Massen in Bewegung: Murgang und Geschiebe ¶

Debris flows radiate both seismic and infrasonic waves. According to previous studies, seismic energy is generated by the solid particle collisions with the riverbed and is dominated by the boulder-rich front, while infrasound is produced by turbulence-induced waves at the flow surface. To further investigate this complex radiation processes, we present the seismo-acoustic analysis of a debris-flow event at Illgraben (Switzerland). Array processing shows that infrasound is preferentially radiated at channel irregularities, acting as predominant acoustic sources because of the intense flow turbulence. The high crosscorrelation observed between the recorded infrasonic and seismic signals suggests that, in addition to the dominant source related to particle impacts, a minor seismic component is produced by the flow waves developing at topographic steps.

This study investigates the runout and depositional behavior of the Tangni debris flow in the Garhwal Himalaya, India, through a series of controlled experiments conducted at the newly developed facility at CSIR-Central Building Research Institute, Roorkee. The experiments focused on evaluating the influence of water content, initial volume, and grain size distribution on debris flow dynamics. Debris flow experiments reveal that small changes in water content can trigger abrupt transitions in flow regimes, significantly altering runout distances and deposition patterns. Additionally, dimensionless parameters were calculated to characterize flow regimes, and granulometric analysis was performed on the deposited material. The results show that runout distance and deposition area increase with higher water content and larger initial volumes, whereas finer content reduces runout. Granulometric analysis revealed a clear segregation of coarse particles, with enrichment at the margins and towards the flow front, indicating that coarse particles enhance the destructive potential of debris flows. Dimensionless analysis identified the experimental flows as predominantly frictional in nature. The findings demonstrate consistency with natural debris flows and align with global experimental studies, providing valuable insights into the behavior of debris flows in the Indian Himalayas. Despite limitations due to scaling effects and reconstruction challenges, this study highlights the importance of experimental approaches in advancing our understanding of debris flows, particularly in regions lacking extensive historical data and monitoring infrastructure.

The Indian Himalayan region is the origin of numerous natural hazards, including landslides, earthquakes, floods, cloudbursts, etc. (Dash et al. 2021; Falae et al. 2021; Kanungo and Sharma 2014). Uttarakhand, located in the northwest Indian Himalayas, is one of the most landslide-prone states in India. In a recently published report by the National Remote Sensing Centre (NRSC) India, all the 13 districts of Uttarakhand state are severely affected by landslides and the Chamoli district ranks 3 among 13 districts of Uttarakhand and ranks 19 among 147 landslide-prone districts in the country according to the landslide density (Jain et al. 2023). Over the past three decades, Chamoli has experienced several destructive mass movement events, including the 2013 Kedarnath tragedy, the Chamoli rock and ice avalanche on February 7, 2021, land subsidence in Joshimath during 2023, etc. In 1999, Chamoli was struck by an earthquake with a magnitude of 6.8 on the Richter scale, with its epicenter near Gopeswar township. In addition to landslides and earthquakes, Chamoli has also faced numerous disasters caused by cloudbursts and flash floods. National Highway 58 connects Uttarakhand with the rest of the country, entering Chamoli district through Karnaprayag and extending to Badrinath. This highway passes through the Alakananda Valley, traversing many landslide-prone zones. In a recent study, Ramiz et al. (2023) found that about 27% of the areas along the highway corridor from Rishikesh to Badrinath is highly susceptible to landslides. Frequent landslides, including debris flows, rockfalls, debris slides, and subsidence, disrupt traffic and deposit materials in the river, increasing the likelihood of river damming (Dash et al. 2022, 2023) and traffic disruption. A similar traffic disruption occurred due to a rock avalanche near Patalganga Langsi Tunnel on the Badrinath Highway of Chamoli district on July 10, 2024. This is an area known for frequent rockfall, and debris slide events, as noted by earlier researchers.

Die Veränderungen im alpinen Permafrost finden meist im Verborgenen statt, haben aber zunehmend bedeutende Konsequenzen für die Berggebiete. Im letzten Jahrzehnt sind in den Schweizer Alpen die Permafrosttemperaturen um bis zu 1 °C angestiegen, die Auftauschichten sind um mehrere Meter mächtiger geworden, im Boden hat das Eis ab- und der Wassergehalt zugenommen und die Blockgletscher haben sich schneller talwärts bewegt als zu Beginn der Messungen vor gut 20 Jahren. Diese grossen Veränderungen im Untergrund bedrohen Hochgebirgsinfrastrukturen und führen zu einer Zunahme von Naturgefahren wie Felsstürzen und Murgängen. In diesem Beitrag beschreiben wir die wichtigsten Mechanismen, die immer schnelleren Veränderungen im Permafrost in der Schweiz und die Herausforderungen, die sich daraus für den Kanton Bern ergeben.

Hidden ice in the Swiss Alps: permafrost is thawing faster
Changes in mountain permafrost are mostly taking place invisibly, but can have increasingly important
consequences for mountain regions. In the past decade, permafrost temperatures in the Swiss Alps increased by up to 1 °C , the active layers thickened by several metres, the ground ice content decreased and the water content increased, and rock glaciers were creeping faster than at the beginning of the measurements more than 20 years ago. These important changes compromise the stability of high mountain infrastructures and lead to an increase in natural hazards such as rockfalls and debris flows. In this contribution, we describe the most important mechanisms, the accelerating changes in permafrost in Switzerland and the resulting challenges for the Canton of Bern.

Recent studies have brought to light large amounts of evidence for enhanced rock slope failure from degrading permafrost rock walls. These failures have previously been thought to be subaerial and triggered by thermal heat propagation from rising air temperatures into the exposed rock faces. However, we have neglected the fact that the dividing line between cold and warm basal states of polythermal glaciers has shifted some hundreds of meters upwards at the same time. This means that previously frozen and ice-filled fragmented rock walls under cold glaciers have suddenly and for the first time in thousands of years been exposed to (i) hydrostatic pressures, (ii) warming and degrading ice in fractures, and (iii) rock mechanical degradation in warming rocks. In this paper, we hypothesize that the transition from cold- to warm-based glaciers, a scarcely observed but widespread phenomenon, caused the massive rock slide. To challenge this hypothesis, we investigated the 3.9 to 4.3×10⁶ m³ rock slide at Bliggspitze on 29 June 2007, which detached from a north-exposed, glacier-covered rock slope at 3200 m above sea level. We (a) have analyzed the glacier transition since 1971 using aerial photographs coincident to meteorological data; (b) compared 2013–2016 ground surface temperature measurements to infer permafrost-prone and cold glacier thermal conditions; (c) categorized springs mapped in summer 2001/2012 according to geomorphological features and mineralization; (d) performed electrical resistivity tomography subsequent to failure on the destabilized rock flank in 2009; (e) conducted rock testing in frozen and unfrozen conditions; and (f) modeled the mechanical impact of hydrostatic pressure, degradation of permafrost, and glacier retreat in a universal distinct element code (UDEC). Aerial photos indicate the existence of a cold glacier from 1971–2003 above the failure volume. On the rock face above the failure volume, ground surface temperature measurements demonstrate permafrost-favorable conditions and underpin the presence of former and present cold-based glacier compartments. Since 2003, the warming of the Northern Bliggferner Glacier has been evident in the lower and upper parts. In 2007, subsequent to the warmest January–June period in a 228-year temperature record in the area of Bliggspitze, the glacier opened massive ice crevasses above the later rock slide, causing frequent ice fall. New springs developed in the former permafrost flank, and some were strong enough to cause debris flows. The high mineralization measured at springs at a proximal distance to the failure volume indicates active-layer thaw. The inversion of electrical resistivity tomography revealed thaw that reached several decameters in depth in the collapsed rock mass 2 years after failure. The tensile strength of tested paragneiss rock samples decreased by −40 % from frozen to unfrozen states, which reflects the mechanical degradation of rock bridges under warming permafrost. In this paper, we demonstrate a new type of rock slope failure mechanism triggered by the uplift of the cold–warm dividing line in polythermal alpine glaciers, a widespread and currently under-explored phenomenon in alpine environments worldwide.