What Hawke's Bay's $265m flood protection projects are - and aren’t - designed to do

Eleven flood resilience projects will give Hawke’s Bay stronger protection than it had during Cyclone Gabrielle.

But despite the $265 million cost, none of them is designed to completely hold back the kind of floodwaters that poured down from the hills on February 14, 2023.

Hawke’s Bay regional councillor Neil Kirton told Hawke’s Bay Today that the programme was one of the region’s largest infrastructure rebuilds in a generation.

It’s being funded through a combination of about $214m from the Government and $50m from Hawke’s Bay Regional Council (HBRC) ratepayers.

“The brief from Government was only to rebuild flood protection infrastructure back to what was lost, with the addition of a $70m project for Wairoa and a $28m project for Whirinaki, protecting Pan Pac,” Kirton said.

The programme aims to restore protection across vulnerable Category 2C areas to the region’s one-in-100-year standard by June 2027.

Pōrangahau is the only project area where flooding on the scale of Cyclone Gabrielle falls within that level of protection, because the flood that hit it in 2023 was a one-in-100-year flood.

“The reality is that every weather event of this intensity will produce different outcomes, given the random nature of storm location and local conditions,” Kirton said.

“We are most unlikely to experience an event producing the same impacts as those occurring in Cyclone Gabrielle. This makes investment in protection a very difficult task.”

Affordability was a key factor.

“Building back the flood protection lost and replacing infrastructure such as bridges and roads has been a monumental task pushing Hastings District Council to the limit of its borrowing.

“Ratepayers across the region cannot afford gold-plated one-in-250-year flood protection. It’s just the reality of our region’s economy.”

Kirton claimed Napier remained the region’s greatest flood risk. He said parts of the city’s flood protection capacity were below a one-in-10-year standard.

He has been a vocal critic of plans for further housing development in lower-lying areas such as Riverbend Rd.

The first of the regional council’s 11 flood resilience projects, the nearly $9m Waiohiki stopbank, was completed last month.

The one-kilometre stopbank runs along the Tūtaekurī River from Napier Golf Course to Redclyffe Bridge and has been designed to the region’s standard flood resilience level of a 1% annual exceedance probability event (essentially a one-in-100-year flood).

Post-cyclone modelling estimated flooding in the Tūtaekurī River during the 2023 disaster exceeded both 100-year and 500-year flood event levels in many areas of Hawke’s Bay.

“Gabrielle flood levels on the upstream side of Redclyffe Bridge were exacerbated by blockages at the bridge,” HBRC said.

The Government funding programme supporting the projects was intended to deliver resilience to the regional one-in-100-year standard, rather than specifically rebuild to Gabrielle-scale protection levels.

“Gabrielle was a massive event. In most areas, it was the largest event on record,” HBRC said.

“The amount of money, time and land needed to give up to 0.2% AEP resilience [one-in-200-year flood protection] would be immense.”

It said that, in extreme events, some areas such as Waiohiki could still flood, although modelling suggested flood depths would likely be reduced compared with 2023.

“In the most frequent and recurring flood events, the stopbank will hold back floodwaters. In larger events, it will provide time for them to evacuate safely.”

HBRC also said the Waiohiki stopbank had been built wider than usual to allow for potential height upgrades if needed.

Updated flood hazard modelling for the Esk Valley, released recently by HBRC, warned that stopbanks could not eliminate flood risk.

“Stopbanks significantly reduce flood risk, but can never completely prevent it,” regional council chairwoman Sophie Siers said.

“In very large events, particularly when climate change is taken into account, floodwaters can still go over the top of stopbanks.

“It’s important our communities understand both the benefits and the limits of flood mitigation.”

The 10 projects still being worked on include flood resilience works at Ōhiti Rd in Ōmāhu, Whirinaki, Pākōwhai, Pōrangahau and Wairoa, as well as stopbank upgrades at Waipawa, Ōmāhu Lower and Brookfields Lower.

Additional projects include upgrades to drainage pump stations and telemetry systems across Hawke’s Bay, and a Havelock North 2C flood resilience project, to be delivered by Hastings District Council.

Alongside the construction programme, the council is carrying out its Reimagining Flood Resilience Project, which is reviewing existing flood schemes and consulting communities about what level of resilience the region should aim for in the future.