Northland flooding: Roads to stay closed until mid-August as new slips form
Friday, 24 July 2020
The sound of boulders and trees moving had workers scrambling from the site of a massive slip on State Highway 1 through the Mangamuka Gorge in the Far North.
The one-in-500-year flood on Friday night and Saturday caused eight significant slips and many smaller ones through the gorge, south of Kaitaia.
And the clean-up will take some time as the land continues to move.
A new slip appears to be forming south of the summit and the three biggest slips on the northern side are still moving, Jacqui Hori-Hoult, Waka Kotahi NZ Transport Agency Northland system manager, said.
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Road clearing crews have moved their equipment away from the summit for now after they could hear the sound of trees and boulders slowly moving.
“There’s a huge safety issue for our workers at the moment and we’ll just have to wait rather than compromise on that,” Hori-Hoult said.
In one area, big trees have been left hanging where the ground has fallen away. In another, half the width of the road has slumped in one place, and this has dropped further in the last couple of days.
Hori-Hoult said road workers would put up a drone on Friday to assess the damage and a call would be made early next week on when the road could open to limited traffic.
The main detour is State Highway 10 through Doubtless Bay, with motorists advised to allow an extra 20 to 30 minutes to Kaitaia.
The Mangamuka Gorge is the only highway still closed completely after the flooding, although a slip has impacted State Highway 14 between Whangārei and Dargaville. The road is down to one lane at Tangiteroria and will remain under stop/go control over the weekend.
Meanwhile, 10 local roads in Whangārei remain closed due to slips and flooding.
Whangārei District Council said it will be mid-August until all the roads are open again, and a number of other roads are down to one lane.
One of the worst-affected roads is Matapouri Rd, which provides access to popular beach Sandy Bay.
There are 16 slips between Kaiatea Rd and Sandy Bay. The road is expected to re-open in early August.
While the clean-up continues in Whangārei, the Northland Regional Council is thankful its $11 million detention dam helped reduce the flood waters by up to half a metre in the CBD.
The Hopua te Nihotetea dam, commissioned in 2016, captured 400,000 cubic metres of floodwater from the Raumanaga Stream, which would have swept through the city’s CBD and parts of Morningside and Raumanga.
The dam reduced the flow in the Raumanga Stream by 30 per cent, from a one-in-90-year storm to one-in-27-year storm.
The dam is not designed to completely stop the CBD from flooding as it only holds water from the Raumanga Stream and two more waterways also flow into the downtown area, council deputy chief executive Bruce Howse said.
“But in floods, much of the harm depends not only on how deep floodwaters are but, crucially, how fast they’re travelling.”
Other local authority works in recent years – including a more than 100-metre long flood wall built in Woods Rd last year and 2014 improvements to free up flows beneath a Rust Ave bridge – had also meant flooding damage had been greatly reduced or, in some cases, avoided altogether.
Meanwhile, fixing flood damage at popular garden attraction Whangārei Quarry Gardens is expected to cost hundreds of thousands of dollars.
The gardens’ usually small waterfall went from 1-2m wide to 30m wide at the height of the rainfall and major slips and flooding have damaged most tracks, manager Guy Hessell said.
Fixing the slips is expected to cost $100,000 to $200,000, with the charitable trust not covered by the Earthquake Commission, he said.
It has a good volunteer base, with a team of about 30 getting stuck in this week to clear debris and fallen trees.
But the costs of engineering work and the replacement of specimen trees will have to be met with donations and possible council support, Hessell said.
On Wednesday, the Government announced $14.2 million funding from the Provincial Growth Fund for Far North roading projects and $12.5 million to help combat future flooding events in Northland.