$5.73m Far North flood protection work passes storm test with flying colours

When opened in August it was hoped the Ngapipito Culvert would help divert water overflowing from Otiria Stream and flooding surrounding land in the Far North’s Matangirau catchment. It passed its first test - Ex-Tropical Cyclone Tam - with flying colours.

Eight months after it opened, a Far North flood prevention scheme has proved its worth, with once flood-prone homes staying high and dry from recent storms.

A small, flood-prone Northland community has withstood a 10-year rainfall event, thanks to the new flood protection works led by Northland Regional Council. Around 300mm of rain fell on the Far North’s Matangirau catchment during Ex-Tropical Cyclone Tam, the most rain recorded in the area in a decade and almost twice the rainfall recorded across Northland.

Flood protection measures were installed last year at Matangirau as part of the $5.735 million Flood-Resilient Māori Communities and Marae project, and opened in August.

The project, funded by the Local Government Flood Resilience Co-Investment Fund and NRC, aimed to reduce flood risks for six flood-affected Māori communities - Kawakawa, Otiria-Moerewa, Kāeo, Matangirau, Whirinaki and Punuruku - and 35 marae across Te Tai Tokerau.

Matangirau resident Robert Rush said before the flood works, his home was always the first to flood when there was heavy rainfall. Their local marae would also always go under water.

Yet after the flood mitigation works, Rush said, the results had been fantastic.

“It’s been a work in progress, especially showing our whānau that the council were only there to help and not to steal our land.

“We’ve had stopbanks and river works done around our homestead and it hasn’t flooded since. We also had some work done just a couple of weeks before ex-Cyclone Tam, which was perfect timing because we didn’t flood during that time either, nor did my grandfather’s house which is near the new marae.”

NRC Te Ruarangi (Māori and council working party) Whangaroa hapū representative and Matangirau haukainga Nyze Manuel agreed that the benefits of the flood works were obvious. She said the mahi of Te Ruarangi had also played a critical role in the activation and front line of Māori communities during these times.

“Well, we’re not under water, so that’s awesome.

“Through our Te Ruarangi network, we were able to get out communications to people about the weather in a fast and efficient way. And as more flood works are done by NRC, we’ll see less flood water in these vulnerable areas.”

Matangirau’s flood mitigation is based on an engineering method called “floodway benching” designed to reduce flood risk for homes and the marae upstream of the Wainui Road Bridge.

A 1960s rebuild of the bridge (which raised the bridge and approaches by about 2 metres above the existing flood plain) unintentionally worsened flooding by creating a “detention dam” effect during heavy rainfall, capturing and holding excess water during heavy rainfall events.

As a child, Rush said he didn’t recall any flooding until the local road and bridge works were completed.

“We’ve had a whole lot of issues and have moaned about that for years, that’s why we built our whare where it is now because it never used to flood there.

“That’s why it was essential to get the flood works done as we’ve been flooded 3-4 times now and are no longer able to insure our house.”

The new benching works aim to reverse this damage by giving floodwaters more space to spread out, allowing more water to flow under the bridge.

This proven approach, used successfully in Awanui, maintains the river channel while adding a higher, wider “bench” for safer floodwater flow.

NRC Rivers manager Joe Camuso said the recent weather event had proven the value of investing early in communities like Matangirau.

While it wasn’t a “miracle” cure for flooding, Camuso said it had made a significant improvement on the impact of heavy rainfall to the area.

“What we’re seeing now is the flow regime is much more efficient, so we’re seeing more water flowing under the bridge, which means less flooding during large rainfall events.

“While this is great, it is only built to withstand up to a 50-year flood event, of which there is only a 2% likelihood each year.”