Landslide boost from entrainment of erodible material along the slope
Abstract
Landslides, debris flows, pyroclastic flows and avalanches are natural hazards that threaten life and property in mountainous, volcanic, coastal and seismically active areas. The granular mass tends to accelerate as gravity pulls it down the slope, and will slow on more gentle slopes, when interaction forces dissipating energy overcome the driving forces. The entrainment of underlying sediments or debris into the gravitational granular flows is suspected to be critical to their dynamics, but direct measurement of material entrainment in natural flows is very difficult. Nevertheless, qualitative and quantitative field observations suggest that material entrainment can either increase or decrease flow velocity and deposit extent, depending on the geological setting and the type of gravitational flow. Based on laboratory experiments on dry granular flows, we show here that erosion of granular material already present on the bed can significantly increase the size and mobility of the flow and possibly generate surges. We present laboratory experiments of granular material flowing over an inclined plane covered by an erodible bed, designed to mimic erosion processes of natural flows traveling over deposits built up by earlier events. The controlling parameters are the inclination of the plane and the thickness of the erodible layer. Different methods are used to prepare the erodible bed, thus leading to various degrees of compaction. We show that erosion processes increases the flow mobility (i. e. runout) by up to 40 % over slopes with inclination close to the repose angle of the grains. The effect is observed even for very thin erodible beds. We demonstrate that the increase of mass of the flowing grains caused by entrainment of the erodible layer is not enough to explain the observed increase in velocity and runout of the granular mass. Erosion efficiency is shown to strongly depend on the slope and on the nature (i. e. degree of compaction) of the erodible bed. Entrainment begins to affect the flow at inclination angles exceeding a critical angle, almost equal to half of the repose angle. Triangular shaped frontal surges are observed at high inclination angles over both rigid or erodible beds. Erosion effects are smaller as the compaction of the erodible granular bed increases and larger as the initial height-to-length ratio and volume of the released mass increase. The avalanche excavates the erodible layer immediately at the flow front, behind which waves travelling downstream that help removing grains from the erodible bed are observed. When increasing the depth of the erodible bed, the excavation depth first increases and then stabilizes to a critical value, and then decreases. Finally, numerical simulations using a 3D visco-plastic model are performed to obtain insight into the physical processes at work during entrainment processes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMEP31E0869F
- Keywords:
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- 1810 HYDROLOGY / Debris flow and landslides;
- 1815 HYDROLOGY / Erosion;
- 8488 VOLCANOLOGY / Volcanic hazards and risks;
- 4316 NATURAL HAZARDS / Physical modeling