Striking the river, road balance
Rivers, what are they good for? Well, they used to be good for making gravel. Now, not so much.
Some rivers in New Zealand have had so much gravel dug out of them over the years they can no longer produce gravel.
Simple supply and demand means, in those regions, the cost of gravel is soaring, and with that, so too is the cost of keeping roads pothole-free.
In Marlborough, where the cost of gravel has doubled, the council is trying self-imposed controls of sorts.
Marlborough Roads, the council’s roading arm, must now get resource consents to extract gravel, where before they could take freely from the region’s rivers. They also used to take from parts of the Marlborough Sounds, but those sources had all but dried up - gravely speaking.
‘‘There are places like the [Marlborough] Sounds, where the gravel resources are very scarce, especially in the Kenepuru [Sound], where there really isn’t any gravel left anymore,’’ Marlborough Roads journey manager Steve Murrin said.
Shifting gravel from Blenheim to places such as the Kenepuru Sound, with its many unsealed roads, made costs ‘‘much higher’’, Murrin said.
Unsealed roads were the number one source of complaints for Marlborough Roads.
Environment Canterbury regional lead for river engineering Shaun Mccracken said the regional council had stopped riverbased gravel extraction in some rivers due to a slow down of river aggradation.
‘‘It’s the process of erosion in the higher parts of the catchment creating sediment that moves downstream, predominantly during floods and is deposited on the river bed,’’ Mccracken said.
‘‘A similar example to the lower-wairau River in Canterbury would be in the Ashley river, we’re not allowing any extraction near Rangiora,’’ Mccracken said.
Back in Marlborough, Gill Construction’s construction manager Roger Earl said river gravel was the best material for road metalling.
Marlborough did not have quarries with rock hard enough for road metalling, he said.
‘‘Everything we make here [at Gill Construction] is for roading, we need to pass a specification in gravel for roads.
‘‘It’s about hardness, cleanliness and weathering. When you go to quarries the clay level is higher,’’ Earl said.
With depleting resources the company had to move further up the river valleys, which meant gravel cartage was more expensive.
‘‘Gill’s will never change to a quarry source for as long as possible, we’re just going further and further up the river. At the moment, we have a minimum of five years where we know we don’t have to go to quarries,’’ Earl said.
Earl believed with ‘‘good management’’ river gravel could be sourced ‘‘forever’’.
It cost the council $600,000 a year to maintain the region’s 630 kilometres of unsealed road. This was expected to increase to $850,000 in 2018/19.
One of the roads that required metalling every year in Marlborough was Tumbledown Bay Rd, in Port Underwood, due to forestry operations in the area.
Resident Marilyn Carter said an ‘‘enormous hole’’ as wide as the road itself had opened up in the last month.
‘‘They do chuck a bit of shingle on the road for us every now and then, it’s been worse than what it is now.’’
The council extracted 170,000 cubic metres of gravel in 2016 to help maintain the region’s road.