Dr. Brian McArdell

Debris flows are one of the most damaging natural hazards in mountainous terrain. Their dynamics are controlled by both surging behaviour and the influence of large boulders. However, a lack of high-resolution field measurements has limited our mechanistic understanding of these important processes. Here, we provide high-resolution in situ debris-flow surge measurements that demonstrate that surges are formed by the spontaneous growth of small surface instabilities into large waves, which amplify the destructiveness of the flow by increasing peak discharge. We use our field measurements to invert for the effective basal friction experienced by the flow, and support this reconstruction using numerical simulations that reproduce the formation and propagation of the surges. Detailed analysis of the inverted frictional data further shows that large boulders in the flow can influence local flow dynamics by increasing basal resistance, but this is not required to drive the surge wave instability. Our analysis provides new insights into debris-flow dynamics and can provide the foundation for improved hazard management of these damaging processes.

Given highly engineered Alpine ecosystems with monocultures and channelized streams, this project proposes radical changes to enable increased ecosystem resilience and societal wellbeing. We propose to rethink 1. management by including ecological and socio-economic aspects and 2. research by integrating natural, engineering, and social sciences. In this inter- and transdisciplinary project, we develop qualitative scenarios as storylines for future Alpine watershed and forest management. These scenarios serve as parameters for, on the one hand, the biophysical modelling of ecological quality (biodiversity, ecosystem function, ecological integrity) in Swiss case study regions. On the other, we assess residents’ preferences for the scenarios in relation to Nature’s Contributions to People and aspects of justice. We thus use science integration and a participatory scenario process to enable integration across disciplines as well as co-create knowledge with stakeholders throughout the research process. While this approach facilitates working across disciplinary boundaries and includes stakeholders from the onset, it also comes with challenges: finding a common language across disciplines and engaging meaningfully with stakeholders takes time and simultaneously does not cater to the traditional metrics in academia.

Interdisciplinary research is essential to address the complex environmental challenges faced by social-ecological systems (SES). However, it is often hindered by difficulties in integrating diverse knowledge and perspectives. Conceptual Frameworks (CFs) can act as boundary objects, facilitating integration in contexts with incomplete knowledge, nonlinearity, and divergent interests. Yet, guidance on developing CFs remains limited. To address this gap, we develop a structured approach and apply it within a research project focused on enhancing the resilience of Swiss Alpine ecosystems. Our approach includes three phases: (1) defining boundary concepts, (2) developing a CF as a boundary object, and (3) using the CF as a boundary object. The resulting CF supports communication, collaboration, and integration across disciplines, advancing SES research that addresses ecological resilience and sustainability. Our approach can be used by other interdisciplinary teams aiming to develop adaptable CFs that facilitate knowledge integration across disciplines.

Debris flows surge down slopes as fully shearing and agitated mixtures, which are frequent and destructive mass movement processes. The prime control on debris-flow hazard is flow volume, which can dramatically increase by the erosion and incorporation of bed sediment. However, an erosion theory for debris-flow volume growth regulated by basal shear and particle collision is currently absent. Here, we establish a mechanistic framework that incorporates basal shear stress, collisional stress, and attendant bed pore-fluid pressure (PP) based on the Coulomb criterion. We identify mechanical controls on bed erosion and validate the proposed model against laboratory experiments and field measurements at Illgraben catchment. The results show that shear and collision stresses collectively regulate the erosion process where debris flows interact with wet sediments with elevated bed PP. In contrast, collision stress is solely responsible for the erosion of relatively dry sediment with low PP because basal shear stress is not enough to overcome shear resistance. The bed PP facilitates progressive scour of the sediment through reducing bed resistance for shear traction and increasing penetration depth in the sediment for collision traction. Our theory can improve predictions of flow volume and hazardous impact for debris flows and offer implications for the erosion processes of other dense granular flows including pyroclastic flows and snow avalanches.

The experimentally based µ(I) rheology is now prevalent in describing the movement of gravitational mass flows. We reinterpret the µ(I) rheology as a Voellmy-type relationship to highlight its connection to grain flow theory and demonstrate its practical applications. Using one-dimensional block modeling and two real-world case studies – the 2017 Piz Cengalo rock–ice avalanche and an experimental snow avalanche at the Swiss Vallée de la Sionne test site – we demonstrate the relationship between the dimensionless number I and the granular temperature R, establishing the equivalence between µ(I) and widely used Voellmy-type grain flow rheologies µ(R). Results indicate that µ(I) rheology utilizes the dimensionless inertial number I to mimic contributions of granular temperature/fluctuation energy to flow behavior. In terms of Voellmy, the µ(I) rheology contains a velocity-dependent turbulent friction coefficient that models shear-thinning behavior. This turbulent friction assumes the production and decay of fluctuation energy are in balance, exhibiting no difference during accelerative and depositional phases of avalanche flow. The constant Coulomb friction coefficient prevents µ(I) rheology from accurately modeling the dispositional characteristics of actual mass flows. The modeled evolution of the snow avalanche using the µ(I) rheology is too slow, lagging 5 s behind the measured values. More importantly, the calculated runout extends approximately 200 m beyond the observed limits, with significant deposit anomalies in the valley. By incorporating non-steady production and decay of fluctuation energy in the µ(R) framework, it becomes possible to achieve a good match with both the measured velocities and the observed runout. Our results highlight the strengths and limitations of both µ(I) and Voellmy µ(R) rheologies, bolstering the theoretical foundation of mass flow modeling while revealing practical engineering challenges.

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.

Debris flows are important processes for the assessment of natural hazards due to their damage potential. To assess the impact of a potential debris flow, parameters such as the flow velocity, flow depth, maximum discharge, and volume are of great importance. This study uses data from the Illgraben observation station in the central Alps of Switzerland to explore the relationships between these flow parameters and the debris flow dynamics. To this end, we simulated previous debris flow events with the RAMMS::Debrisflow (Rapid Mass Movement Simulation::Debrisflow) runout model, which is based on a numerical solution of the shallow water equations for granular flows using the Voellmy friction relation. Here, the events were modelled in an effort to explore possible controls on the friction parameters μ and ζ, which describe the Coulomb friction and the turbulent friction, respectively, in the model. Additionally, sediment samples from levee deposits were analysed for their grain size distributions (14 events) and their mineralogical properties (4 events) to explore if the properties of the fine-grained matrix have an influence on the debris flow dynamics. Finally, field data from various debris flows such as the flow velocities and depths were statistically compared with the grain size distributions, the mineralogical properties, and the simulation results to identify the key variables controlling the kinematics of these flows. The simulation results point to several ideal solutions, which depend on the Coulomb and turbulent friction parameters (μ and ζ, respectively). In addition, the modelling results show that the Coulomb and turbulent frictions of a flow are related to the Froude number if the flow velocity is <6-7 m s^-1. It is also shown that the fine-sediment grain size or clay-particle mineralogy of a flow neither correlates with the flow's velocity and depth, nor can it be used to quantify the friction in the Voellmy friction relation. This suggests that the frictional behaviour of a flow may be controlled by other properties such as the friction generated by the partially fluidised coarse granular sediment. Yet, the flow properties are well-correlated with the flow volume, from which most other parameters can be derived, which is consistent with common engineering practice.

Debris flows and floods generated by rainfall runoff occur in rocky mountainous landscapes and burned steeplands. Flow type is commonly identified post-event through interpretation of depositional structures, but these may be poorly preserved or misinterpreted. Prior research indicates that discharge magnitude is commonly amplified in debris flows relative to floods due to volumetric bulking and increased frictional resistance. Here, we use this flow amplification to develop a metric (Q*) to separate debris flows from floods based on the ratio of observed peak discharge to the theoretical maximum water discharge from rainfall runoff. We compile 642 observations of floods and debris flows and demonstrate that Q* distinguishes flow type to ∼92% accuracy. Q* allows for accurate identification of debris flows through simple channel cross-section surveys rather than through qualitative interpretation of deposits, and therefore should increase the performance of models and engineered structures that require accurate flow-type observations.

Debris-flow volumes can increase along their flow path by entraining sediment stored in the channel bed and banks, thus also increasing hazard potential. Theoretical considerations, laboratory experiments and field investigations all indicate that the saturation conditions of the sediment along the flow path can greatly influence the amount of sediment entrained. However, this process is usually not considered for practical applications. This study aims to close this gap by combining runout and hydrological models into a predictive framework that is calibrated and tested using unique observations of sediment erosion and debris-flow properties available at a Swiss debris-flow observation station (Illgraben). To this end, hourly water input to the erodible channel is predicted using a simple, process-based hydrological model, and the resulting water saturation level in the upper sediment layer of the channel is modelled based on a Hortonian infiltration concept. Debris-flow entrainment is then predicted using the RAMMS debris-flow runout model. We find a strong correlation between the modelled saturation level of the sediment on the flow path and the channel-bed erodibility for single-surge debris-flow events with distinct fronts, indicating that the modelled water content is a good predictor for erosion simulated in RAMMS. Debris-flow properties with more complex flow behaviour (e.g., multiple surges or roll waves) are not as well predicted using this procedure, indicating that more physically complete models are necessary. Finally, we demonstrate how this modelling framework can be used for climate change impact assessment and show that earlier snowmelt may shift the peak of the debris-flow season to earlier in the year. Our novel modelling framework provides a plausible approach to reproduce saturation-dependent entrainment and thus better constrain event volumes for current and future hazard assessment.

Les laves torrentielles peuvent gagner considérablement en volume et donc en potentiel de menace par l'érosion le long de leur parcours. Il est prouvé que ces processus d'érosion sont liés aux conditions de saturation des sédiments avant l'événement. De tels facteurs d'influence hydrologiques n'ont toutefois pas encore été explicitement pris en compte dans la modélisation des laves torrentielles. Le présent article présente une approche permettant de combler cette lacune.
Le programme de simulation utilisé (RAMMS) est en mesure de bien représenter les caractéristiques d'écoulement et d'érosion pour une majorité des laves torrentielles. De plus, dans ces cas, il est possible d'établir une relation claire entre l'érodabilité et la saturation précédente du parcours d'écoulement. La méthode présentée constitue une base prometteuse pour une évaluation plus complète des processus de danger ainsi que de leur influence par le changement climatique.

Murgänge können durch Erosion entlang ihres Fliessweges erheblich an Volumen und damit an Gefährdungspotenzial gewinnen. Diese Erosionsprozesse hängen erwiesenermassen mit den Sättigungsbedingungen des Sediments vor dem Ereignis zusammen. Solche hydrologischen Einflussfaktoren werden bei der Murgangmodellierung bisher jedoch nicht explizit berücksichtigt. Im vorliegenden Beitrag wird ein Ansatz vorgestellt, um diese Lücke zu schliessen.
Das verwendete Simulationsprogramm (RAMMS) ist in der Lage, für eine Mehrheit der Murgangereignisse die Fliess- und Erosionseigenschaften gut abzubilden. Darüber hinaus kann in diesen Fällen ein klarer Zusammenhang zwischen der Erodierbarkeit und der vorhergehenden Sättigung des Fliesspfades festgestellt werden. Die vorgestellte Methode stellt eine vielversprechende Grundlage für eine umfassendere Beurteilung von Gefahrenprozessen sowie deren Beeinflussung durch den Klimawandel dar.

Glacier lake outburst floods (GLOFs) initiate with the rapid outburst of a glacier lake, endangering downstream populations, land, and infrastructure. The flow initiates as a mud flow; however, with the entrainment of additional solid material, the flood will often transform into a debris flow. As the run-out slope flattens, the coarse solid material deposits and the flow de-waters. The flow transforms back into a muddy, hyperconcentrated flow of fine sediments in suspension. These flow transitions change the flow composition dramatically and influence both the overall mass balance and flow rheology of the event. In this paper, we apply a two-phase/layer model to simulate flow transitions, solid–fluid phase separations, entrainment, and run-out distances of glacier lake outburst floods. A key feature of the model is the calculation of dilatant actions in the solid–fluid mixture which control flow transitions and phase separations. Given their high initial amount of fluid within the flow, GLOFs are sensitive to slope changes inducing flow transitions, which also implies changes in the flow rheology. The changes in the rheology are computed as a function of the flow composition and do not need any adaptation by ad-hoc selection of friction coefficients. This procedure allows the application of constant rheological input parameters from initiation to run-out. Our goal is to increase the prediction reliability of debris flow modeling. We highlight the problems associated with initial and boundary (entrainment) conditions. We test the new model against the well-known Lake 513 (Peru, 2010), Lake Palcacocha (Peru, 1941), and Lake Uchitel in the Aksay Valley (Kyrgyzstan) GLOF events. We show that flow transition modeling is essential when studying areas that have significant variations in slope.

In vielen Einzugsgebieten finden sich entlang von Wildbächen zahlreiche vegetationslose Steilhänge. Aus diesen Flächen wird durch Oberflächen- und Rinnenerosion sowie Steinschlag und flachgründige Rutschungen kontinuierlich Lockermaterial in die Gerinne verlagert. Während Hochwasserereignissen wird das Material dann als Geschiebe talwärts transportiert und kann zu Überschwemmungen und Übersarungen in besiedelten Gebieten führen. Im Rahmen einer Machbarkeitsstudie wurden auf einer unverbauten Teilfläche eines Rutsch- und Erosionsgebietes bei Dallenwil NW Untersuchungen zu Erosion in steilen Bacheinhängen durchgeführt. Ziel war dabei, wichtige Einflussgrössen auf die Materialverlagerung zu ermitteln und eine Quantifizierung auf verschiedenen Skalen zu ermöglichen. Zum Einsatz kamen dabei eine mobile Beregnungsanlage, Sedimentfallen, Drohnen sowie ein terrestrischer Laserscanner. Die Beregnungsanlage stiess im steilen Untersuchungsgebiet mit grobkörnigen Oberflächen an ihre Grenzen. Dennoch konnten insgesamt 24 Versuche auf Flächen von je 25 mal 25 cm durchgeführt werden. Die Parameter Oberflächenabfluss und Deckungsgrad der Vegetation erwiesen sich dabei als wichtige Einflussgrössen für den Abtrag von Bodenmaterial. Für die Messung von Erosion in den bachnahen Hängen wurden jeweils im Sommerhalbjahr anfänglich vier, und später zwei Sedimentfallen mit Flächen von je etwa 30 m2 betrieben. Diese Einrichtungen haben sich im Grossen und Ganzen bewährt. Im sehr steilen Untersuchungsgebiet mit den verschiedenen, teils enorm aktiven Verlagerungsprozessen traten jedoch auch Probleme auf, insbesondere infolge von Materialeinträgen von ausserhalb der abgegrenzten Flächen in Rinnen und durch Steinschlag. Wie zu erwarten war, erwies sich die Regenmenge hinsichtlich Intensität der Verlagerung als zentrale Einflussgrösse. Bei der Ermittlung von Erosion auf der Skala des ungefähr 0.6 ha grossen Untersuchungsgebietes mittels digitalen Terrainmodellen wurden ausreichende Genauigkeiten erreicht. Der LiDAR-Sensor eignete sich besser als die photogrammetrische Auswertungen und das Laserscanning, insbesondere aufgrund der geringeren Beeinträchtigungen durch die Vegetation. Bereiche mit Erosion bzw. Ablagerung waren durch die halbjährlich durchgeführten Erhebungen klar erkennbar. Beispielsweise konnten auch grössere Abbrüche oder flachgründige Rutschungen identifiziert werden. Vom Frühsommer 2021 bis März 2024 wurden im Untersuchungsgebiet insgesamt etwa 1730 m3 Bodenmaterial erodiert. Die Ablagerungen im Bachbett betrugen für den gleichen Zeitraum rund 370 m3. Anhand der DTM-Auswertungen konnten ferner saisonale Regimeänderungen in den Rinnen mit mehrheitlich Ablagerung im Winter und Erosion im Sommer nachgewiesen werden. Durch die mehrjährige Beobachtung des Untersuchungsgebietes ergaben sich neben den Messwerten wertvolle Beobachtungen und Erkenntnisse über die verschiedenen Verlagerungsprozesse in den steilen Bacheinhängen und über deren Zusammenwirken.

In many catchment areas, steep unvegetated slopes along torrent channels are subject to surface and rill erosion, as well as rockfall and shallow landslides, which deliver sediment to the torrent channels. During floods, the sediment is transported downstream and can lead to flooding and overbank sedimentation in populated areas. As part of a feasibility study, investigations into erosion on steep slopes were carried out on a hillslope without construction or other mitigation measures near Dallenwil NW. The aim was to determine dominant factors controlling sediment delivery and to quantify the rates at various spatial scales. A mobile sprinkler system, sediment traps, drones and a terrestrial laser scanner were used. The sprinkler system was at the upper end of its range of applicability on the slopes and with coarse-grained surfaces. Nevertheless, 24 measurements could be carried out on areas of 25 x 25 cm each. Surface runoff and vegetation cover proved to be important factors influencing the removal of sediment by rainfall. To measure erosion on the hillslopes close to the stream, initially four and later two sediment traps, each with an area of around 30m2, were operated during the summer months. These installations proved to be generally successful. However, problems were encountered on the steep and sometimes extremely active hillslopes, especially due to sediment from upslope areas entering the plots through gullies and rockfall. As expected, the amount of rainfall was the main factor influencing the intensity of transport. Sufficient accuracy was achieved in measuring erosion at the scale of the ~0.6 ha study area using a comparison between digital terrain models. The LiDAR sensor was more suitable than the photogrammetric evaluations and laser scanning, particularly due to the lower interference from vegetation. Areas of net erosion or deposition were clearly visible in the semiannual surveys. For example, major slumps or shallow landslides could be identified. From early summer 2021 to March 2024, a total of ~1730m3 of sediment was eroded from the study area. Deposits in the streambed were ~370m3 for the same period. Based on the DTM evaluations, seasonal regime changes in the channels with mainly deposition in winter and erosion in summer could also be detected. In addition to the measured values, the observation of the study area over several years provided valuable observations and insights into the various sediment transport processes on the steep stream slopes and their interaction.

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.

Pore fluid plays a crucial role in many granular flows, especially those in geophysical settings. However, the transition in behaviour between dry flows and fully saturated flows and the underlying physics that relate to this are poorly understood. In this paper, we report the results of small-scale flume experiments using monodisperse granular particles with varying water content and volume in which the basal pore pressure, total pressure, flow height and velocity profile were measured at a section. We compare the results with theoretical profiles for granular flow and with flow regimes based on dimensional analysis. The runout and the centre of mass were also calculated from the deposit surface profiles. As the initial water content by mass was increased from zero to around 10%, we first observed a drop in mobility by approximately 50%, as surface tension caused cohesive behaviour due to matric suction. As the water content was further increased up to 45%, the mobility also increased dramatically, with increased flow velocity up to 50%, increased runout distance up to 240% and reduced travel angle by up to 10° compared to the dry case. These effects can be directly related to the basal pore pressure, with both negative pressures and positive pore pressures being measured relative to atmospheric during the unsteady flow. We find that the initial flow volume plays a role in the development of relative pore pressure, such that, at a fixed relative water content, larger flows exhibit greater positive pore pressures, greater velocities and greater relative runout distances. This aligns with many other granular experiments and field observations. Our findings suggest that the fundamental role of the pore fluid is to reduce frictional contact forces between grains thus increasing flow velocity and bulk mobility. While this can occur by the development of excess pore pressure, it can also occur where the positive pore pressure is not in excess of hydrostatic, as shown here, since buoyancy and lubrication alone will reduce frictional forces.

Estimating flow velocities is key to assessing hazards associated with debris flows. One approach to post-event velocity estimation is the superelevation method, which uses debris-flow mudlines to measure the cross-channel surface inclination, or superelevation, produced by centripetal forces acting on the flow in a bend. Flow velocities are then calculated using a subjective parameterization of the forced vortex equation modified to include a debris-flow specific correction factor. Subjective parameterization of this equation leads to substantial variability and uncertainty in the resulting flow velocities. We present an analysis of the reliability of the superelevation method using a large UAV-based data set of 14 debris flows with front velocities of ∼0.8–6.5 m s⁻¹ and cross-channel surface inclinations of ∼0.6–8.5°, as well as a validation for a single debris flow measured using high-resolution, high-frequency 3D lidar data fused to video imagery. The validation event indicates that when the flow surface inclination can be measured directly, the forced vortex equation produces excellent results without needing a correction factor for Froude numbers ranging from 0.7 to 1.5. This finding indicates that the main challenge with the superelevation method lies in obtaining accurate measurements of superelevation from the mudlines, and that a correction factor may serve to compensate for measurement difficulties rather than variable flow properties. For very small and highly subcritical flows, the superelevation method may generate a large overestimation of flow velocities.

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.

We present the combined efforts of a research network designed to address the many challenges in the experimental modelling of debris flow phenomena. The approach has been to use apparatuses of different functional arrangement and at different scale with identical and commonly sourced flow materials from the highly idealised (dry, coarse and uniform) to the highly complex (well graded, segregating, fluid saturated). Here we briefly present some key findings of the network and point to the research questions that are currently being addressed. This complementary view of experimental debris flows helps to constrain methodological artefacts/scale effects and to identify key processes responsible for the diverse appearance and often high mobility of debris flows.

Debris flows entrain sediments and water along their flow path and grow significantly in size. Because the entrainment process isn’t well understood and data is rare, hazard and risk assessment with numerical models is challenging. It is known, however, that both for debris flows and rock avalanches, interstitial pore water in flow path substrate can cause increases in pore water pressures when overridden by the flow, which enhances erosion. The entrained water likely also plays a role in the process transition from rock avalanche to debris flow, like in the Pizzo Cengalo event in 2017. We present a framework for producing entrainment maps serving as an input for runout modelling, here illustrated using RAMMS. The entrainment maps consist of spatially-distributed entities with properties such as max erosion depth, and soil water content inferred from land cover and lithology maps. This study serves as a basis for producing duration curves of subsurface water available for entrainment and include it into the entrainment maps. Such hydrologically-informed entrainment maps will be useful to assess the probability of certain runout distances.