EGU21-9549
https://doi.org/10.5194/egusphere-egu21-9549
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

The role of sedimentology in the mobility of debris avalanches: Evidence from their deposits and granular flow experiments

Symeon Makris1, Irene Manzella1, Paul Cole1, and Matteo Roverato2
Symeon Makris et al.
  • 1University of Plymouth, Faculty of Science and Engineering, School of Geography, Earth and Environmental Sciences, United Kingdom of Great Britain – England, Scotland, Wales (symeon.makris@plymouth.ac.uk)
  • 2Department of Earth Sciences, University of Geneva, Geneva, Switzerland

Debris avalanches and lahars are among the most destructive and hazardous mass flows in volcanic environments making them important to understand from a hazard assessment perspective. Sedimentological characteristics of their deposits are important for assessing their propagation and emplacement mechanisms. Here, we compare the sedimentology of nine volcanic debris avalanches and eight lahars, by the descriptive statistics: median grain size, sand, gravel and finer particle proportion, skewness, and sorting.

Results suggest that lahars and debris avalanches diverge in their grain size distribution evolution during propagation, even when sourced from the same material. Increasing bimodality, evolution to negative skewness, with decreasing sediment size, accompanied by very poor sorting suggest comminution of particles due to particle-particle interactions in debris avalanches. Instead, preferential deposition of the coarsest particles and improved sorting suggest that the decrease in grain size of lahars is the result of debulking. The divergence is mainly caused by the high water content in lahars, which introduce different processes during propagation. This suggests, in agreement with previous studies, that debris avalanches can be considered as dense granular flows where the effect of inertial collisions of solid fragments are more important than fluid effects.

Present findings and previous sedimentological studies suggest that both volcanic and non-volcanic debris avalanches exhibit bimodal grain-size distributions, at least locally, in areas of high shear accommodation. Following these results, an experimental campaign has been carried out to test the effect of bimodality on the propagation of granular flows. These experiments are flows of bidisperse granular material on an initial inclined plane, with a horizontal accumulation surface at the bottom.  Findings confirm that the bimodality of the grain size distribution generates a more efficient shearing arrangement, which can increase the mobility of granular flows in the same way recorded in debris avalanche deposits.

How to cite: Makris, S., Manzella, I., Cole, P., and Roverato, M.: The role of sedimentology in the mobility of debris avalanches: Evidence from their deposits and granular flow experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9549, https://doi.org/10.5194/egusphere-egu21-9549, 2021.