It is widely recognized that the effects of a phase shift of fine sediment in large-scale debris flows are likely to be large. Therefore, in numerical simulations, it is essential to describe fine sediments in the fluid phase, and not in the solid phase. Recently, the "Kanako" numerical simulator has been widely used for a variety of objectives, particularly because it has a graphical user interface. However, to date, there is no widely available numerical simulation model for large-scale debris flows that includes the effects of phase shifts. Here, we present a modified version of Kanako to describe this phase shift for fine sediment. In the new numerical simulator, which we refer to as Kanako-LS, we assume that sediments can be classified into two groups in terms of sediment diameter (fine and coarse), and define the critical diameter of the sediment (Dc) as the smallest diameter at which sediments behave as a solid phase. Then, we test the applicability of Kanako-LS using an example of debris flows triggered by a deep-seated rapid (catastrophic) landslide in Japan. Our results suggest that Kanako-LS may be useful for a variety of types of large-scale debris flow, particularly if the amount of fine sediment and the magnitude of the interstitial fluid turbulence are sufficient.
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.
Deep catastrophic landslides (DCLs) sometimes lead to large-scale debris flows with serious impacts on human life and infrastructure. However, no adequate information about DCL-triggered debris flows, such as the topography of eroded and deposited areas or the grain size distribution, exist. We compiled published data and obtained additional new data for the topographic characteristics and grain size distributions of 10 recent DCL-triggered debris flows in Japan. We compared these data with previously published data of small-scale debris flows, steep-slope failures, and large-scale debris flows. We examined the effects of topography and DCL volume on erosion and deposition due to debris flow as well as on grain size distribution. The longitudinal gradient of the lower end of the deposited area decreased with increasing landslide volume, and about half of DCL-triggered debris flows deposited material where the longitudinal gradient of the lower end of the deposited area was less than 2°. However, the minimum longitudinal gradient of the eroded section due to debris flow was not affected by the landslide or the debris flow volume. We found that the travel distance of debris flow, including DCL-triggered debris flow, might also be a function of landslide and/or debris flow volume and that the grain size of debris flows triggered by DCLs spanned more than eight orders of magnitude.
Landslide-induced debris flows can travel long distances and seriously damage infrastructures (Iverson & George, 2014;Tai et al., 2019). Therefore, predicting areas most at risk of future landslides is essential in mitigating the effects of such disasters. In the last decades, landslide mobility has been widely evaluated using relationships between the travel distance of landslides (L) and the height between the landslide scar and the lower end of the deposited area (H) (e.g., Hsü, 1975;Legros, 2002;Scheidegger, 1973). The value of H/L has been revealed to be related to the landslide scale (i.e., the area or volume of the landslide) in various environments, not only on Earth but also on other planets (e.g.,
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