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

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.

Automated summary

An extremely heavy winter storm in July 2022 caused massive alpine mass movements in the Southern Alps of New Zealand, including snow avalanches, debris flows, and erosion caused by rain runoff. The largest avalanche since 1986 occurred in the Kitchener avalanche path, which tested the effectiveness of a diversion berm designed for a 1:100-year event. Results from a lidar survey and numerical modeling provide insights into future hazards posed by intense alpine precipitation, especially on a winter snowpack.