Wairoa flooding

To understand why the Wairoa District Council (WDC) commissioned its own Cyclone Gabrielle report, we have to go back a year or so.

In a letter written to HB Regional Council (HBRC) chair Hinewai Ormsby in March, 2023, Wairoa Mayor Craig Little labelled HBRC’s levels of response and assistance in the immediate aftermath of Cyclone Gabrielle as “pathetic’’, adding, “Our region will become uninsurable if there is no firm signal from HBRC that it is changing its focus to planning and infrastructure that can handle these events and mitigation.”

He put this question: “The outcome of the cyclone on our district was extreme flooding along local waterways, particularly the Wairoa River and through the North Clyde township. I would like to know what caused this extreme situation. A huge focus is needed on future flood control in the Wairoa township.”

So that’s the backdrop for the Wairoa Cyclone Gabrielle Review, undertaken for WDC by Andrew Newman of Strome Advisory Limited. A former chief executive of HBRC, Newman says he sought to produce a readable, common sense document that sorted fact from fiction.

Newman believes that, in the absence of strong communication from the likes of HBRC, conjecture filled the void, leading to widespread beliefs that the flooding in Wairoa was a result of issues such as forestry slash and flood flow releases from the Waikaremoana Power Scheme . In both instances, Newman’s review – backed by further independent analysis and peer review – found neither was a significant factor.

As for the “mitigation’’ Little mentioned in his letter to Ormsby, Newman says the issues are complicated and there’s no “silver bullet’’ to fix them.

He says the North Clyde area, for instance, has flooded several times over the last hundred years and there’s every reason to assume it will do again. “So, taking the North Clyde area, which is the area that flooded in Gabrielle, the propositions that are on the table, from the work that the regional council, the community and WPS are doing are the sort of classic types of hard flood infrastructure,’’ Newman told BayBuzz.

“A lot of it’s confidential, but I can talk about it generally. It could be stopbanks, it could be flood channels etcetera. So, some hard infrastructure of that type will help, but it won’t remove the risk entirely.

“If I was sitting there in North Clyde and saying ‘Well, we want to stay here in North Clyde’, I’d also be saying from the individual homeowner’s point of view, that potentially lifting the houses would be something you’d look at seriously.

“That’s not uncommon in places outside of New Zealand, in fact it’s extremely common.

“Queensland’s a classic, but we don’t typically do it here, so we’ve got to get over our institutional, cultural hang up about adapting. “A lot of those houses in North Clyde are on piles already so lifting seems, in principle, a sensible thing to do.’’

Wairoa is a complex district Newman says, with a tidal river and rugged, often unstable, hill country with a high concentration of forestry. Add in a very wet La Nina weather pattern prior to Cyclone Gabrielle, which saturated the soils and caused waterways to run high, and you had the recipe for the disaster that occurred.

Because there is historical precedent for flooding in the district, Newman says very careful consideration needs to be given to mitigation and perhaps rethinking the previously accepted wisdom.

Newman quotes figures indicating intact poplars and willows, which were deemed good flood protection alongside rivers, contributed 38% of the woody debris that filled waterways and impacted infrastructure.

“In a major storm event, with those super saturated soil conditions, those trees lined on the riverbanks become part of the hazard, actually,’’ said Newman. “They’re not a flood protection tool, they’re an additional hazard. In fact, in terms of risk to infrastructure such as bridges, assets, roofs, highways, powerlines, you name it, they are, in my mind, arguably the biggest risk. Especially the poplars.

“You’ve got to take your blinkers off and look at it for what it is.’’

Pine is the other big contributor, in more ways than one.

For starters, radiata was planted to stabilise hillsides following Cyclone Bola, in 1988. Newman says the evidence of Cyclone Gabrielle – and also Cyclone Hale which preceded it by a month – showed the pine wasn’t able to stop landslides. Newman’s report indicates that wind from Cyclone Hale actually toppled stands of pine that were then swept away in Gabrielle.

“What I’m saying in this report is that if we bring it back to the central issue of flood risk, my view is that – if that’s your objective – don’t put radiata forests in areas that can collapse on you. Like seriously collapse on you,’’ Newman said. “On the evidence of trying that [post-Bola] we’re now seeing the consequences. It’s a learning and everyone had the best intentions.’’

The remedy is harder to diagnose, says Newman.

“My view is that radiata pine is actually not a long run viable land management solution,’’ he said. “Longer growing species might work in principle potentially. But it’s a bit of a grey, it’s not well researched. It is a conundrum.’’

Establishing native forests, for instance, will be time consuming, expensive and potentially not even work, Newman says, because of weed and goat issues, plus the highly-erodible nature of the soil.

In areas where pine has been planted for commercial harvest, Newman says better management practices are required. “The problem with radiata pine is that the older it gets, particularly if it’s not well tended and well managed, it starts collapsing,’’ said Newman.

“So radiata pine is a species you have to manage and, within reason, you have to extract. You can’t leave it there forever. If you’re taking a really long term view of it, you have to extract it. But I’ve seen parts of Wairoa hill country, in extreme erodible zones, where harvesting will be really, really difficult.’’

Then there’s the issue of the Wairoa River and what can be done to lessen its threat.

Stopbanks could be a solution, Newman says, but they can’t stand alone. They have to be part of a system of flood-mitigation initiatives.

One thing that’s not a viable component of that system is dredging, given the tidal nature of the river. “In my view – and this would be the view of various flood engineers that I involved in this piece of work – dropping the riverbed level though dredging would be expensive, complicated and unlikely to work over any durable period of time,’’ Newman said.

It remains to be seen what sort of flood protection scheme is implemented.

In the meantime, Newman says there are things HBRC can do to rebuild trust within Wairoa.

In his letter of March, 2023, Little expressed dissatisfaction with the communication from HBRC, which he reiterated in the Government Inquiry into the Response to the North Island Severe Weather Events. Newman’s report did not address the immediate cyclone response, but he does have some views on how things have been handled since.

“To relate to the community’s concerns, you actually have to listen to how they articulate an issue and address it explicitly. I think therein lies some of the gap, actually,’’ said Newman.

“I really tried to write this report in a way that was helpful. That if people within various organisations sat back and said this is the issue, is it well described? Is it credible?

“The relationship issues are not that hard to address, there’s some pretty obvious answers. “Less of a centralised view of the world and let’s trust competent people in these areas to help us with our jobs, whether they’re employed by us or not.’’

Public Interest Journalism funded by NZ On Air

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