Journal Article

Unprecedented Winter Rainfall Initiates Large Snow Avalanche and Mass Movement Cycle in New Zealand's Southern Alps/Kā Tiritiri o te Moana

Bartelt, Perry, Conway, Jono P., Cox, Simon C., Cullen, Nicolas J., Miller, Aubrey D., Bühler, Yves, Sirguey, Pascal, Bogie, Don, Redpath, Todd A. N.

2023

Overview

An exceptional July 2022 winter storm brought 550 mm of precipitation to the Southern Alps of New Zealand. A series of alpine mass movements occurred during the storm, including a widespread snow avalanche cycle, debris flows, and erosion from rain runoff. We detail the sequence of events in the Kitchener avalanche path. Here, two large snow avalanches were followed by a debris flow. Substantial erosion of deposition and the underlying alluvial fan were induced by runoff from over 300 mm of rain falling after the first avalanche. The Kitchener path saw the largest avalanche since 1986, testing the utility of a diversion berm constructed for a 1:100‐year event. Results from a unmanned aerial vehicle lidar survey and numerical modeling characterize the rain‐on‐snow hazard sequence. In particular, the rain‐on‐snow event occurred on a deep mid‐winter snowpack, offering insights into future hazards posed by increasingly frequent extreme alpine precipitation. Plain Language Summary Intense rain and snow in alpine regions can trigger snow avalanches, debris flows and other mass‐movements, posing risks to people and infrastructure. This study documents an extreme hazard sequence triggered by a record‐breaking winter rainstorm in the Southern Alps of New Zealand. A drone was used to map deposition from snow avalanches and a debris flow that ran along a diversion berm designed to protect Aoraki/Mount Cook Village from a 1:100‐year avalanche. The mapping was used to calibrate avalanche modeling software that could replicate key characteristics of the snow avalanches and provide insight into flow dynamics. The documentation and results from the modeling can be used to plan for future hazards triggered by intense rain falling on a winter snowpack, which is particularly important in the face of a warming climate. Key Points Record‐breaking winter storm bringing rain to high elevations triggered a series of alpine mass movements in New Zealand's Southern Alps Snow avalanches were successfully diverted away from Aoraki/Mount Cook Village in the largest avalanche cycle observed in decades Event documentation and modeling help anticipate future hazards for such rain‐on‐snow induced mass movements on deep mid‐winter snowpacks

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Alluvial deposits / Alluvial fans / Atmospheric precipitations / avalanche / Avalanches / Berms / Debris flow / Deposition / erosion / extreme rainfall / Extreme weather / Global warming / hazard / Hazards / Landslides / Lidar / Mass movement / Modelling / Precipitation / Rain / Rainfall / Rainstorms / Runoff / Sequencing / Snow / Snow avalanches / Snowpack / Storms / Unmanned aerial vehicles / Weather hazards / Winter / Winter storms