Abstract
The frictional resistance of rock and debris is supposed to induce stress anisotropy in the unsteady, non-uniform flow of gravitational mass flows, including debris flows. Though widely used in analytical models and numerical simulation tools, concurrent measurements of stresses in different directions are not yet available for natural flow events. The present study aims to investigate the relation of longitudinal and bed-normal stress exerted by two natural debris flows impacting a monitoring barrier in the Gadria creek, Italy. For that, a force plate in front of a barrier was used to continuously record forces normal to the channel bed, whereas load cells mounted on the vertical wall of the barrier recorded forces in flow direction. We observed an anisotropic stress state during most of the flow events, with stress ratios ranging between 0.1 and 3.5. Video recordings reveal complex deposition and re-mobilization patterns in front of the barrier during surges and highlight the unsteady nature of debris flows. These first-time in-situ measurements confirm the assumption of stress anisotropy in natural debris flows for gravitational mass flows, and provide data for model testing.
Highlights
Debris flows are saturated sediment–water mixtures flowing along a channel at high speed that can endanger human life and infrastructure (e.g., Ballestero-Canovas 2016; Schlögl et al 2021)
The present study aims to investigate the relation of longitudinal and bed-normal stress exerted by two natural debris flows impacting a monitoring barrier in the Gadria creek, Italy
Video recordings reveal complex deposition and remobilization patterns in front of the barrier during surges and highlight the unsteady nature of debris flows. These first-time insitu measurements confirm the assumption of stress anisotropy in natural debris flows for gravitational mass flows, and provide data for model testing
Summary
Debris flows are saturated sediment–water mixtures flowing along a channel at high speed that can endanger human life and infrastructure (e.g., Ballestero-Canovas 2016; Schlögl et al 2021). The combination of high impact forces due to the bouldery front and the high mobility of the more fluid body pushing ahead highlights the particular devastating characteristic of these hazards in mountainous areas. For the mitigation of debris-flow hazards, predictions of analytical and numerical flow models are needed to assess relevant quantities like run-out length (Hungr 1995; Takahashi 2014), superelevation (Scheidl et al 2015), run-up heights (Iverson et al 2016; Faug 2015; Mancarella and Hungr 2010), and impact forces (Faug 2021; Jiang and Towhata 2013; Li et al 2020; Vagnon and Segalini 2016; Tan et al 2019). A common feature of the above models derived to predict these parameters is that, due to the high solids content, a non-isotropic stress state should be considered, resulting from the frictional properties of the granular matter involved (Hungr 2008)
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