Making room for rivers: The flooding conversation Hawke’s Bay needs to have

Hawke’s Bay rivers run a course much more constrained from how they were before settlements grew around them. Now that severe flood events are becoming more frequent, environmentalists say the solution is to pull back and make more room for rivers to flow and flood again. But there’s a cost. James Pocock reports.

It would cost hundreds of millions, maybe billions if it was taken to its fullest extent. And it could tear apart some riverside communities.

But with Cyclone Gabrielle making it clear just what is ahead for Hawke’s Bay as climate change continues, the widening of the region’s stopbanks, and the returning of land to the rivers, is back on the table.

Aerial images from just after the end of World War II show that only 80 or 90 years ago the Tūtaekuri and Ngaruroro rivers in particular used to flow across a wider braided path, particularly once they hit the Heretaunga Plains.

The cost of purchasing all the land since taken for farms, properties, homes, could well be huge and some won’t be willing to leave.

Going back to pre-human habitation is all but impossible, with Napier and Hastings both on the original floodplains for these rivers.

But environmentalists say solutions which restore something of a natural flow and floodplain may be the best way to mitigate the increasing risk of floods and billions of dollars of damage to infrastructure for future generations.

Tom Kay, Forest & Bird’s freshwater advocate, has long supported the idea of “making room for rivers”, widening their channels to bring them back to how they naturally flowed, and he says it is more relevant than ever for Hawke’s Bay since Cyclone Gabrielle.

Kay argues we have historically engineered our rivers using diggers and bulldozers to straighten and narrow their channels and line them with stopbanks or groynes to protect communities from flooding.

He said that while engineering may have worked for smaller floods, it could increase the risk during more serious events.

“Traditional hard-engineered solutions provide a false sense of security. It’s given us the idea that it’s safe to build homes and communities right up to the edges of rivers,” Kay said.

He said at a meeting organised by Forest & Bird on Thursday that a study had been done of two Dutch rivers, the Rhine and the Meuse after their floodplains were widened.

It found that widening was able to reduce river levels by 30cm during floods and reduced the probability of dyke failure by two to five times.

The study found that if river levels could be lowered by 50cm during floods, it cut the risk of dyke failure by more than 10 times.

Kay pointed to examples in New Zealand where local government bodies were enacting the concept, including the $700 million Riverlink project on the Hutt River which required 141 properties to be acquired.

The flood protection aspect of Riverlink involves widening the river channel to 90 metres to protect from up to a one-in-440-year flood event.

Without the widening, that event could cause an estimated $1.1 billion worth of damage to the community, according to the Hutt City Council.

Hawke’s Bay Regional councillor Sophie Siers, Cyclone Recovery Committee chairwoman, said making more room for rivers was not a new concept.

She said central government was still exploring the options of managed retreat and red zoning and would be responsible for purchasing land if they decided to go ahead with it.

She said councillors and staff were considering more than just widening room for rivers, and they were undertaking a “detailed and intelligent analysis” of the options.

“Whatever we decide, it has to be intergenerational, a solution that brings safety and security for our children and children’s children,” Siers said.

Dr Graeme Smart, principal scientist for hydrodynamics and natural hazards for the National Institute of Water and Atmospheric Research, said while visiting Esk Valley on Thursday that rivers had created much of New Zealand’s landscape that was now built upon.

“The natural solution is to let the river do its thing - the river built many parts of New Zealand,” Smart said.

“Most flat areas of New Zealand are old riverbeds or lake deposits, and it is just cheaper to build on the flat areas. That’s where we are doing it. But that brings in the hazard of flooding.”

Smart said flooding got worse when stopbanks failed due to the design scheme being exceeded.

”Because people have had a false sense of security thinking they are protected from the flooding until a really big one happens and there is a lot more infrastructure [damage] and a lot more losses,” he said.

“But you can design a scheme if you put enough money into it to contain the river.”