Gravel supplies hit the rocks
After years of excavation, riverbeds run low on stones. Anan Zaki reports.
The cost of keeping roads potholefree has doubled in some parts of New Zealand as over-excavation of river beds has led to a chronic shortage of gravel.
Rivers don’t just offer a multitude of recreational opportunities, they also supply most of the gravel used to keep our roads in good working order.
But some rivers in New Zealand have had so many tonnes of stones dug out of them they are no longer producing gravel.
In Marlborough, where the cost of gravel has doubled, is trying self-imposed sorts.
Marlborough Roads, the council’s roading arm, must now get resource consents, 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.
‘‘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’’, he said.
Unsealed roads were the number one source of complaints for Marlborough Roads.
Environment the council controls of 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, the process gravel builds up in rivers.
‘‘A river like the Waimakariri, for example, has a very large supply rate of gravel from the Alps,’’ where McCracken said.
‘‘The Ashley River, which is more foothills based, doesn’t have the same supply of sediment and the aggradation rate is much slower.’’
River-based gravel accounted for 20 to 30 per cent of Canterbury’s extracted gravel, with the rest coming from quarries, he said.
In Gisborne, concerns were raised following high gravel demand for regional forestry roads, which prompted the Gisborne District Council to conduct a study to assess the state of its gravel.
A council report in early February proved gravel resources had taken a hit and in some rivers, was fully used up.
Gisborne District Council environmental science manager Lois Easton said more careful river management was needed.
Eric Souchon, managing director of Waikato-based roading contractor H.G. Leach and Co, believed councils needed to focus on balancing the use of quarries rivers for their gravel.
The company focused on quarrying but had extracted gravel from Gisborne rivers in the past.
‘‘It’s not about going out and ripping out every bit of it, it’s got to be planned. Prudent river management and gravel extraction can work hand-in-hand quite easily,’’ he said.
‘‘Absolutely rivers are not an endless source of rock, they regenerate and replenish over hundreds of years. Rivers are not the only answer, it’s part of the answer.’’
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 this purpose, 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 and weathering. When you go to quarries the clay level is higher.’’
Given 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,’’ Earl said.
‘‘At the moment, we have a minimum of five years where we know we don’t have to go to quarries.’’
Earl believed that with ‘‘good management’’ river gravel could be sourced ‘‘forever’’.
It cost the council $600,000 a year to maintain the region’s 630km of unsealed road. This was expected to increase to $850,000 in 2018/19.
It extracted 170,000 cubic metres of gravel in 2016 to help maintain the region’s roads.
The state highways were maintained by the New Zealand Transport Agency.