Wairoa flooding: Did sediment build-up in river contribute to floods?

Hawke’s Bay Regional Council says sediment build-up along the Wairoa River bed has barely changed in three decades, despite residents’ concerns it may have contributed to the recent floods.

It comes as the council said it was urgently reviewing the cause of last Wednesday’s flood, and the Government announced an urgent review of Wairoa’s flood response.

About 118 homes were flooded in the lower part of the town in northern Hawke’s Bay when the Wairoa River overflowed and floodwaters breached those homes. Hundreds of other properties were also affected.

Long-term residents say they have never experienced flooding to that extent in that part of town before, and many believe Hawke’s Bay Regional Council (HBRC) should have started work earlier to open the bar at the Wairoa River mouth that can take two days to open with machinery.

HBRC has defended its decision not to begin work opening the bar until the day before the floods.

Council said there would “likely be insufficient flow in the river to keep a new opening in place” if work had started earlier.

Long-term Wairoa resident Chris Wilson said he believed there was another issue with the river that may have contributed to the flooding.

“Sedimentation of the riverbed is the elephant in the room.”

He said from his observations, there appeared to be build-up of sedimentation along the riverbed over the decades, and he grew up swimming and yachting in that river.

“I first noticed the rapid build-up over 20 years ago, compared to the first half of my life, when the river seemed to be in relative equilibrium.”

He said it was once a navigable river for shipping, but appeared to have changed significantly.

“The questions I’d like put to the scientists at HBRC [include] does their data confirm this accelerated siltation?”

Wairoa has been cleaning up since the Wairoa River overflowed into the lower part of town. Photo / Paul Taylor

After Cyclone Gabrielle in February last year, he asked HBRC to supply that data on the depth and topography of the river.

HBRC gave him plenty of data, however, he said it was raw data and hard to make sense of if you weren’t a scientist, and he would love to see it made accessible to the public with an explanation.

Other residents have also echoed Wilson’s concerns that the river may have become shallower and more prone to flooding.

However, HBRC asset management group manager Chris Dolley said that was not the case, according to the council’s surveys, which date back to 1988.

“Data shows that the bed has remained at a stable level since Cyclone Bola [in 1988]. There was also little change post-Gabrielle [in February 2023], except for around Town Bridge,” he said.

“The bank-full width of the river has remained stable for a number of decades and the bed levels of the river have had no evidence of change.

“The beds of rivers of this nature generally scour down during a big flood, then move back to an equilibrium in the years between large events.”

Wairoa River flooded the lower part of the township last Wednesday. Photo / RNZ

The regional council began undertaking cross-sectional surveys of the river in 1988 — on a six-yearly cycle — and some historic surveys by other boards date back to 1949.

Dolley said there had been a “slight increase” to the riverbed post-Cyclone Gabrielle, upstream and downstream of Town Bridge at State Highway 2.

“[This] may be due to the presence of sheet piling resting on the bed of the river from the sheet pile wall that was destroyed during Cyclone Gabrielle.”

Hawke’s Bay Regional Council does not dredge the Wairoa River, but is now reassessing whether it should start that work.

“Dredging the river has been considered, however, there is little evidence that this would be effective,” Dolley said.

Before the latest flood, the council had also been assessing how various river mouth configurations could help prevent floods.

“The river mouth is dynamic. It regularly moves to the west over time depending on a number of conditions, mostly associated with river flow and swell direction,” Dolley said.

“Various positions of the mouth could be less advantageous for river discharge, but provide more protection against storm surge in the river.”

Wairoa resident Robert Campbell said he believed the river had changed over time and the erosion from the land went into the river.

”The shallowness is noticeable now and the bar did not move about so much in the 70s and 80s,” he said.

“There was more whitebait and kahawai.

“Everyone used to fish, almost all year ... the river was part of our staple, if you had too much, you just drove around and gave it to family, friends, strangers. We all did it, Pākehā and Māori.

“Things have changed so much.”

Gary Hamilton-Irvine is a Hawke’s Bay-based reporter who covers a range of news topics including business, councils, breaking news and cyclone recovery. He formerly worked at News Corp Australia.