Christoph Graf

Die Ereignisse Ende Juni 2024 haben gezeigt, dass neue, konsistente Methoden zur Bewertung von Murgang- Schutzmassnahmen benötigt werden. Die Wahl geeigne­ter Gefahrenszenarien und die Modellierung der Mur­gänge sind zentral, um Schutzmassnahmen effektiv zu planen. Ein neues numerisches Zwei-Schichten-Modell, das feste und flüssige Phasen trennt, wurde eingesetzt, um die komplexe Dynamik von Murgängen besser nachzu­bilden. Zwei Fallstudien im Val Greva (GR) und in Sorte (GR) zeigen realistische Ergebnisse und verdeutlichen die Bedeutung der Erosion für die Mobilität von Mur­gängen. Die Ergebnisse weisen darauf hin, dass künftige Schutzmassnahmen differenziertere Informationen zum Fliessverhalten erfordern. Eine stetige Verbesserung der Modelle durch eine gründliche Analyse vergangener Er­eignisse ist entscheidend, um die Genauigkeit und Zu­verlässigkeit der Gefahrenbewertung zu gewährleisten.

In high mountain areas, the disposition for debris flows is increasing, as rapid glacier retreat and permafrost degradation is favouring higher availability of loose sediments. The probability of occurrence and magnitude is increasing, too, as triggering events such as heavy thunderstorms, long-lasting rainfalls, intense snowmelt or rain-on-snow events are likely to occur more often and intensely in future decades. Hazard assessment is thus crucial, as records of past events on which local magnitude frequency relationships and debris flow scenarios can be based on are often scarce or inexistent. In this study, we present a multi-methods approach including precipitation runoff modelling, field surveys and empirical approaches for hazard scenario building and runout modelling in pro- and periglacial catchments. Numerical modelling is carried out using RAMMS, which was calibrated to the studied catchment (Le Dar, western Swiss Alps) based on data from an event recorded in summer 2005.

Die Fachleute Naturgefahren (FAN) in der Schweiz bildeten 2021 eine Arbeitsgruppe (AG) um zu untersuchen, wie in Zukunft eine hohe Qualität der Gefahrenbeurteilung von Wildbächen in der Schweiz erhalten, sichergestellt und optimiert werden kann (FAN, 2023). Der Bericht dazu beinhaltet eine Standortbestimmung bezüglich der Gefahrenbeurteilung von Wildbächen in der Schweiz und macht Empfehlungen im Hinblick auf mögliche zukünftige Entwicklungen. Dazu wurden vorhandene Lücken bzw. erwünschte Neu- oder Weiterentwicklungen in den Bereichen Forschung, Ausbildung (Lehre und Weiterbildung) und Gefahrenbeurteilung in der Praxis identifiziert und diskutiert, sowie basierend auf dieser Bestandesaufnahme Empfehlungen für Massnahmen formuliert. Nach der Publikation des Schlussberichts der AG wurden zwei Folge-Workshops durchgeführt: Ein erster zum Themenbereich Forschung und Ausbildung (Lehre und Weiterbildung) und ein zweiter zum Themenbereich Praxis und Umsetzungsaspekte. Bei diesen Workshops wurden die Resultate des Berichts vorgestellt und mögliche (konkretere) Massnahmen mit den Teilnehmenden diskutiert.

Différents systèmes de surveillance sont utilisés dans le monde entier pour l'observation et la saisie de paramètres importants des laves torrentielles, mais ils ont tous une résolution limitée dans le temps et/ou dans l'espace. Dans cet article, nous présentons une nouvelle méthode de surveillance basée sur des scanners laser 3D (LiDAR), qui fournit des données à haute résolution temporelle et spatiale sur les laves torrentielles. Dans l'Illgraben, plusieurs scanners LiDAR de ce type ont été installés le long du chenal et ont déjà enregistré onze événements de laves torrentielles. Grâce à ces données 3D, nous avons pu mesurer avec une grande précision la vitesse d'objets individuels dans le mélange des laves torrentielles et déterminer les débits et les volumes. Nos analyses montrent que la technologie LiDAR peut apporter une contribution essentielle à une description plus détaillée de la dynamique des laves torrentielles, d'autant plus que les premiers résultats indiquent des différences significatives, par exemple dans la détermination du débit, entre cette méthode et les méthodes conventionnelles.

Zur Beobachtung und Erfassung wichtiger Kenngrössen von Murgängen kommen weltweit verschiedene Monitoring-Systeme zum Einsatz, die alle eine zeitlich und/oder räumlich beschränkte Auflösung haben. In diesem Artikel präsentieren wir eine neue Monitoring- Methode basierend auf 3D-Laserscannern (LiDAR), die zeitlich und räumlich hochaufgelöste Daten von Murgängen liefert. Im Illgraben wurden mehrere solche LiDAR-Scanner entlang des Gerinnes installiert und haben bereits elf Murgangereignisse erfasst. Anhand dieser 3D-Daten konnten wir Geschwindigkeiten einzelner Objekte in der Murgangmischung mit hoher Genauigkeit messen sowie Abflüsse und Volumina ermitteln. Unsere Analysen zeigen, dass die LiDAR-Technologie einen wesentlichen Beitrag zur detaillierteren Beschreibung der Murgangdynamik leisten kann, zumal erste Resultate auf massgebliche Unterschiede etwa in der Bestimmung des Abflusses zwischen dieser Methode und konventionellen Verfahren hindeuten.

In-situ measurements of debris-flow properties are crucial for understanding their movement mechanisms and quantifying their impact. Here we present the first results of a field monitoring campaign, at Illgraben, Switzerland, to measure debris-flow parameters using high temporal (10 Hz) and spatial resolution LiDAR sensors at several locations along the channel. The point cloud data is projected onto video images to enhance visualization and aid in the interpretation of the measurements. We process the data using machine vision and deep learning based algorithms, and show that this system can accurately measure front and flow surface velocity, flow depth and bed elevation change, as well as the size, style of motion (e.g. rotating or floating without rotation) and trajectories of individual particles. This system thus provides a promising new method for inferring the internal dynamics of debris flows.

Surging debris flows are among the most destructive natural hazards, and elucidating the interaction between coarse-grained fronts and the trailing liquefied slurry is key to understanding these flows. Here, we describe the application of high-resolution and high-frequency 3D LiDAR data to explore the dynamics of a debris flow at Illgraben, Switzerland. The LiDAR measurements facilitate automated detection of features on the flow surface, and construction of the 3D flow depth and velocity fields through time. Measured surface velocities (2-3 m s^-1) are faster than front velocities (0.8-2 m s^-1), illustrating the mechanism whereby the flow front is maintained along the channel. Further, we interpret the relative velocity of different particles to infer that the vertical velocity profile varies between plug flow and one that features internal shear. Our measurements provide unique insights into debris-flow motion, and provide the foundation for a more detailed understanding of these hazardous events.

Debris flows are a severe hazard in mountainous regions. However, cost-effective long-term studies of debris flows are seldom, which leads to substantial uncertainties in hazard mitigation methods. This paper investigates whether cost-effective remote sensing techniques can be applied to assess the hazards of mountain torrents and to gather accurate long-term information on the development of the watershed. Torrents that are prone to debris flows are often devoid of vegetation and can thus be well surveyed using photogrammetric methods based on uncrewed aerial vehicle (UAV) surveys. The possibility of extracting automatically torrent parameters from high-resolution terrain models, such as cross-sectional area or slope, is explored. The presented methodology yields continuous and automatically derived parameters along the torrent, which is a major advantage over pointwise field surveys. Cross-validation with field measurements reveals a strong agreement. These parameters are very accurate along highly incised sections, while they are severely limited along sections with steep adjacent hillslopes and/or dense vegetation. We show that these kinds of assessments greatly gain from UAV data followed by automatic parameter extraction. The extracted parameters provide insights so that key sections and weak points can be identified and accurately assessed in the field. We find that UAV data can contribute to a comprehensive, reproducible and objective assessment of torrent processes and predispositions. However, ground-based fieldwork is still essential and further research on remote sensing-based hazard assessment of torrents prone to debris flows is crucial.

Debris flows are a hazard in mountainous regions. Cost-effective, long-term studies of debris-flow torrents, however, are rare, leading to uncertainties in hazard assessment and hazard prevention. Here, we address the question of whether cost-effective remote sensing techniques can be applied for assessment of mountain torrents and possibly further gather accurate, long-term information on the evolution of the catchment. Torrents prone to debris flows are often devoid of vegetation in the near channel area and hence can be well captured with photogrammetrically derived methods using uncrewed aerial vehicle (UAV) surveys. The possibility of automatically extracting specific torrent parameters from high-resolution terrain models, such as cross-section area or gradient, is investigated. The presented methodology yields continuous and automatically derived geometrical parameters such as torrent bed width, inclination and cross-section area, which is a major advantage compared with point-based, often dangerous field surveys. Their cross-validation with field measurements shows strong agreement. Those parameters are accurate along sharply incised sections with strong limitations along sections with steep adjacent slopes and/or dense vegetation. The information along the torrent allows fast identification of key sections and weak spots which can be precisely evaluated in the field. The study highlights that proper classification of real ground points poses the key challenge. We show that photogrammetric routines to derive a high-resolution digital terrain model (DTM) are limited in the case of dense vegetation coverage. In such cases, LiDAR surveys have clear advantages even though they are also limited by very dense vegetation. We find that UAV data can be used for an objective method of estimating debris-flow torrent geometric properties. And the introduced approaches therefore build a stepping stone towards a more comprehensive, reproducible and objective assessment of torrent processes and predispositions. However, ground-referencing fieldwork remains essential, and further research on remote sensing supported hazard assessment of debris-flow-prone torrents is indispensable.

Accurate debris-flow modelling depends on the ability to simulate surging and pulsing behaviour. However, our understanding of these phenomena is starved of observational constraints. Here we propose the use of seismic measurements, which resolve the arrival of coarse-grained roll wave fronts in debris flows at Illgraben, Switzerland. Roll waves likely play a key role in flow pulses but are typically only observed with point measurements like depth gauges. We compare in-torrent force plate measurements with near-torrent seismic records and discuss how these data can test existing roll wave theories.

The mobility of debris flows is highly variable, which makes runout prediction challenging. Numerical runout models are often used to estimate inundation area, flow depth and velocity for debris-flow analyses. Runout models typically use initiation conditions that disregard the fact that debris flows commonly occur in multiple surges. In this work, we modified the Dan3D runout model to enable a more realistic hydrograph initiation condition. We analyzed a 2019 debris flow at Mount Currie in southwestern British Columbia to examine the effects of complex inflow hydrographs on model outputs. The impact area, flow depths and velocities were sensitive to the inflow hydrograph, particularly depths and velocities higher on the fan. The results from the case study demonstrated that, in addition to variations in material and rheological parameters, the simulated debris flow runout is sensitive to variations in the inflow hydrograph. The numerical model results are also interpreted using an empirical runout relationship to assess the plausibility of the numerical results.

Debris flows affect people and infrastructure around the world, and as a result, many numerical models and modelling approaches have been developed to simulate their impacts. Observations from instrumented debris-flow channels show that variability in inflow depth, velocity, and discharge in real debris flows is much higher than what is typically used in numerical simulations. However, the effect of this natural variability on numerical model outputs is not well known. In this study, we examine the effects of using complex inflow time series within a single-phase runout model utilizing a Voellmy flow-resistance model. The interactions between model topography and flow resistance were studied first using a simple triangular hydrograph, which showed that simulated discharges change because of local slopes and Voellmy parameters. Next, more complex inflows were tested using time series based on 24 real debris-flow hydrographs initiated from three locations. We described a simple method to scale inflow hydrographs by defining a target event volume and maximum allowable peak discharge. The results showed a large variation in simulated flow depths and velocities arising from the variable inflow. The effects of variable-inflow conditions were demonstrated in simulations of two case histories of real debris flows, where the variation in inflow leads to significant variations in the simulation outputs. The real debris-flow hydrographs were used to provide an indication of the range of impacts that may result from the natural variability in inflow conditions. These results demonstrate that variation in inflow conditions can lead to reasonable estimates of the potential variation in impacts.

Les laves torrentielles et les coulées de boue mettent en danger les personnes et les biens dans les régions montagneuses. Dans le cadre de la gestion intégrée des risques, des mesures de protection structurelles, organisationnelles ou du génie biologique peuvent être envisagées, en plus de la prise en compte des dangers naturels dans l’aménagement du territoire. La brièveté des délais d’alerte pour les laves torrentielles et les coulées de boue, leur impact élevé ainsi que les forces destructrices correspondantes sont des raisons suffisantes pour que les mesures structurelles doivent assumer une fonction de protection importante.
Les filets de protection contre les laves torrentielles et les coulées de boue ne sont disponibles sur le marché suisse que depuis quelques années et complètent l’éventail des mesures possibles. Ce type de protection se caractérise par la perméabilité des filets qui retiennent les matériaux solides les plus grossiers tout en laissant passer l’eau et les matériaux en suspension, et drainent ainsi les laves torrentielles et les coulées de boue. En plus de la rétention du charriage, il existe d’autres possibilités d’utilisation des barrières à filet, notamment pour la stabilisation d’un lit de torrent ou pour dévier le processus de danger. [...]

Debris flows and hillslope debris flows endanger people and property in mountainous regions. Within the framework of integral risk management, structural, organizational, or nature-based protection measures can be used, in addition to the considerations of natural hazards in spatial planning. The short warning times for debris flows and hillslope debris flows as well as their high impact and corresponding destructive forces are an important reason why structural measures must assume an important protective function.
Debris flow and hillslope debris flow protection nets have only been available on the market in Switzerland for a few years and complement the range of possible measures. A characteristic feature of this type of measure is the permeability of the nets, which retain the coarser solids, but allow water and suspended solids to flow through and thus drain the material. In addition to bedload retention, there are other possible applications for net barriers, e.g. for stabilizing the river bed or for redirecting the hazard process. [...]

Murgänge und Hangmuren gefährden in gebirgigen Regionen Personen und Sachwerte. Im Rahmen des integralen Risikomanagements können neben der Berücksichtigung von Naturgefahren in der Raumplanung bauliche, organisatorische oder ingenieurbiologische Schutzmassnahmen getroffen werden. Die kurzen Vorwarnzeiten für Murgänge und Hangmuren und deren hohen Einwirkungen mit entsprechender Zerstörungskraft sind ein wichtiger Grund dafür, dass bauliche Massnahmen eine wichtige Schutzfunktion übernehmen müssen.
Murgang- und Hangmurenschutznetze sind in der Schweiz erst seit einigen Jahren auf dem Markt verfügbar und ergänzen das Massnahmenspektrum. Eine charakteristische Eigenschaft dieses Massnahmentyps besteht in der Durchlässigkeit der Netze, welche die gröberen Feststoffe zurückhalten, Wasser und Schwebstoffe jedoch durchfliessen lassen und somit eine Drainage des Murgang- und Hangmurenmaterials bewirken. Neben dem Geschieberückhalt bieten sich weitere Einsatzmöglichkeiten für Netzbarrieren an, z. B. bei der Stabilisierung einer Bachsohle oder zur Umlenkung des Gefahrenprozesses.

Rock glaciers, tonguelike bodies consisting of angular rock debris frozen in interstitial ice, may flow with velocities varying from a few centimeter up to several meters per year. Recent ground warming may generate an accelerated slope movement related to permafrost creep.
Newer generations of commercial off-the-shelf drones like DJI Mavic are cost-effective, lightweight and still have a sufficient pay load reserve for flights at 2800 m.a.s.l. This allows that a two men crew can carry the necessary reference and drone equipment in a high alpine environment and a flight campaign can be conducted within one day.
Using different cameras with similar optical resolution allows to generate DSMs with comparable accuracy. The DSM resolution should be 10 cm or better to achieve robust results. Motion tracking of rock glacier with tie point matching algorithms SIFT or SURF combined with an appropriate filter method allows to distinguish different movement patterns within local neighborhood. Compared to point-wise GPS-based methods or image-based cross-correlation approaches, tie point matching allows to detect inhomogeneous movement patterns, which are typical for rock glaciers in a high alpine environment.

Dynamic debris-flow runout models are applied by practitioners (1) to generate hazard maps, (2) to help design mitigation measures such as dams and warning systems, (3) to explore potential impacts of rare events such as pro-glacial lake failure, and (4) to illustrate the hazard process to local decision makers and stakeholders. Automated observations of debris flows in several torrents have shown a large degree of variability in the flow process, ranging from fast muddy debris floods to relatively slow debris flows with granular fronts, at any given torrent. It is not yet possible to predict with certainty which type of debris flow can be expected in any given catchment. The prediction problem is amplified by the fact that topographic data are generally coarse (2m horizontal resolution in Switzerland) compared with typical channel widths (1–10 m) and because channel topography changes with time. These challenges and their impact on hazard assessment is illustrated using recent examples of typical applied projects. Herein we use the RAMMS debris-flow runout model. However, the general procedures presented here are applicable to other similar runout models. Common points in these examples include the accurate assessment of potential erosion, deposition, and avulsion along the channel, as well as the systematic modification of the friction coefficients in the model to account for variations in water content, sediment size, channel-bed roughness, and other properties of the flow. We mention the main challenges in these steps, as the exact procedure is still not clearly regulated in Switzerland. In general, it is desirable to work with several scenarios to account for multiple flow surges and the erosion and deposition produced by each surge, and uncertainties in the expected debris-flow type (granular vs. muddy). Thus, for the generation of appropriate scenarios, experience in the field and with the use of runout models is essential.

Debris flows represent one of the most dangerous types of mass movements, because of their high velocities, large impact forces and long runout distances. This review describes the available debris-flow monitoring techniques and proposes recommendations to inform the design of future monitoring and warning/alarm systems. The selection and application of these techniques is highly dependent on site and hazard characterization, which is illustrated through detailed descriptions of nine monitoring sites: five in Europe, three in Asia and one in the USA. Most of these monitored catchments cover less than ∼10 km² and are topographically rugged with Melton Indices greater than 0.5. Hourly rainfall intensities between 5 and 15 mm/h are sufficient to trigger debris flows at many of the sites, and observed debris-flow volumes range from a few hundred up to almost one million cubic meters. The sensors found in these monitoring systems can be separated into two classes: a class measuring the initiation mechanisms, and another class measuring the flow dynamics. The first class principally includes rain gauges, but also contains of soil moisture and pore-water pressure sensors. The second class involves a large variety of sensors focusing on flow stage or ground vibrations and commonly includes video cameras to validate and aid in the data interpretation. Given the sporadic nature of debris flows, an essential characteristic of the monitoring systems is the differentiation between a continuous mode that samples at low frequency ("non-event mode") and another mode that records the measurements at high frequency ("event mode"). The event detection algorithm, used to switch into the “event mode” depends on a threshold that is typically based on rainfall or ground vibration. Identifying the correct definition of these thresholds is a fundamental task not only for monitoring purposes, but also for the implementation of warning and alarm systems.

Im Illgraben bei Leuk (VS) wurde 2003 die weltweit erste Murgangwaage gebaut. Sie lieferte ab Sommer 2004 bis zu ihrer Zerstörung im Juli 2016 wertvolle Daten über die dynamischen Kräfte von Murgängen. Im Rahmen eines Investitionsprojektes der Eidg. Forschungsanstalt WSL wird nun eine ähnlich konzipierte Messeinrichtung am selben Standort unter der Kantonsstrassenbrücke über den Illgraben gebaut. Erfahrungen aus dem bisherigen Betrieb fliessen zusammen mit den Ergebnissen aus den Tests im Labor in eine optimierte optimierte Version ein, welche uns ab Mai 2019 weitere interessante Einblicke in die Fliessdynamik von Murgängen ermöglichen wird.

Identification of debris flows using satellite data, the results of numerical simulation of Irtash rainfall flow (Akhangaran River basin, Uzbekistan) and of possible glacial flow in the case of lakes outburst (Isfayramsay River basin, Kyrgyzstan) using RAMMS software are considered in the paper.