Brian McArdell

International audienceGullies are kilometre-scale alcove-channel-fan systems that occur on steep slopes in mid-latitude to polar regions on Mars. They have formed by repeated deposition of hundreds to thousands of individual mass flows, which have been proposed to range from dry grain flows to aqueous debris flows. The most likely candidate for triggering these flows is slope failure in the gully-alcoves, which could be induced by sublimation of CO 2-ice (resulting in a dry flow) or by active-layer detachment (resulting in a wet flow). To understand the current-and paleo-climatic conditions on Mars, and the role of volatiles therein, the conditions during triggering of the gully-forming mass flows need to be understood. While final flow deposits can be observed on some gully-fans, morphological expressions of initial failures are generally absent. Terrestrial observations tell us that mass flows can erode large amounts of bed material when flowing down a slope, and therefore small-volume initial failures could form large mass-flow deposits on the gully fans. As such, final deposit volumes cannot directly inform us about initial failure volumes and conditions. We modified the RAMMS debris flow and avalanche model to permit its use under Martian conditions, and then used this model to back-calculate and infer the initial conditions of mass flows in multiple gullies across Mars for both dry and wet initial conditions. These simulations enabled us to constrain the initial failure volume as well as the flow conditions of mass flows in Martian gullies. We used the calculated initial flow volumes to constrain: 1) the total number of flow events required to form the studied gullies; 2) the amounts of CO 2 or water needed to trigger mass flows in these gullies, and 3) combined those to constrain the climatic conditions required for gully formation. In addition, we compared the frictional parameters of the flows in Martian gullies to those of a wide range of terrestrial mass flows, to determine which types of terrestrial mass flows best represent flows in Martian gullies and would form the best Martian analogues in future studies

Observations of debris-flow events all over the world cover a wide range of phenomenologically similar processes, consisting of different concentrations of water, fine and coarse sediment, and frequently wooden debris. For this reasons, empirically derived coefficients to be used in prediction models to estimate debris-flow dynamics often show a wide degree of scatter. Two of such empirically derived concepts, originally developed for pure water flows, are presented in this study, showing similar deviations from hydrostatic stress assumption in subcritical flow conditions. The first concept is used to estimate debris-flow velocities, based on superelevation data. Based on our experimental results as well as observations from real debris-flow events at the field monitoring station at Illgraben (canton Valais, Switzerland) we show that the empirical coefficient used in the superelevation equation to account for non-Newtonian flow effects correlates with the Froude number – the dimensi...

Wahrend der Unwetter im August 2005 wurden in Guttannen (BE) uber 500 000 m3 Geschiebe durch einen Murgang im Talboden abgelagert. Ein Ruckstau bzw. eine Verlagerung der Aare aus ihrem ursprunglichen Gerinne mit anschliessender Uberflutung des Dorfes waren die Folgen. Ausloser fur dieses ausserordentliche Ereignis stellten nebst den intensiven Niederschlagen auch die begunstigenden hydro geologischen Verhaltnisse im Anrissgebiet dar (Geologie, Geschiebepotenzial und Permafrost). Mittels eines Fliess- und Massenbewegungsmodells (MSF) sowie eines dynamisch- physikalischen Modellansatzes(RAMMS) wurde das Ereignis nachmodelliert, um einerseits Ruckschlusse bezuglich Plausibilitat und andererseits Hinweise auf die Kalibrierung des Modells zu erhalten. Die Resultate korrelieren dabei gut mit den tatsachlichen Ablagerungscharakteristiken. Die Modelle konnen daher fur unterschiedliche Fragestellungen (z.B. bei der Gefahrenkartierung) als wertvolle Unterstutzung dienen. Der Einsatz von RAMMS...

Debris flows typically entrain sediment along the flow path, thereby increasing the volume of the flow. The entrainment process is poorly understood, yet vitally important in hazard analysis. Here we investigate debris flow entrainment by comparing direct measurement of entrainment with entrainment estimated from sediment budget calculations at the Illgraben catchment in Switzerland. A sensor for measuring debris flow erosion

Recent channel changes caused by rock avalanches in the source area of a catchment near Preonzo, Switzerland, resulted in the unexpected bypass of existing mitigation structures. Two debris flow deflection dams were built in the source area to re-direct debris flows back into the old channel which connects to an existing debris flow retention basin. The torrent is monitored by an automated debris-flow observation station which provides information on front velocities and flow depths of torrential events such as debris flows and flash floods. Following construction of the deflection dams, the station was moved to the new channel and successfully recorded several debris-flow events. Numerical simulation was used to evaluate the effectiveness of such mitigation measures. Using the RAMMS (Rapid Mass Movements) simulation software, which describes the flow of debris using the 2D shallow water equations for granular flows and a Voellmy relationship for the friction, we describe simulation...

Infrasonic and seismic signals generated by debris flows and snow avalanches are observed by microphones and seismometers, respectively, in near field. The properties of the signals obtained are presented. For debris flows, infrasonic and seismic signals are correlated and their amplitudes show a relationship with flow depth and precipitation data. During the passing of a debris flow several surges identified

Avalanche deposits consist of rounded granules composed of aggregates of snow and ice particles. The size of the granules is related to vertical shear gradients within the flow; studying the granule-size distribution may be useful in understanding the flow and stopping of avalanches. We applied a sediment-size sampling method to measure snow granule-size distributions at different depositional environments on two dry and two wet avalanche deposits at three field sites. The granule-size distributions are approximately log-normal, similar to many natural sediment deposits. The median granule size in the wet and dry avalanches varies between 65 and 162 mm. Wet avalanches tend to produce more large granules than dry avalanches, indicating both smaller flow velocities and near-surface shear gradients. Granule size is similar in frontal lobes and levee deposits, suggesting that levee formation occurs independently of the size segregation at the avalanche front.

... Christoph Hegg,* Brian W. McArdell and Alexandre Badoux WSL, Swiss Federal Research Institute for Forest, Snow and Landscape, CH-8903 Birmensdorf ... Together with studies on the influence of aerial pollutants on the forest ecosystem initiated in the 1980s (Gassmann et al ...

Title: Change detection using terrestrial laser scanning in steep terrain and complex surface geometry - Survey planning, data processing and validation. Authors: Schürch, Peter;Densmore, Alex L.; Rosser, Nick J.; Lim, Michael; McArdell, Brian W. ...

To predict hazard-endangered areas and debris-flow velocity, a variety of physically-based numerical simulation models have been developed. In these models, the relatively large sediment particles such as boulders move as a laminar flow, but the interstitial fluid between sediments behaves like a turbulent flow. Moreover, several recent models assumed that fine sediments act as a fluid. This behavior of fine sediment is referred to as the “phase-shift” of fine sediment. However, because it is difficult to observe the phase-shift of fine sediment in the field, adequate data on the phase-shift of debris flow are still lacking. In the last two decades, intensive monitoring for debris flow has been conducted all over the world, and observations have dramatically increased. For example, in the Illgraben catchment, Switzerland, observations of bulk density, pore pressure, flow depth, front velocity, and temporal and spatial patterns of erosion due to debris flows are available. So, we use...

A new kind of mitigation measure against debris flows are flexible barriers. Compared to similar barriers against rockfall, where the impacting rock is well modelable by a rigid body, the interaction between the two-phase medium debris flow and the flexible barrier is quite unknown. It is therefore the aim to develop a load model that can be used for both dimensioning in practice and numerical simulation.

This study describes an investigation of channel-bed entrainment of sediment by debris flows. An entrainment model, developed using field data from debris flows at the Illgraben catchment, Switzerland, was incorporated into the existing RAMMS debris-flow model, which solves the 2-D shallow-water equations for granular flows. In the entrainment model, an empirical relationship between maximum shear stress and measured erosion is used to determine the maximum potential erosion depth. Additionally, the average rate of erosion, measured at the same field site, is used to constrain the erosion rate. The model predicts plausible erosion values in comparison with field data from highly erosive debris flow events at the Spreitgraben torrent channel, Switzerland in 2010, without any adjustment to the coefficients in the entrainment model. We find that by including bulking due to entrainment (e.g., by channel erosion) in runout models a more realistic flow pattern is produced than in simulati...

This study describes an investigation of channel-bed erosion of sediment by debris flows. An erosion model, developed using field data from debris flows at the Illgraben catchment, Switzerland, was incorporated into the existing RAMMS debris-flow model, which solves the 2-D shallow-water equations for granular flows. In the erosion model, the relationship between maximum shear stress and measured erosion is used to determine the maximum potential erosion depth. Additionally, the maximum rate of erosion, measured at the same field site, is used to constrain the erosion rate. The model predicts plausible erosion values in comparison with field data from highly erosive debris flow events at the Spreitgraben torrent channel, Switzerland in 2010, without any adjustment to the coefficients in the erosion model. We find that by including channel erosion in runout models a more realistic flow pattern is produced than in simulations where entrainment is not included. In detail, simulations w...

Does the forest provide protection from landslides? Evidence from the WSL Shallow Landslide Database During strong rainfall events, shallow landslides and debris avalanches (hillslope debris flows, or open-slope debris flows) are triggered and sometimes lead to considerable damage. Analysis of damage-causing events show that there are fewer landslides in forested areas compared to non-forested areas, which indicates the generally positive influence of forest vegetation on slope stability. However, these effects depend on the condition of the forest stand and quantification of the effects is difficult. Event documentation contributes to a better understanding of the relevant processes. The information obtained is not only important for the preparation of hazard maps, but also provides valuable insight for assessing the hazard protection provided by the forest. Data from the landslide database of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) were used to ev...

<p>The flow resistance in granular mass flows can be due to frictional contacts or collisional interactions between particles. For a constant number of particles, the transition from a frictional to a collisional regime is expected to depend on flow velocity and is associated with an increase of volume and a decrease of bulk density, an effect termed dilation. The relation between velocity, dilation and flow resistance is not well understood. Here we present results of steady, non-uniform flows of ceramic beads (d = 4 mm) in a rotating drum, a setup allowing observations and averaging of parameters measured over an extended period of time. We systematically varied flow mass between 12.3 and 49 kg and flow velocity between 0.2 and 1.2 m/s, while continuously measuring basal normal stress and flow depth. Flow resistance was assessed by calculating average bulk shear stress from torque measurements at the axis of the drum as well as from the deviation of the center of mass from the vertical. Additionally, the flows were captured by high-speed video recordings through the transparent side wall. We find bulk densities at the deepest section of the flow decreasing from 1430 kg/m³ at low velocities to 1370 kg/m³ at the highest velocity for the largest flow mass. At the same time flow resistance increased linearly. When the flow mass was reduced, also bulk density decreased, indicating the importance of overburden pressure for dilation. Video recordings revealed that shear is concentrated in depth zones of lower volume fraction. Our results shall contribute to a better understanding of the transition from a frictional to a collisional flow regime and may help to assess the importance of dilation for gravitational mass flows.</p>

<p>Debris flows can grow greatly in size and hazardous potential by eroding bed material, but effective hazard assessment and mitigation is currently hampered by limited understanding of erosion and deposition dynamics. We have collected high-resolution pre- and post-flow topography with drone-based photogrammetry in the Illgraben channel in the Swiss Alps. We present erosion and deposition patterns as a result of six debris flows and intensive subcatchment activity over a 3.3 km long unconsolidated reach with check dams, and interpret these erosion and deposition patterns with in-situ flow measurements. We show that the spatio-temporal patterns of erosion and deposition in natural debris-flow torrents are highly variable and dynamic. We identify a memory effect where erosion is strong at locations of strong deposition during previous flows and vice versa. Large sediment inputs from subcatchments initially result in new channel erosion through the subcatchments deposits and at the same time upstream deposition as a result of backwater effects. It is generally believed that erosion increases with debris-flow magnitude, but we show that there is a limit to debris-flow bulking set by channel geometry. Large flows that overtop their channel deposit large amount of sediment in levees and on overbanks, leading to net deposition despite strong thalweg erosion, and thus a decrease in flow volume. These findings provide key guidelines for flow volume forecasting, emphasizing the importance of memory effects and the need to resolve both erosion and deposition for accurate flow volume estimation.</p>

<div> <div> <div> <p>Large mass movements cause the surface of the earth to deform, depending on the spatial distribution and magnitude of the mass movement and the response of the ground. In volcanology, tilt measurements are used to study earth surface displacement during volcanic processes such as dyke intrusions and magma chamber collapses. Broadband and long period seismometers also record tilt signals at periods of tens to hundreds of seconds, with the horizontal components being most sensitive to tilt. To obtain tilt from seismic recordings the signal from true ground motion and from apparent ground motion due to tilt have to be seperated. Nevertheless, seismometers have shown similar sensitivities as tiltmeters and are, depending on the type of tiltmeter and study site, less cumbersome to install. In this study, we explore the capability of tilt measurements from surface tiltmeters and broadband seismic sensors to determine debris flow parameters like mass, density and flow velocity. We focus on seismic broadband data recorded within a few meters of the Illgraben torrent in Switzerland. Illgraben’s catchment is one of the most active mass wasting sites in the European Alps, producing several debris flows per year. Our seismic records show clear tilt signals from more than ten debris-flow events in 2018 and 2019, which we compare to data from large-scale laboratory experiments at the U.S. Geological Survey (USGS) debris-flow flume at which broadband seismometers and tiltmeters were installed for six 8-10 m<sup>3</sup> experiments in 2016.</p> <p>To explain our observations, we present a model for the loading response of a layered elastic half-space to a moving surface load. This model can be used to invert our observed tilt signals for the surface load, i.e., the mass, density and/or geometry of the debris flow. To verify our model, we use nearby force plate and flow height measurements at both study sites. We discuss to what extent and under which assumptions, compared to force plate installations, the relatively simple and inexpensive tilt measurements can be used to determine debris flow parameters, which to date require sophisticated equipment.</p> </div> </div> </div>

In mountain areas, mass movements, such as hillslope debris flows, pose a serious threat to people and infrastructure, although size and runout distances are often smaller than those of debris avalanches or in-channel-based processes like debris floods or debris flows. Hillslope debris-flow events can be regarded as a unique process that generally can be observed at steep slopes. The delimitation of endangered areas and the implementation of protective measures are therefore an important instrument within the framework of a risk analysis, especially in the densely populated area of the alpine region. Here, two-dimensional runout prediction methods are helpful tools in estimating possible travel lengths and affected areas. However, not many studies focus on 2D runout estimations specifically for hillslope debris-flow processes. Based on data from 19 well-documented hillslope debris-flow events in Switzerland, we performed a systematic evaluation of runout simulations conducted with t...

Three years of geotechnical seasonal field monitoring including soil temperature, suction and volumetric water content plus geophysical measurements, lead to a preliminary ground model and assessment of slope stability for a steep scree slope in the Meretschibach catchment, near Agarn village in the Swiss Alps. Building on data reported in a previous paper, which focused on preliminary ground characterisation and seasonal field monitoring, this current research aims to understand whether a surficial failure in the scree slope, triggered by rainfall and depending on bedrock conditions, would represent a relevant natural hazard for Agarn village. A final year of field data is included as well as site-specific sensor calibration, a Ground Penetrating Radar (GPR) profile, and laboratory triaxial testing to provide strength parameters. A bedrock map is presented, based on GPR, with a realistic ground model of the entire scree slope. Furthermore, a preliminary numerical analysis, performe...

<p>Debris flows are episodic strongly impacting gravitational currents of generally high density, consisting of mixtures of water and debris in varying proportions and occurring in steep mountain torrents with volumes commonly exceeding thousands of m<sup>3</sup>. Despite the observation that debris flows are among the most dangerous processes in mountain environments, the moderate flow velocities (typically < 10 m/s) make early warning in principle possible if the flows are detected early upon formation.</p> <p>Seismic and infrasound studies of debris flows rapidly increased in the last decade but focused mostly on event detectability and application as early-warning systems. Seismic networks, arrays of infrasound sensors and the combined use of a collocated single seismic and infrasound sensors, have turned out to be efficient systems for detecting the occurrence of debris flows in near-real time with a good reliability.</p> <p>However, open questions remain about the possibility to infer source characteristics and event magnitude from recorded geophysical signals. This requires theoretical source models of elastic energy radiated both in the ground, in the form of seismic waves, and in the atmosphere, in the form of infrasound.  Seismic waves are believed to be generated by both large sediment particles impacts on the channel bed and by turbulent structures within the debris flow. Infrasound is instead believed to be generated by standing waves at the free surface of the flow, but their source processes are not yet fully understood.</p> <p>Here we present preliminary results of a study performed at the Illgraben catchment (Switzerland), in the 2017-2019 period, combining infrasound and seismic signals with direct in-torrent measurements of flow depth and velocity. Seismic and infrasound signals are analyzed using both spectral analysis and array techniques, in order to achieve an improved understanding of the dynamics of the source mechanisms of the two wavefields. Comparison with in-situ measurements is used to extrapolate empirical relationships between signal features, e.g. amplitude, spectral content or waveform characteristics of both seismic signals and infrasound, and flow characteristics.</p> <p>The results obtained will possibly be used to develop an efficient monitoring system for remote detection and the early warning of debris flows using seismic signals and infrasound generated by the process.</p>

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