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The Illgraben is a 10 km² steep side valley located in Switzerland. This active debris flow catchment supplies 5-15% of the total sediment load of the Rhône River upstream of Lake Geneva. The 30-80° steep catchment slopes host frequent rock falls and slides. From 2012 to 2014, a network of up to ten Nanometrics Trillium Compact 120s broadband seismometers, sampled by Digos DataCube³ext loggers at 200 Hz (and later by centaur), was deployed in and around the catchment to monitor distributed geomorphic activity.

Debris flows can grow greatly in size and hazardous potential by eroding bed and bank materials. However, erosion mechanisms are poorly understood because debris flows are complex hybrids between a fluid flow and a moving mass of colliding particles, bed erodibility varies between events, and field measurements are hard to obtain. Here, we identify the key controls on debris-flow erosion based on a field data set that combines information on flow properties, bed conditions, and bed and bank erosion. We show that flow conditions and bed wetness jointly control debris-flow erosion. Flow conditions describing the cumulative forces exerted at the bed during an event best explain erosion. Shear forces and particle-impact forces are strongly correlated and act in conjunction in the erosion process. A shear-stress approach accounting for bed erodibility may therefore be applicable for modeling and predicting debris-flow erosion. This work provides a foundation for developing effective debris-flow erosion models.

Understanding the initial and flow conditions of contemporary flows in Martian gullies, generally believed to be triggered and fluidized by CO2 sublimation, is crucial for deciphering climate conditions needed to trigger and sustain them. We employ the RAMMS (RApid Mass Movement Simulation) debris flow and avalanche model to back calculate initial and flow conditions of recent flows in three gullies in Hale crater. We infer minimum release depths of 1.0–1.5 m and initial release volumes of 100–200 m3. Entrainment leads to final flow volumes that are ∼2.5–5.5 times larger than initially released, and entrainment is found necessary to match the observed flow deposits. Simulated mean cross‐channel flow velocities decrease from 3–4 m/s to ∼1 m/s from release area to flow terminus, while flow depths generally decrease from 0.5–1 to 0.1–0.2 m. The mean cross‐channel erosion depth and deposition thicknesses are ∼0.1–0.3 m. Back‐calculated dry‐Coulomb friction ranges from 0.1 to 0.25 and viscous‐turbulent friction between 100 and 200 m/s2, which are values similar to those of granular debris flows on Earth. These results suggest that recent flows in gullies are fluidized to a similar degree as are granular debris flows on Earth. Using a novel model for mass flow fluidization by CO2 sublimation we are able to show that under Martian atmospheric conditions very small volumetric fractions of CO2 of ≪1% within mass flows may indeed yield sufficiently large gas fluxes to cause fluidization and enhance flow mobility., Key Points We use RAMMS to back calculate initial and flow conditions of recent flows in three gullies in Hale craterRecent flows in gullies are fluidized to a similar degree as are wet granular debris flows on EarthVery small volumetric fractions of CO2 of ≪1% may yield sufficiently large gas fluxes to fluidize contemporary flow in Martian gullies

We studied the run-up behavior of debris flows in a small-scale experimental flume using various material compositions, approach flow velocities, and geometries of the obstacle. The experiments were performed with a straight and 4 m to 6 m long flume channel with a circular cross-section of 15 cm top width. The debris flows were released from a head tank. We used three debris-flow mixtures, three channel slopes and either a vertical wall or an adverse slope of 30�� as an obstacle. Additional tests were performed using water without sediment. The inclined channel was followed by a 30 cm long horizontal transition reach. Immediately upstream and along this reach we measured the shape of the approaching debris-flow surge with four laser sensors, and determined also the approach flow velocity and depth. The run-up conditions were recorded with a high-speed video camera. The measured run-up conditions were compared with four different theoretical models. The observed run-up conditions differed to some extent between debris-flow mixtures and clear water flows, and there were also some differenced among the debris flow mixtures depending on the relative proportion of coarse particles. The observed run-up heights were generally within the range predicted by the theoretical models, but none of them appears to be universally applicable to the entire range of investigated flow conditions. The commonly used energy principle is not always a conservative method to estimate run-up heights, as has been reported in previous studies.

Here, we present validation tests of the fluid dynamic solver presented in von Boetticher et al. (2016), simulating both laboratory-scale and large-scale debris-flow experiments. The new solver combines a Coulomb viscoplastic rheological model with a Herschel–Bulkley model based on material properties and rheological characteristics of the analyzed debris flow. For the selected experiments in this study, all necessary material properties were known – the content of sand, clay (including its mineral composition) and gravel as well as the water content and the angle of repose of the gravel. Given these properties, two model parameters are sufficient for calibration, and a range of experiments with different material compositions can be reproduced by the model without recalibration. One calibration parameter, the Herschel–Bulkley exponent, was kept constant for all simulations. The model validation focuses on different case studies illustrating the sensitivity of debris flows to water and clay content, channel curvature, channel roughness and the angle of repose. We characterize the accuracy of the model using experimental observations of flow head positions, front velocities, run-out patterns and basal pressures., Geoscientific Model Development, 10 (11), ISSN:1991-9603, ISSN:1991-959X

International audience; Trois modèles bidimensionnels (2D) de simulation des laves torrentielles sont appliqués à deux important événements de laves torrentielles, bien documentés, qui ont causé d'importants dépôts de matériau solide sur le cône de déjection. Les modèles sont fondés sur une rhéologie de Voellmy traduisant une pseudo-turbulence et des contraintes de frottement au fond, une formulation quadratique de la rhéologie incluant les contraintes de Bingham, collisionnelles et turbulentes et une rhéologie de Herschel-Bulkley représentant un fluide viscoplastique. Les paramètres rhéologiques ou de frottement des modèles sont supposés a priori ou calibrés en référence aux observations de terrain. Les trois modèles sont capables de reproduire raisonnablement les caractéristiques des dépôts sur le cône de déjection, après un calage utilisant les données historiques. Une représentation précise de la topographie du chenal et du cône de déjection est particulièrement importante pour obtenir une bonne reproduction des caractéristiques des dépôts observés.

Rapporto Finale del Progetto EU THARMIT

Grains of a single size within a mixed-size bed are entrained over a range of flows. Within this range some grains exposed on the bed surface are active (entrained at least once over the duration of a transport event), while the remaining surface grains are immobile, a condition we define as partial transport. We demonstrate the existence and domain of partial transport using observations of grain entrainment on time series of bed photographs of flume experiments with a widely sorted sand/gravel mixture. The active proportion of the bed surface increases with bed shear stress τ0. At a given τ0, 90% of the active grains are entrained when the cumulative mass transported exceeds approximately 4 times the active mass on the bed. Mobilization of grains in a size fraction increases from 10% to 90% over a range of τ0 of a factor of 2. The bounds of this range increase with grain size Di so that at a given τ0, sizes over a range of a factor of 4 are in a state of partial transport. Fractional transport rates are independent of Di for fully mobilized fractions and decrease rapidly with Di for partially mobile fractions. Partial transport is associated with substantial transport rates of finer, fully mobile sizes, limits both the rate and size distribution of grain exchange with the bed subsurface, and may be the dominant transport condition in many gravel-bed rivers.

Twenty-eight coupled observations of flow, transport, and bed surface grain size distribution were made in a laboratory flume using a wide range of flows and a sediment with a very poorly sorted, bimodal grain size distribution. These observations permit the transport rates of individual size fractions to be scaled by the proportion of each size immediately available for transport on the bed surface. The key to our observations is the use of a sediment in which each size fraction has been painted a different color, which permits reliable, repeatable, and nondestructive measurement of the bed surface grain size distribution from photographs of the bed surface. At a given flow, the fractional transport rates may be divided into two parts: a finer-grained portion within which fractional transport rates are a function only of their proportion on the bed surface and total transport rate, and a coarser-grained portion for which fractional transport rates also depend on the proportion of individual grains within a fraction that remain essentially immobile throughout the experimental run. We define the latter condition as one of partial transport and observe that the grain size separating partial and fully mobilized transport consistently increases with flow strength. Complete mobilization of a size fraction occurs at roughly twice the shear stress necessary for incipient motion ofthat fraction. Zones of partial and full mobility are quite distinct when fractional transport rates are scaled by the bed surface grain size distribution, although a region of partial transport is evident when these data and other experimental and field observations are scaled by the bulk grain size distribution of the sediment bed. Critical shear stresses for the incipient motion of individual fractions in our experimental sediment vary over an order of magnitude, a result strongly in contrast to many earlier observations, but consistent with our observations of incipient motion in sediments with bimodal grain size distributions.