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[1] Experimental studies indicate that crystal-bearing magma exhibits non-Newtonian behavior at high strain rates and solid fractions. We use a zero-dimensional (0-D) inversion model to reevaluate rheological parameters and shear heating effects from laboratory data on crystal-bearing magma. The results indicate non-Newtonian behavior with power law coefficients of up to n = 13.5. It has been speculated that finite strain effects, shear heating, power law melt rheology, or plasticity are responsible for this non-Newtonian behavior. We use 2-D direct numerical crystal-scale simulations to study the relative importance of these mechanisms. These simulations demonstrate that shear heating has little effect on aggregate (bulk) rheologies. Finite strain effects result in both strain weakening and hardening, but the resulting power law coefficient is modest (maximum n = 1.3). For simulations with spherical crystals the strain weakening and hardening behavior is related to rearrangement of crystals rather than strain rate related weakening. Finite strain effects were insignificant in a numerical simulation with naturally shaped crystals. Strain partitioning into the melt phase may induce microscopic stresses that are adequate to provoke a nonlinear viscous response in the melt. Large differential stresses and low effective stresses revealed by the simulations are sufficient to cause crystals to fail plastically. Numerical experiments that account for plastic failure show large power law coefficients (n ≈ 50 in some simulations). We conclude that this effect is the dominant cause of the strong nonlinear viscous response of crystal-bearing magmas observed in laboratory experiments.

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Abstract. The erosion of and depositions on channel bed surfaces are instrumental to understanding debris flow processes. We present an overview of existing field methods and highlight their respective advantages and disadvantages. Terrestrial laser scanning (TLS), airborne laser scanning (ALS), erosion sensors, cross sections (CS) and geomorphological mapping are compared. Additionally, two of these approaches (i.e. TLS and CS) are tested and applied in the channel reaches of the torrent catchments. The results of the comparison indicate that the methods are associated with variable temporal and spatial resolution as well as data quality and invested effort. TLS data were able to quantify small-scale variations of erosion and deposition volumes. While the same changes could be detected with CS and geomorphological mapping, it was only possible with lower precision and coarser spatial resolution. The study presents a range of potential methods that can be applied accordingly to address the objectives and to support the analyses of specific applications. The availability of erosion data, acquired mainly by TLS and ALS, in combination with debris-flow monitoring data, provides promising sources of information to further support torrent risk management.