The role of the phase shift of fine particles on debris flow behavior: an numerical simulation for a debris flow in Illgraben, Switzerland

Abstract

Physically-based numerical simulation models have been developed to predict hazard area relating to debris flows. Since fine sediments are expected to behave as a part of the fluid rather than solid phase in stony debris flows, several models have recently included this process of the phase-shift from solid to fluid in the context of fine sediment. However, models have not been fully tested regarding the ability to reproduce a variety of debris flow characteristics. We therefore tested (1) applicability of a numerical simulation model for describing debris flow characteristics and (2) the effect of phase-shift of fine sediment on debris flow behaviors. Herein we applied a numerical simulation model to a well-documented dataset from the Illgraben debris-flow observation station in Switzerland. Based on the stony debris flow concept, we physically modeled effects of the phase-shift of sediment on transport capacity and flow resistance. We successfully reproduced the observed bulk density, erosion and deposition patterns, front velocity, and erosion rate, although we had to tune the ratio of fine sediment that behaves as a fluid. Considering the effects of the phase-shift of sediments, we conclude that physically-based numerical simulation models can describe a variety of debris flow behaviors.

Long‐Runout‐Landslide‐Induced Debris Flow: The Role of Fine Sediment Deposition Processes in Debris Flow Propagation

“…(2019), and Uchida et al. (2021). The Manning's coefficient in mountain rivers vary greatly from basin to basin (Asano et al., 2018).…”

“…Kanako-LS is based on shallow water equations for granular flows and a 1D model. The physical, mathematical, and computational bases of Kanako-LS are shown in detail in Appendix C and described by Uchida et al (2013Uchida et al ( , 2021. Kanako-LS is a single-phase model that evaluates erosion and deposition rates and riverbed shear stress based on the stony debris flow theories proposed by Takahashi and colleagues (e.g., Takahashi, 2009;Takahashi & Nakagawa, 1991).…”

“…The coefficient of the erosion rate was set to the general value of 0.0007 (Takahashi, 2007). Although the coefficient of the deposition rate affects the simulation result, it was set to 0.05, following Takahashi (2007), Takahashi and Nakagawa (1991), Nishiguchi et al (2014), Nakatani et al (2016), Suzuki et al (2019), and Uchida et al (2021). The Manning's coefficient in mountain rivers vary greatly from basin to basin (Asano et al, 2018).…”

“…In contrast, relatively fine sediments exhibit turbulent flows with interstitial water. Processes involving behavioral transitions from part of the solid phase to part of the fluid phase of the debris flow are termed “liquefaction” (Hotta et al., 2013), “phase shifting” (Egashira et al., 2016), “phase‐shift” (Uchida et al., 2021), and “hydrodynamic suspension” (Iverson & George, 2014). Fine sediments behaving as fluid phases are called “suspended sediment” (Kaitna et al., 2016), “fluid phase sediment” (Sakai et al., 2019), “phase‐shifted sediment” (Uchida et al., 2021), and so on.…”

“…Thus, previous modeling efforts often set the friction coefficient of solid materials to be relatively small and/or the fluid density of the interstitial fluid to be large to describe large‐scale fluidity (e.g., Egashira et al., 2016; Iverson & George, 2016; Nishiguchi et al., 2011; Tai et al., 2019; Uchida et al., 2013). Moreover, various studies revealed that the increase in the fine sediment fraction enhanced the fluidity in stony debris flows (e.g., D'Agostino et al., 2010; de Haas et al., 2015; Hürlimann et al., 2015; Iverson et al., 2010; Kaitna et al., 2016; Sakai et al., 2019; Sosio et al., 2007; Uchida et al., 2021). It has been considered that fine sediments moved as turbulent flows with interstitial water (e.g., Iverson, 1997).…”