20 million tonnes of silt, sand dumped on Hawke's Bay floodplains during Cyclone Gabrielle

Newshub can reveal around 20 million tonnes of silt and sand was dumped on Hawke’s Bay’s floodplains during Cyclone Gabrielle. 

It’s the findings from aerial laser surveys carried out by scientists who are mapping the land to see how it’s changed.

They estimate the cyclone created about 300-thousand landslides in Gisborne and Hawke’s Bay, which now threaten to slip more debris into rivers in future floods.

Hillside slips in Esk Valley show significant damage.
Hillside slips in Esk Valley show significant damage. Photo credit: Newshub

"There is a lot of concern about particular floodplain pockets, areas upstream..higher in the headwaters, where sediment is being stored”  University of Canterbury river scientist James Brasington said.

He’s part of a team of researchers from the University of Canterbury and Lincoln University who have been extensively mapping the land and rivers across Hawke’s Bay and Gisborne since the cyclone struck in February. 

"We want to get a proper figure for how much material was eroded, how much was deposited on the flood plains. Then we will know how much went out to sea, and how much is left in the catchment".

What they know so far is that about 20 million tonnes of silt and sand was dumped on Hawke’s Bay flood plains by the cyclone. 

"Here in Esk Valley, there was over 800 hectares of sediment deeply laid - about 10,000 tonnes per hectare, and that pattern was quite widely spread over other catchments".

The scientists are using a LIDAR laser and imaging system, attached to a helicopter, that captures detailed 3D information about the terrain and vegetation cover.  

It’s safe for eyes, and can do up to 1.5 million pulses of light a second. The information is sent directly to a computer on board that James’s colleague Justin Stout is operating as they fly back and forth over 2000 square kilometres of land. 

“Before we start flying we build a plan in the office that tells us how fast to fly, and how fast the laser needs to move, that dictates points on the ground," he said.

In the immediate aftermath of Gabrielle, the team created detailed 3D models to measure the location and size of stopbank failures on the Ngaruroro and Tutaekuri Rivers. 

They mapped the flood impacts across Hawke’s Bay and Gisborne to find the depth and distribution of silt and sand. Work that is being partly funded by the government’s 10.8 million dollar fund for urgent scientific research.

That information was used to create flood hazard simulation models, and by overlaying it with LIDAR information from 2020, they’ve created 4D models showing the difference in river valley levels after Gabrielle.

The blue parts of the 4D map in Esk Valley show where its floor is much higher, in some places the silt is six metres deep.
The blue parts of the 4D map in Esk Valley show where its floor is much higher, in some places the silt is six metres deep. Photo credit: Newshub

"I've never seen the scale of vertical change that we’ve looked at with Gabrielle. Packets of sediment that are moving through here are actually shocking. They’re scary".

Now they’ve gathered data on how much sediment is already on the floodplains,  they’re trying to calculate how much is still to come down in future floods.

"There is more material up in the catchment, the possibility for it to work its way down over time, a little bit like a pulse of sediment, a wave of sediment coming down, is a threat we need to be aware of".

Because it could result in properties and infrastructure being buried in silt yet again. 

"All these landslides are activated and still going - going to continue to supply sediment for years to come," Stout said. 

He says the risk from landslides is far from over.

“In every small flood we will see it move," Stout said.  

Which is a concern for residents who’ve spent months digging their properties out from mountains of silt.

Steve Wheeler is worried about the landslides upstream, and the debris from pine forests. 

"It’s frightening. Absolutely frightening, there is a phenomenal amount of material up there."

But it’s not stopping him from staying put.

"Friends of ours say you’re nuts, you’re mad - walk away.  But we can't run away, this is our piece of paradise".

Residents will soon have more information about the risks they face, as researchers work to understand them.  

"Piecing together that puzzle we can begin to provide an answer to if there is a future threat we need to be aware of," Brasington said.

Because this won’t be the last cyclone we see..

"What we saw here was not a one off," he said, warning that with climate change causing higher rainfalls, these extreme events create much larger flows of sediment from land to rivers, to oceans.

"We hypothesise that flows through the catchment are highly connected, so that much more of the eroded material is transported down to the lower floodplain areas and ultimately into the sea".