Work to start on new Awanui River spillway

River management engineer Barney Brotherhood surveys eroding stopbanks behind Juken New Zealand’s Kaitaia triboard mill, where the regional council is planning a new spillway. Photo / NRC

An estimated 90,000 tonnes of soil will be removed in coming months to transform a paddock into a 400-metre-long flood scheme spillway behind Juken New Zealand's Kaitaia triboard mill.

Pending approval of resource consents and archaeological authority, the work is expected to be carried out between February and May as part of the second stage of the Northland Regional Council's multimillion-dollar upgrade of the Awanui flood scheme.

Forestry and wood processing company Juken is allowing its land to be used for the upgrade which will also give better flood protection to its Whangatane Dr triboard mill, one of the district's largest employers.

The regional council's Te Hiku representative Colin ''Toss'' Kitchen said the existing spillway was literally a stone's throw from the mill.

The new spillway would be roughly 20m wide and 400m long and will be built 100m to the southeast on what is currently a paddock.

Roughly 90,000 tonnes of soil, or about 50,000cu m, would be removed.

"This work will account for more than a third of the total $1.02 million expected to be spent upgrading the scheme during the current construction season."

While the new spillway would be about 2km downstream of Kaitaia itself, it would form an integral part of the upgraded scheme by helping to carry potentially damaging floodwaters away from the town more efficiently and quickly.

The existing spillway stopbanks behind Juken New Zealand's Kaitaia triboard mill. Photo / NRC

Kitchen said much of the aging scheme was built about a century ago so the regional council's focus until recently had been on maintenance and immediate repairs.

"However, this upgrade will help future-proof the scheme for a number of years – including predicted effects from climate change – as well as deliver a considerably higher level of protection for Kaitaia and surrounding areas."

All going well, the council expects the latest phase of construction to take 8-10 weeks.

The new spillway will be given a few months to stabilise before the spillway is diverted through it next year. The current spillway behind the triboard mill will be retired as a wetland.

Improving existing stopbanks and building new ones will allow the Awanui River, including critical sections upstream of the triboard mill, to carry up to 15 per cent more floodwaters.

The scheme is designed to protect urban Kaitaia in a ''once in a century'' type flood and a 1:20 year event in surrounding rural areas. It is a key project in the council's Long Term Plan 2018-2028.

Seventy per cent of the work is funded by ratepayers Northland-wide via a regional flood infrastructure rate, with the balance funded locally through a targeted Awanui River Management Rate.

The JNL spillway will be the second new one built in two years as part of the upgrade. An emergency spillway opposite the slow-moving Bell's Hill slip site was built last summer on the site of the former Firth concrete works in Kaitaia.

The regional council has been monitoring and managing the Bell's Hill slip site for many years in case it slips into and blocks the Awanui River.

"While that spillway is expected to probably only carry floodwaters once or twice a year, crucially – should the Bell's Hill slip ever collapse unexpectedly and block the Awanui River's existing flow path – it's big enough to carry the river's entire flow," councillor Kitchen said.