Climate Change Impacts on Alpine Mass Movements

Abstract/Description

The impact of a warming climate on snow avalanche hazards varies by region and season, influenced by factors such as the timing and intensity of rainfall on alpine snowpacks. For instance, winter rain on unconsolidated snow creates more complex hazards than spring rain on a consolidated snowpack. Snow avalanche flow dynamics are primarily driven by temperature and water content of the released and eroded snowpack.

Aotearoa New Zealand (NZ), characterized by a maritime climate, frequently experiences large rain events (>100 mm) falling to high elevations. The Southern Alps experience steep precipitation and temperature gradients, resulting in large avalanches that often release as cold powder avalanches in the high alpine and rapidly transition into warm, wet flows as they descend to valley floors. Limited meteorological and avalanche observational data in NZ have so far constrained our understanding of how climate change influences avalanche flow regimes.

This study provides the first detailed insights into NZ’s avalanche flow regimes from naturally occurring events. Over four seasons, we inventoried 250 avalanches across nine paths in the Hooker Valley, Aoraki Mount Cook National Park, using a time-lapse camera network and meteorological data. We focused on 197 events (79% of total) from two adjacent paths with similar slope gradients and aspects. These paths reach to just over 2,300 m and descend directly into a paraglacial lake at 870 m.

Time-lapse observations reveal that while avalanches often initiate as storm or wind slabs, they quickly transition into warm, wet flows, frequently traveling below the snowline over bare ground or old debris before terminating in the lake. Sediment and rock entrainment further increases flow density and slows velocity. Although springtime rain reaching high elevations is common in NZ, frequent mid-winter intense rainfall events have been observed to occur in the release zones, creating complex release and flow dynamics.

NZ’s avalanche patterns provide valuable insights for other regions globally, where warm and wet avalanche cycles are increasingly observed in mid-winter. Understanding these shifting flow regimes is crucial for future hazard mitigation, including updates to hazard management plans and infrastructure design to accommodate denser, more dynamic avalanche conditions.

Abstract/Description

Debris flows are a significant hazard to people and infrastructure in many mountainous areas. Quantification of intensity and spatial distribution of this hazard, however, remains challenging. This is due to the often-encountered lack of high-resolution spatial and temporal data and the complexity and wide-range of processes involved. This applies to current climatic conditions, but especially for future conditions where uncertainty in extreme precipitation and sediment availability is even larger. We present the outline of a national-scale, GIS-based approach to assess debris flow disposition with a statistical model under current extreme precipitation conditions and the best available estimates for 2065 and 2080 extreme precipitation and sediment loads. Debris flow disposition is assessed using properties derived from delineation of channels and their corresponding upstream contributing area, based on high spatial resolution (1m) elevation data. The disposition model is trained on a combination of available Swiss debris flow databases (such as StorMe). Runout is computed from disposition areas with calibrated RAMMS:Debrisflow software. Our method is applied to the network of the Swiss national railway (SBB) and results in a spatially distributed maximum debris-flow height. The results indicate sections of the railway network that are prone to debris flow hazard and serve as input to a risk assessment for the SBB. By adapting this approach, it could be applied also in other regions in the Alps and worldwide.

Abstract/Description

Mountain landscapes are influenced by geomorphological processes such as debris flows. These affect both their infrastructures and ecosystems, and the assessment of their magnitude and frequency is key to land management and risk assessment. On the other hand, the reconstruction of their past activity remains a challenge due to the scarcity of continuous historical records. To fill this gap, a dendrogeomorphological study was conducted on an alluvial cone in the Pineta Valley (Central Pyrenees, Spain). A total of 924 tree samples (851 cores and 73 cross-sections) from 758 disturbed trees were collected, allowing the identification of growth disturbances linked to past events. These data were complemented by detecting geomorphological changes from multitemporal LiDAR analysis over the last three and aerial picture recognitions, which provided additional insights into surface modifications and supported the tree-ring-based reconstructions. Analyses of the unprecedented number of trees analyzed suggest a complex history of debris flow activity, with variations in both magnitude and frequency over time. These results reveal trend changes potentially related to climatic conditions and dissimilar sediment connectivity. Using dendrochronology with LiDAR-based analysis of the terrain, a detailed spatio-temporal reconstruction was carried out, providing insight into the dynamics and triggers of debris flows. This integrated approach contributes to better hazard assessment and to identifying factors and periods of different levels of activity. The use of tree ring records combined with high-resolution topographic data highlights the importance of integrating different methodologies for the reconstruction of past geomorphological processes. These findings provide insight into the dynamics of debris flows in mountain landscapes and highlight the importance of using tree rings in conjunction with remote sensing for risk assessment.