Satellite imagery has captured the enormous devastation that Gabrielle wrought on our landscapes, revealing flooded valleys, transformed coastlines and thousands of landslides.
Nearly a fortnight on from the historic disaster, scientists have been able to observe valleys covered with hundreds of metres of silt, beaches at their most eroded state in 80 years and pine plantations flattened by gale-force winds.
One of those researchers, climate scientist Dr Nathanael Melia, said the imagery laid bare shortcomings in land use decisions – especially around the slash-covered catchments of Tairawhiti.
Melia, who runs the consultancy firm Climate Prescience, had been tracking Gabrielle on February 13 as it began flooding East Coast rivers monitored by council-run webcams.
“The webcams went offline in the early hours, and due to the widespread power outages, I knew that helicopter photos and satellite imagery would be the only way to verify the impacts the weather community had been forecasting.”
He turned to the Sentinel Hub EO Browser - providing high-resolution data captured by two satellites launched by the European Space Agency – to assess local impacts from high winds, flash flooding and forestry slash.
The imagery quickly confirmed the extent of damage to a pine forest at Motuoapa, near Turangi, where one video posted to social media showed trees snapped in half.
While the satellite data showed blocks near Turangi had been flattened by windthrow, fortunately, that damage hadn’t extended across the entire Central North Island and its billions of dollars of forestry.
Next, he turned to the East Coast, where he’d expected to find heavy damage from slash and debris flow from having studied previous events.
Imagery captured on February 17 showed swollen rivers around Ruatoria, Tokomaru Bay, and Tolaga Bay, some of which had changed course and broken their banks.
While cloud prevented the satellite view to freshly-harvested catchments, he was shocked to find a huge number of slips even amid unharvested areas of forestry around Tokomaru Bay.
“It suggested that the rainfall totals here were likely far higher than in the numerous previous events in the last five years, but maybe compounded by the wind.”
Two days later, satellites captured the scale of flooding and slipping in Hawke’s Bay’s Esk Valley, where stricken residents had to be rescued from rooftops in the hours after Gabrielle tore through.
Melia said the Esk Valley typically spanned 10m to 30m wide, yet silt left by the flooding stretched 500m to 1km across the entire valley floor.
“The surrounding agricultural land has the answer for where the cubic kilometres of silt came from,” he said.
“My eye-balled estimate is that 30 per cent of the pasture topsoil has slipped off the land and flowed into the Esk River Valley.”
In all, he said the imagery had brought into clearer focus what an extreme weather event like Gabrielle could do to New Zealand’s diverse landscapes.
“It has also starkly illustrated, in space-borne technicolour clarity, the shortcomings of land-use decisions that have been made,” he said.
“Whether it be placing pine forests in areas where the wind can be topographically turbocharged, pine forests in the steeps of Tairawhiti, or agricultural pasture on the valleys of Hawke’s Bay.”
The widespread damage had also been shown in aerial photographs showing landscapes scarred by thousands of slips and landslides.
Meanwhile, University of Auckland researchers had been mapping the country’s changing shoreline long before the storm hit, drawing on more than half a century of aerial photographs and satellite images.
“Even before the cyclone had passed, we had prepared to ‘task’ high-resolution satellites to capture images of the North Island’s east coast,” Associate Professor Murray Ford said.
Months of La Niña-influenced weather, bringing more easterly winds and large swells to eastern coasts, had already taken their toll – meaning the heavy impacts of Gabrielle were ultimately “super-imposed” on eroded beaches.
Ford and other colleagues working under the collaborative Resilience to Nature’s Challenges project were still piecing together what Gabrielle’s extra blow had meant for our shores.
“What can we say so far about how this event has changed the shape and structure of our coastlines and coastal areas – particularly in those worst affected areas in the East Coast.”
In many areas, they’d observed foredunes – or dune ridges that run parallel to the shore – being noticeably scarped.
“On the ground, this looks like a vertical face to the dunes, or a sort of mini-sand cliff caused by the waves eating away at the dune during high tide.”
While it was often hard to unpick Gabrielle’s specific impacts in the satellite data, some of the imagery showed striking before-and-after contrasts – notably at Great Barrier Island’s Kaitoke Beach.
“From these images, we can clearly attribute significant changes to the dunes to the cyclone.”
At other northern beaches - such as around Ruakaka, which was hit by massive waves - they could see changes in the order of five to 10 metres.
“Several beaches are certainly at their most eroded state for decades, in places the shoreline has not been this eroded over the past 80 years,” Ford said.
“This is obviously a big concern for coastal communities that have houses and key infrastructure at risk.”
Ford said the problem would likely be exacerbated if more major storms blew in over the coming months.
“Going into the winter with beaches already in an eroded state obviously places them at heightened risk of further erosion.”