Source: Northland Regional Council
Millions of dollars of improvements to the Awanui flood scheme in recent years proved their worth in the recent storm, allowing the Awanui River to contain vastly more floodwaters than a 1950s flood that inundated Kaitaia township.
Joseph Camuso, the Northland Regional Council’s Rivers and Natural Hazards Manager, says at its peak the Awanui River was flowing at 300 to 320 cubic metres a second during the recent storm.
“That compared to a flow of 220 cubic metres a second in January 1958, an event which saw floodwaters up to a metre deep inundating Kaitaia township.”
Mr Camuso says the improved scheme is performing as designed and had essentially just spared Kaitaia township from millions of dollars’ worth of potential damage, as well as the potential risk to people.
A $12.5M grant from the government as part of its Covid-19 recovery response package – the bulk of it for the Awanui scheme – has slashed the time needed to complete the upgrade and reduced the cost to the community. It is now due for completion in just one more year instead of in 2027 as originally planned.
Mr Camuso says in the most recent works – carried out during a fourth construction season over summer and autumn – had seen about $3.5M of work carried out.
The work included approximately $500,000 removing obstructions, topping up stopbanks and setting stopbanks back in the lower Whangatane Spillway.
Although this area did see overtopping in the recent event, the flows were above of the 1:20 year target design. More work is scheduled in this lower reach this coming season.
Other work carried out this year included a $1.22M contract to increase the flood capacity of the Whangatane Spillway between State Highway 10 and Quarry Road.
“This involved ‘benching’ or shifting or reconfiguring stopbanks to create a wide, flat area on the inside of them which can carry extra floodwaters.”
This year’s work had also focused on maximising the capacity at Quarry Rd Bridge.
Meanwhile, another $500,000 of work was also carried out from Switzer Bench to Bells Produce.
“The goal of that work was to better split flood flows between the Whangatane Spillway – which is designed to carry roughly two-thirds of the water in a big flood – and the Awanui River, which carries the remaining third.
Mr Camuso says in normal, lower flow conditions all the water goes to sea via the river. However, in heavy rain the higher flows enter the spillway – built in the early 1900s – which cuts by 12 kilometres (and roughly halves) the length of the journey the floodwaters must take to reach the sea.
Finally, this year, work was carried out at Bedgood Park and Dunn Street, including work to stabilise an existing stopbank, benching, as well as topping up stopbanks.
The upgrade programme is designed to help future-proof the scheme – including predicted climate change impacts – as well as deliver a considerably higher level of protection for Kaitaia and surrounding areas.
