Professor says Lake Taupō should be lowered for flood protection

Thursday, 23 August 2018

University of Waikato Associate Professor Earl Bardsley has challenged the status quo when it comes to the management of Lake Taupō.

New Zealand's largest lake should be managed at a much lower level to protect thousands of properties from flooding, says a University of Waikato professor.

Associate Professor Earl Bardsley, from the Faculty of Science and Engineering, has challenged the status quo when it comes to the management of Lake Taupō.

At present, the lake level is controlled by Mercury which has resource consents allowing it to influence lake levels by 1.4 metres [355.85-357.25 metres above sea level].

This allows it to manage its hydro-electric power stations on the Waikato River - stations that produce about 10 per cent of the country's electricity needs.

Earl Bardsley is planning a major research project into water levels at Lake Taupō.

* Erosion could see car park under water

* Mercury spends up on ageing Waikato River hydro schemes**

Phil Gibson said Bardsley's proposal assumed that there was a flood risk.

Bardsley said there was no need to have such a large operating range and he thinks the lake could potentially be managed at a level much lower than it is, with a smaller operating range to avoid fluctuations in lake levels.

That would provide a buffer against flooding in the lower reaches of the Waikato River and fix problems with erosion around the lake itself, without adversely affecting the country's power supply, he said.

He raised the example of the 1998 floods which affected large swathes of land below the Lake Karapiro hydro reservoir, including some parts of Hamilton. These types of flood events could become more common as a result of climate change, he said.

Mercury general manager Hydro/Wholesale, Phil Gibson, said Bardsley's proposal assumed that there was a flood risk.

'You go from very low [risk] to non-existent.'

He pointed to cyclones Debbie, Donna and Cook which came through the Taupō catchment area last year, all within a space of 6-7 weeks. That pushed the lake up to within 9mm of its maximum allowable operating level.

'Had absolutely fantastic outcomes for the residents of Hamilton city and the lower Waikato as we worked with the [Waikato] Regional Council to carve out space in the river reservoirs to capture that water and release it.

'Just an example of a very dynamic response to an unplannable weather system, three cyclones in a row where we managed to tick all the boxes.'

He said a lot of the catchment area for the Waikato River was below Lake Karapiro where the river flattened off considerably over the last half of its journey.

Bardsley said he was not advocating for a review of current consents just yet but he hoped to be running an extensive research programme.

'I have a PhD student coming in from overseas, starting early next year, who will carry out a three-year thesis study on the possibility of reducing the consented lake level operating range. I hope the thesis will be supported by MBIE funding.

'If the results indicate such operation is possible, I would hope that would be taken into account in a later review of the lake level consents.'

He said it was true that the prospect of major floods had a low probability based on past records.

'But we have to allow for the possibility that climate change will reduce the return periods of such events. Also, though 1998-type events are rare, they have massive impact when they do happen.'

Waikato Regional Council infrastructure manager Brent Sinclair was given a copy of Bardsley's simulations and research to date. He said the consent conditions can be reviewed every five years and 'one such opportunity' existed this year.

'These resource consents, which among other things set the operating range for Lake Taupō water levels, were determined by the Environment Court after hearing considerable expert evidence,' Sinclair said.

Gibson believed Bardsley was taking a very Taupō-centric view of the catchment.

'There is a much broader consideration for us and as I said to you the other day on the phone we are talking about New Zealand's most critical catchment.

'It's a very, very volatile and large catchment. It stretches all the way from Whanganui to Rodney and affects close to 2 million Kiwis. There is Lake Taupō but also the broader Waikato.'

Bardsley said operating Lake Taupō to always be in flood control mode had 'obvious advantages' but there was also a question as to the impact of removing hydro storage from Lake Taupō.

'As it happens, a case can be made that Taupō hydro storage serves no real purpose beyond allowing short-term water releases to match within-day variations of power demand.'

During winter, he believed it would be better to release Lake Taupō water at about the same rate as it flows in, except for when buffering flood inflows.

'In other words, it can be argued that operating Lake Taupō in permanent flood control mode is also best for electricity supply.'

Gibson said the 'limited influence' they had over the level of Lake Taupō was integral to a hydro catchment.

'[It] provides massive benefits for the whole Waikato community, not just Taupō foreshore, which is an important stakeholder but not the only one we consider.'

Other users of the hydro infrastructure included recreational user - boaties and fishers - and irrigation for farmers.