Flooding at the bottom of Turntable Hill on SH1 near Moerewa, during July's heavy rain. Photo / Peter de Graaf
Flooding at the bottom of Turntable Hill on SH1 near Moerewa, during July's heavy rain. Photo / Peter de Graaf
An $850,000 project to reduce flooding at a notorious highway bottleneck near Moerewa is just about finished.
In heavy rain Ōtiria Stream regularly swamps the bridge at the bottom of Turntable Hill, making State Highway 1 impassable.
If the alternative route via SH11 is also flooded the Far North can be virtually cut off from the rest of New Zealand until floodwaters recede.
However, Northland regional councillor Justin Blaikie, who chairs the Taumarere Flood Management Working Group, said the almost-completed flood mitigation project should significantly reduce highway closures.
Blaikie said the $850,000 project had been 100 per cent funded by Waka Kotahi NZ Transport Agency because it would help keep the bridge open during extreme weather.
"While only small, the roughly 100m-long section of SH1 that forms the approach to this bridge is a critical piece of Northland's roading infrastructure and a vital link to areas north of Moerewa."
A vehicle stuck in floodwaters at the bottom of Turntable Hill, Moerewa, during Cyclone Wilma in 2011. Photo / Michael Cunningham
The work started in earnest in January with almost 1km of the stream's banks benched to create controlled overflow areas for floodwaters.
While the work would not prevent all flooding at the bridge, Blaikie said road closures due to flooding should be fewer and farther between.
Previously, the bridge would flood at least once every two years.
"Obviously, the amount of rain that actually falls in any given year can vary tremendously and no one has any control over that, but in theory, the area should now cope with the sort of rainfall you'd normally expect to occur only once in a decade."
Blaikie said the project was a great example of collaboration between local and central government to solve a problem in a cost-effective way.
He believed it was the first time Waka Kotahi had paid for something that wasn't actual roading infrastructure.
Ngāti Hine kaitiaki Wiremu Keretene, of Moerewa, watched over the project. Photo / Peter de Graaf
Blaikie also paid tribute to Ngāti Hine kaitiaki Wiremu Keretene, who watched over the project, and lead contractor Far North Roading Group, which was challenged by the hard, rocky ground.
That was evidenced by the many tonnes of rock that accumulated at the site as work progressed.
"The rocks came from three lava spurs that pinched the river, like fingers,'' Blaikie said.
"The top layers were relatively soft rock, easy to remove with a digger, but the rock got much harder the deeper into the lava flow, probably due to slow cooling deep in the lava versus relatively quick cooling at the surface."
While some of the rock had been used to armour the approaches to the bridge and line critical stretches of the stream bank, an estimated 3000cu m of surplus rock would be earmarked for other flood mitigation work in future.
The council and the flood working group are now working on options for preventing the kind of flooding that swept through Moerewa in July.
That could involve building a $5 million spillway to divert water from the floodplain upstream from Ōtiria, then return it to Waiharakeke Stream further downstream where the channel is big enough to handle the volume.
