Farmers and council at odds over management of Tasman's flood-hit Upper Motueka River

A group of Tasman farmers and the district council are working to find a balance between managing the mighty Motueka River with the environmental impacts of gravel extraction.

The district's farmers and growers were dealing with the trail of destruction left by the flooded upper Motueka and Wangapeka Rivers, following two serious flooding events within two weeks at the top of the South Island since late June.

A private stopbank at the Tapawera bridge burst during the one-in-100-year floods in late June, dragging swathes of woody debris and river rock through nearby properties.

This included at Dion McGaveston's 200 hectare sheep and beef farm, which he told RNZ looked "more like a riverbed" after the event.

McGaveston and around 30 local farmers had been dealing with the Tasman District Council for years on ways to mitigate bank erosion and flood protection in the area.

Getting on top of gravel build-up

Hop farmer Dean Palmer, of the Upper Motueka River management committee, said locals wanted high amounts of gravel removal, ideally around 200 cubic metres each year, then a shift to more sustainable levels.

"Our main issue is the way it's being managed," Palmer said.

"We've met several times, going back 10 years, and all the land owners will say, 'it's all the gravel and the river needs to be extracted to give it flood flow capacity'.

"When a flood comes, there's no capacity to carry water down the river and it all carries in our margins and therefore scours big pieces, takes farm land, wrecks fences."

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A 2022 council report showed 649,000 cubic metres of gravel had built up since 1988 - most of it within the five years to 2022, said Palmer.

"It's 100,000 [cubic metres] of additional gravel in there every year, and we've only been taken out [around] 20,000 per annum," he said.

He said extraction volumes at the site were 8200 cubic metres in 2021, 12,200 in 2022, 37,300 in 2023 and 36,000 permitted in 2024 - but only 24,000 cubic metres were extracted.

"Those numbers are a mere fraction of the 650,000 build-up that was there in 2022," Palmer said.

He said that caused some of the problems locals were facing.

"Too much, too late. We've been driving this for so long and it's so frustrating, the lack of traction we've got," Palmer said.

"It's no surprise to anyone that we have way too much gravel in the river and therefore there is no capacity to carry the flood."

District council managing 'concerning' loss of riverbed material

Tasman District Council rivers and coastal structures lead David Arseneau said it was working with the group to widen the river and plant vegetation to help catch debris during floods.

It had extracted 50,000 cubic metres since 2022, with a further 30-40,000 cubic metres planned this year in efforts to manage erosion damage, he said.

"We carry out this work under a global resource consent that requires us to maintain the riverbed within a Mean Bed Level Envelope (MBLE), which was established based on our historical river cross-section surveys," he said.

"This prevents over-extraction while also enabling any extraction required if too much gravel accumulates.

"As well, the consent has conditions around endangered or vulnerable nesting birds that impact work in the Upper Motueka River, as they require the braided river gravel beaches for nesting and nest during the summer months which is also the ideal time for extraction."

However, Arseneau said extraction work was not a silver bullet flooding solution, as it was widely believed to be.

"We do not extract gravel to provide a target flow capacity in the river channel itself, that is, to prevent flooding of adjacent property," he said.

"The reason for this is that you would need to extract such an enormous amount of gravel from the river to have any noticeable benefit during the kinds of floods we've just experienced. It just doesn't provide enough flow capacity [measured by the channel's cross-sectional area] to make a meaningful difference.

"As well, if you extracted that amount of gravel from the river you would be left with a narrow, entrenched canal with riverbanks that are constantly being eroded, requiring continual maintenance and expensive bank protection."

Arseneau said stopbanks were needed to control large floods.

He said recent surveys via LiDAR showed a "concerning" loss of riverbed material at the site, where the river corridor had constrained over time, so it was not able to carry out significant extraction under its consent.

"The results of these surveys in the Upper Motueka River are concerning for us, showing significant loss of riverbed material from year to year. Over the entire Upper Motueka River managed section, there was a loss of 345,000 cubic metres of material from 2022 to 2023, and a further loss of 142,000 cubic metres from 2023 to 2024.

"If we're looking just at the section of river from the Tapawera Bridge to the Wangapeka River confluence [the area in which Dion and Dean's properties are located], we've lost 147,000 and 77,000 cubic metres over those two periods, respectively, and the Mean Bed Level is such that we cannot carry out significant extraction under our consent."

While Palmer said the council could earn royalties from contractors for the high quality extracted aggregate, Arseneau said there was "depressed aggregate demand from the industry" due to economic conditions and the long transport distance.

This winter, the district council will re-assess how floods have changed the rivers using LiDAR.

New supports for flood-affected farmers and growers

The Government announced a new half-million-dollar support package to support rural communities in Nelson and Tasman on Wednesday.

It included a $300,000 contribution to the Mayoral Relief Fund for the rural sector, $100,000 with Federated Farmers to the Farmers' Adverse Events trust and $100,000 with Horticulture New Zealand.

It followed $100,000 announced in June, a fifth of which went to the Top of the South Rural Support Trust.

Farmers and growers in need of assistance should contact their local Rural Support Trust branch on 0800 787 254.

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