Paula Fern: Dam reservoir ticking timebomb

Paula Fern.

Among the many concerns raised over the Ruataniwha dam project, no one has yet satisfactorily answered the most pressing civil risk.

Given it will be built on a significant faultline which is overdue to slip, how do we prevent the changing weight of the reservoir from triggering a major earthquake?

The Mohaka Fault is one of the three most active in New Zealand. The reservoir the dam will create, with an estimated weight of 93 million tonnes at maximum capacity, will sit directly above it.

An early regional council report assessing the consequences of the dam breaching in the event of an earthquake gave it a "high" risk rating, mainly because of the potential fatalities that would occur. We have been told the chances of this happening are low, because of the design of the dam.

However, given the hands-on experience of one gentleman at the Waipawa public meeting on March 10, who spoke of the "rotten rock" he had encountered in the area while manning a bulldozer for many years at Smedley Station, perhaps this aspect needs review. Certainly in the speaker's opinion he struggled to see how the local rock would provide a solid base for any large construction and especially not a dam.

But as disastrous as a breach could be, it is not the main concern when it comes to earthquakes.

The Modified Mercalli Intensity Scale rates seismic events from 1 (hardly felt) to 12 (catastrophic). According to a GNS document referenced on a Civil Defence site, complete with mapped data, the Mohaka fault in the vicinity of the proposed dam and reservoir is expecting an earthquake on this scale of 11; what is called a "very disastrous" earthquake, whereby most buildings are destroyed.

(It's now hard to find, so here's the Civil Defence site referred to: http://www.hbemergency.govt.nz/assets/Facing-Risks/Facing-the-Risks- Chap-2-Earthquake-HazardsPt2.pdf )

It isn't the dam itself that could cause this devastating earthquake to prematurely occur, it's the fluctuating weight of the reservoir directly on the fault, which can cause what is called Reservoir Triggered Seismicity.

The changing pressure of filling and drawdown of water has been linked to increased seismic activity overseas, and in the case of the Zipingpu Dam was identified as the probable cause of the Wenchuan earthquake on May 12, 2008 which caused the death of more than 69,000 people ,with a further 18,000 never accounted for.

Such a quake in this case would not just affect those in close proximity, such as the residents of Waipawa, but have far-reaching impacts over the entire Hawke's Bay and beyond.

The council and its investment company, HBRIC, have continually downplayed these risks without providing any reasonable explanation as to why they should be ignored. Again, at the Waipawa meeting, HBRIC chairman Andrew Pearce merely stated this had been addressed during the Board of Inquiry process and there was nothing to worry about - but did the board of Inquiry look into any of this information? In the closing submission for the regional council and its investment company, their counsel Sainsbury Logan & Williams made reference to my representation on these matters, correctly calling me a "lay submitter" and then arguing the Board should, as a "matter of principle", ignore lay submitters in favour of the expert evidence that HBRC/HBRIC had paid for. That they did makes it hard to maintain faith in a system that so easily dismisses the concerns of its citizens.

And while those "experts" said that a Concrete Faced Rockfill Dam (CFRD) was a good option for the site, they used the Zipingpu dam as an example of a CFRD that withstood a catastrophic quake - but failed to mention it is held by many to be the cause of the tragedy it survived.

Now the regional council has changed the design for a cheaper option, unproven under these conditions. Does the Board know?

One expert also communicated to Tonkin and Taylor in regards to site selection that there were no ideal dam sites in the area. International experts on Large Dams have stated if significant movement is accepted as a reasonable possibility during the lifetime of a dam, the best advice is to select an alternate site because "the fact that no dam, foreseen to successfully survive the shearing action of a fault slip in its foundation, has ever been exposed to actual test under such event".

Thus the Board did not, in my opinion, properly question the geologic risks associated with the changing impacts of the reservoir on the fault line, nor did it adequately address the consequences for both the dam and the region in the event of a major earthquake being triggered.

So now we wait. If this does proceed I would dearly like to be wrong, but I wonder if the council and dam supporters who have been so scathing publicly of anyone who questions any aspect of this project are prepared to have red water on their hands.

- Paula Fern is a Waipawa mother and ratepayer, active in environmental issues.

- Views expressed here are the writer's opinion and not the newspaper's. Email: editor@hbtoday.co.nz