A West Coast glacier town faces a version of the cascading hazard behind this week's catastrophic floods on the Nepal-Tibet border — a river capable of damming high in the mountains, then unleashing a torrent without warning.
Hundreds are confirmed dead and nearly 2500 missing – including five New Zealanders – after a huge collapse of ice and rock high in the Himalayas dammed a river before unleashing it in a single catastrophic surge on Wednesday.
New Zealand researchers say the sequence that caused so much devastation — a blockage forming high in a steep catchment, then failing suddenly and sending a disproportionate flood downstream — is a modelled risk on the Callery River, which flows into the Waiho at Franz Josef.
The comparison comes with an important caveat: the trigger being modelled for the Callery is a major rupture of the Alpine Fault, rather than the glacier and rock collapse that caused this week's Himalayan disaster.
NZ's highest-risk catchment for this kind of event
The Callery's catchment reaches back into the Southern Alps but, unlike the neighbouring flood-prone Waiho, it winds through steep, narrow gorges easily blocked by rockfall.
University of Canterbury research modelling of the landslide dam hazard has identified the Callery as New Zealand's highest-risk catchment for this kind of event, exposing an estimated 570 permanent residents and more than 3000 tourists at Franz Josef.
The trigger being modelled for the Callery is a major rupture of the Alpine Fault, not a glacier collapse such as the one behind this week's disaster.
Earth Sciences NZ mountains to sea chief scientist Simon Cox said the risk to the Southern Alps was different to the Himalayas, largely because our highest mountain regions were much less populated.
"Events such as this demonstrate the importance of considering cascading hazards. When a large landslide or ice avalanche enters a river or lake, it can entrain water and sediment, temporarily dam a river, generate waves, or produce debris flows and floods that transfer the effects many kilometres downstream."
What about climate change?
Cox said climate change, although not the "simple or sole trigger" for the event in Nepal-Tibet, was generating conditions that could destabilise high mountain rock and ice, making events like this more likely.
"Climate change is affecting the stability of snow, ice and rock in high mountain environments. Warming can reduce ice support on steep slopes, increase meltwater, and change freeze-thaw and rainfall patterns. These changes can weaken slopes and make large collapses more likely in some locations."
He suggested a systematic assessment was needed to determine which mountainous areas may become more prone to these hazards in the future.
Victoria University of Wellington glaciologist Dr Lauren Vargo, who studies New Zealand's glaciers, said the country does get landslides in valleys with glaciers above them, and the Nepal disaster was a reason to better understand that risk at home — while cautioning against assuming an equivalent event here.
"This makes me think that we need to learn more about this, and understand if there is a risk, or how much of a risk there is, here in New Zealand," Vargo said.
She said the clearest difference between the Callery and the Nepal disaster was scale, not just the presence of similar hazards.
"If that were to happen from the Franz Josef, or from that Callery River valley, it could be devastating for the town of Franz Josef — but beyond that is the sea. It's a pretty different scale of risk compared to the event that's happened in Nepal," she said.
'Hazard is not often the first event'
Associate Professor Jon Tunnicliffe, a river scientist at the University of Auckland, said the value of the comparison lies in the pattern, not the scale.
"New Zealand lives with its own versions of cascading mountain hazards — slope failures, landslide dams, debris floods and large sediment pulses moving through river systems," Tunnicliffe said.
"The settings and scales differ, but the underlying lesson is the same: the hazard is often not the first event. It is what that event sets in motion."
He said the priority now was combining monitoring with planning, since it's not possible to watch every unstable slope.
"We will never monitor every unstable slope, so communities need mapped runout corridors, evacuation plans, appropriate land-use controls, and resilient roads, bridges, power and communications."
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