Work begins on $15.5m project to protect Kaitaia from floods

Regional council engineer Sher Khan, left, and manager Joseph Camuso with the remains of the old concrete works buildings cleared at Kaitaia as part of a $15 million upgrade of the Awanui flood scheme. The rubble will be used as part of the bed of a new emergency spillway. Photo / NRC

Work is about to begin on a $15.5 million flood prevention scheme which aims to safeguard Kaitaia from Northland's riskiest river.

The Awanui River, which rises in the ranges south of Kaitaia before flowing through the town and emptying into Rangaunu Harbour, is often cited as Northland's biggest flood risk because of Kaitaia's low-lying topography. It also passes a massive, slow-moving slip on Bell's Hill which, if it were to collapse, would block the river and send the water coursing through the town.

Northland Regional Council's 2018-28 Long Term Plan includes a $15.5m programme of repairs and upgrades to the existing Awanui flood scheme.

Once completed the scheme will protect urban Kaitaia from so-called ''once in a century'' type floods and surrounding areas from ''one in 20-year floods''.

Earlier this month the NRC awarded two contracts worth almost $500,000 - marking the start of the eight-year project.

Te Hiku constituency representative Mike Finlayson said both contracts had been awarded to Kaitaia Contracting, with work due to begin in earnest in the New Year and expected to run until the end of March.

The first $213,000 contract was for repairs to a 500m stretch of undermined stopbank behind Te Ahu and the second, for $280,000, was for construction of a new emergency spillway opposite the Bell's Hill slip.

Finlayson says the NRC had been working through the highest priority repairs needed for the decades-old scheme.

"The council has been monitoring and managing the Bell's Hill slip for many years, concerned it could potentially slip into, and block, the Awanui River."

The emergency spillway will be built on the recently cleared 14,600sq m former Firth concrete plant site bought by the NRC in June.

"The spillway will probably only carry floodwaters once or twice a year, but will be big enough to carry the entire flow from the Awanui River should the Bell's Hill slip collapse unexpectedly and block the river's existing flow path."

A potential bonus for local residents would be that during dry periods the spillway and grassed area around it could be used as recreational area.

Finlayson said a 2.4km stretch of Awanui River downstream of Bell's Hill had also been surveyed by drone over winter. The data would be used in detailed design of the wider upgrade.

The scale and cost of the full upgrade meant the work, much of which was weather-dependent, would be carried out in stages until 2027.

"Future flood risks will be mitigated largely through extensive modifications and improvements to stabilise existing stopbanks, which will allow the river to carry up to 15 per cent more floodwaters. Without the added protection from this upgrade, a 1:100-year flood in urban Kaitaia could cause tens of millions in damage and put lives at risk."

Seventy per cent of the work would be funded by ratepayers Northland-wide via a new regional flood infrastructure rate. Just under a third would be funded locally via a roughly 60 per cent increase to the existing targeted Awanui River Management Rate.

"This 70/30 spilt means that for roughly $7 extra annually per ratepayer across the region, schemes like Awanui become much more affordable at a local level for those communities protected by existing – and often ageing – flood infrastructure work," Finlayson said.