OPINION: The recent floods in Edgecumbe have certainly been an unexpected disaster and portions of the town have been flooded that have not been flooded before – Edgecumbe is a flood prone area so floods are not exactly unexpected It is good to hear the pumps are now in place and the flood waters are receding as the pumps do their job.
Surprise! Surprise! An Inquiry had also been announced which will be lead by Sir Michael Cullen. Inquiries seem to be the standard response to any kind of disaster in New Zealand. However, the best of luck to them and like everyone I hope they come up with high-quality suggestions for the future.
The first thing to establish is that this was a 500-year event, and I think the standard for planning is generally a 1 in a 100-year event. So, the whole infrastructure was stressed beyond anything it was designed for. It is not surprising that it did not completely hold up.
One of the obvious reactions to this is this maybe we should be designing at a higher level – which is a bit the way we have gone with earthquakes. Maybe be the standard should be 1 in 200 or even higher.
The immediate problem with that is cost. The higher the standard the higher the cost and that has always been the uncomfortable trade-off for local bodies, engineers and so on. Just how much are we prepared to pay for things that only happen occasionally? And is building more and more expensive infrastructure necessarily the best solution. Maybe we should build where the risks are inherently lower?
The frequency of extreme events
The other issue is more difficult and that is to put recent events in Edgecumbe in perspective. This is the second severe flood the town has had in only a few years and the recent flooding up north – in North Auckland and in the Coromandel is still fresh in people’s minds. There is a feeling abroad that these severe events are becoming more frequent and more extreme and that means we should change our approach. Climate change generally gets the blame for this but I think we have to carefully consider the impact of changes humans have made to the landscape through the conversion of bush and so on.
I have not done the research so 
So, I think one of the things I would look to the Inquiry to do is to look closely at the history and I guess be upfront about the nature of the history and any trends it shows. Looking at the history means going back decades – maybe to when records began – not just going back a few years.
Collapse of the stop bank
In the case of Edgecumbe two things seems to have happened. First, the stop bank partly collapsed and that allowed the river to flow through the town. I think the question of why is fair cop for Dr. Cullen and the inquiry team. A stop bank can overflow if the design limits are exceeded, and that may well have happened if the stop bank had stayed intact. But breaching of the stop bank is another matter, and I suspect that where one finger will be pointed at is the question of whether it was built strongly enough, ie to design requirements, and/or was the design flawed?
The Matahina dam – planning (?) for the unexpected
The other interesting element in the Edgecumbe story is the Matahina dam. The dam was built to essentially “stockpile” water for a variety of uses including irrigation and it was well recognised at the time that it would also be a powerful weapon in flood control – basically the flow of water down the river could be regulated to some extent taking advantage of the capacity if the dam lake to store water. I don’t know the details of the management plan that was agreed but no doubt it involved compromises of various sorts so that all potential users were satisfied if not entirely happy.
In practice – and bearing in mind this was an event of unexpected severity – the level of water in the dam was not lowered in advance far enough and there was thus limited ability to moderate the flow of water down the river. Nevertheless, my guess would be that the agreed management plan was followed and this will be the first line of defense for anyone taken to task.
However, the interesting aspect of this is what could have actually been done under the circumstances, irrespective of the management plan, and how those in authority may have or could have reacted. This is all about taking action in the face of unexpected events. Ideally, there should have been a great deal of contingency planning and this might we have included questions like “what happens if you get 1 in a 500-year event, or even more?” The likelihood of this is irrelevant to the question – the key question is always “however unlikely what would you do?”
There may well have been contingency planning to that level and the Cullen inquiry will no daunt find out. But there may not have been. If there is no contingency planning then the people close to the event are genuinely dealing with the unexpected and the guiding rule under these circumstances is to think outside the square and if thought justified throwing the rule book in the bin. In this particular case who knows what would have happened if a decision had been taken to empty the dam lake early and provide more capacity to absorb the effect of increased river flow.
By Bas Walker
This is another of Bas Walker’s posts on GrownUps. Please look out for his articles, containing his Beachside Ponderings.