological Values

Meridian Energy’s proposed Project Aqua flushing flows would have recreational as well as ecological benefits for the Waitaki River.

Project Aqua is a proposed canal-based hydro-electric scheme in the Waitaki Valley near Oamaru. It would generate enough renewable electricity to power the equivalent of about 375,000 households in an average rainfall year and 250,000 households in a very dry year such as occurred in 1992 (a 1 in 20 year event).

Project Aqua must be commercially viable and environmentally sustainable for Meridian Energy to proceed even if all the required consents and approvals are granted. This means it must cost less than other forms of generation it within the 4.5 – 5 cent per kilowatt range. A major potential benefit of Project Aqua is that it could enable irrigation in the Waitaki district.

As part of its applications for consent for Project Aqua, Meridian Energy has accepted the recommendations from NIWA to release four flushing flows down the Waitaki River each year. These would run from the existing Waitaki Dam to the coast. During the flushing flows, the minimum river flow would be increased from between 100 and 140 cumecs to between 400 and 500 cumecs, for 24 to 48 hours.

The flushing flows are designed to remove silt and undesirable periphyton growths from the river. The timing of the flushing flows (three between 24 January and 30 April and one between 1 July and 10 August) take into consideration the avoidance of the early winter spawning season, the spring rearing season for juvenile salmonids and the breeding season for specialist braided river breeding birds.

Meridian Energy spokesperson Alan Seay says as well as maintaining the aquatic ecosystem values of the river, the flushing flows could also provide a “big flow” experience for jet boaters.

“We realise that jet boaters love to experience the river when there are high flows. These flushing flows would provide the “big river” jet boating experience and could be timed to coincide with high use jet boating events,” says Alan Seay.

When timing the flushing flows, Meridian Energy would also take into account the rainfall in the tributaries to enhance the downstream distribution of juvenile fish, the naturally occurring high river flows and the spring high tides in March and April so as to avoid possible dewatering of whitebait eggs.

As well as flushing flows, Meridian Energy would use its best endeavours to ensure that floods of at least 900 cumecs occur down the river every two to three years. These floods would mobilise and transport gravel and maintain braiding patterns. Meridian Energy would also do its best to manage the duration and frequency of flood events to help minimise the destabilising effects on fish, in particular on salmon and trout stocks.

“Our proposed flow regime would provide a much greater flow stability than currently exists and lessen the amount of sediment movement in the river, meaning habitats would not be removed. Expert fisheries advisers from NIWA and Cawthron Institute have found that it should be possible to maintain the salmon and trout populations in the lower Waitaki River under the proposed Project Aqua flow regime, provided recommendations for mitigation and monitoring are actioned,” says Alan Seay.

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