Rivers have been given room to flood safely in the Netherlands for two decades and an expert in New Zealand thinks we should be doing the same.
Forest and Bird freshwater advocate Tom Kay is touring the country, giving Making Room for Rivers presentations to communities and local government groups on how accommodating a river prone to flood (rather than hem it in with engineering) can help manage flood risk for communities, and preserve ecosystems.
Redclyffe Bridge, Hawke's Bay, post Cyclone Gabrielle Photo: Chantal Pagel
Listen to the full conversation
We’ve taken space away from our rivers and they will take it back when they need it, he says.
“Rivers aren't fixed things, they're trying to move and adjust all the time to everything that's happening upstream, or the sediment that's coming down, the gravels... the rainfall.
"And they're trying to adjust to that and to think that we are controlling the river is a bit of a myth.”
Building stop banks higher and higher is foolish, and recent evidence shows this, he says.
“You only have to look back not very long through history and see every couple of decades, there's usually a big flood, in different towns, and we kind of forget that those floods have happened.
“We say, build the stock banks higher put the river back where it was. And we do that and then we then we get hurt again. And we usually get hit harder.”
Taradale in Hawke’s Bay was devastated recently by Cyclone Gabrielle, despite years of building stop banks higher and higher, he says.
“That stop bank was still overtopped by the flood. And when we have higher stop banks, those waters behind the stop bank are flowing faster, they're deeper because the stop bank is higher.
“So, then when the water comes over that stop bank, or bursts through it or anything like that, it comes out of that channel with huge amounts of energy.”
That energy was released to the flood plains of Hawke’s Bay with catastrophic consequences, he says. The Dutch have a different approach.
“We talk about flood protection systems in terms of maybe a 1-in 20-year stock bank, or a one-in-100-year stock bank, that idea of a 1 percent chance every year that the stock bank will be over topped.
“In the Netherlands, they talk about one-in-1000 years. They realised we can't just keep building these walls higher and higher.”
The Dutch retreated and gave rivers room to adapt, he says.
“Some studies that were done afterwards showed if you could lower the level of a flood in a river by 50 centimetres, you could reduce the potential, or the probability, of failure of the stop banks by 10 times.
“So just that little bit of extra width to lower those floodwaters means that the likelihood of them bursting is just so much lower.”
It is pointless to keep throwing money trying to control something we simply can’t control, he says.
“Stop banks, it's a bit like when you build a road and more cars come, you build stop banks, more people move in behind it, we develop in behind those stop banks, because we have this perception that they provide this level of protection that will just provide for every flood. And that's just totally untrue.”
Greater Wellington Regional Council appears to have got the message, he says and it is giving the Hutt River back the space it needs.
“The Hutt River has been squeezed through time it's been narrowed, its had space and taken away from it and it's got stop banks lining it and a lot of people will be familiar with that.
“Greater Wellington Regional Council identified this pinch point in the river down at Melling, basically, the narrowest point and if the river overtopped the stop banks there it would do over a billion dollars’ worth of damage. “
There are about 600 houses and five schools in the area, so “crazy amounts of damage would be done”, he says.
The council bought properties along the river and intend to return it to nature, Kay says.
“This project should be starting this year, they've put in the consents to widen the river by 90 metres.They're going to build the stop bank higher on the on the Lower Hutt side as well.
“So, it's sort of a combination of things, but they're also going to try and turn Lower Hutt back to face the river so that people remember that they live on a floodplain you know, and they connect with that river again.”
There is great potential to so this at much less cost with the great braided rivers of the East Coast, he says.
“We haven't developed really intensely, like we have, for instance, in Lower Hutt and Petone. And there's a lot of potential give some space back to the river there. And there's a lot of hope there. And there's huge potential then for fish and macro invertebrates and birds to have those areas back that they need to live, we've got 76 percent of our fish species are threatened.
“We've got so many threatened bird species, and groundwater levels are dropping, in these places they're going to get drier, you know, we could do great things to restore these ecosystems and kind of benefit everyone from an ecological perspective, but also from a flood resilience perspective.”
The cost of not doing this will greatly exceed the cost of planned retreat, he says.
“The cost of insurance payouts from the Ashburton floods was something like $40 to $60 million.
“And then we had Nelson, and then we had Auckland, and then we had Cyclone Gabriel and the cost of that - $9 to 14.5 billion Treasury has estimated - and I don't know if that includes the social cost, the disruption to people's lives, the anxiety.”
The time to act is now, he says, while flooding is front of mind around the country.
“There's never a cheaper time to retreat than straight after a disaster, because otherwise you're just throwing money into something again, that's eventually going to be taken away.
“So, now's the time to transform our thinking and make some room for rivers.”