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.
Debris flows often cause substantial losses of human life as well as economic losses. Damage can be estimated using numerical simulation models that describe the debris flow process. Some models can be used to determine the possible effects of sabo dams and have been practically employed to plan sabo dam arrangement. However, the existing simulation systems currently do not have efficient user interfaces, making it difficult for non-experts in debris flow simulations to run simulations without the aid of specialists. We developed a system that produces one-and two-dimensional debris flow simulations and is equipped with a graphical user interface. The system is based on an integration model and employs one-dimensional simulations of gully areas and two-dimensional simulations of alluvial fan areas; it then considers mutual influences in boundary areas between gullies and alluvial fans. Data can be input using a mouse and viewed on the monitor, and users can see real-time visualized images of a debris flow during a simulation. The interface enables users to run a debris-flow simulation without expert knowledge of the model, enabling better solutions for sabo engineering.
Landslide dams are caused by large-scale slope collapse. Predicting their failure is important because hazardous flooding may result downstream when landslide dams burst. Accurate prediction of a landslide-dam formation which means height and length via landslide maps that show two-dimensional figure but does not consider landslide depth is needed for a countermeasure against the sediment disaster caused by landslide dam. In this study, past landslide dam failures were investigated. Relationships between landslide dam formation and the topographical and geological characteristics of the region were established to improve landslide dam analysis for more accurate prediction of dams' structural integrity.
Accurate models of rainfall infiltration are needed for analysis and prediction of slope failure induced by heavy rainfall. In this study, a numerical model was developed to simulate two-dimensional rainwater infiltration into an unsaturated hillslope, the formation of a saturated zone, and the resultant changes in slope stability. This model was subsequently used to analyze the effects of soil porosity parameters (i.e., saturated soil water content u s and effective soil porosity [ESP]) on the occurrence of slope failure, the moisture conditions of the displaced material, and the movement of debris flow on weathered granitic hillslopes. We conducted the simulations by imposing various conditions of rainfall, initial wetness of the slope, soil thickness, and slope gradient. Results showed that when the surface soil of a slope has a relatively large ESP value, it has a greater capacity for holding water and therefore delays deeper water infiltration into the subsurface. Consequently, the increase in pore water pressure in the subsurface at a greater depth is also delayed. In this manner, the greater ESP value contributes to delaying slope failure. Under small storm conditions, slope failure tends not to occur when the surface soil has a relatively large ESP value. However, a greater ESP tends to increase the water content of the displaced matter, which results in faster and longer travel distances, and more deposition of debris flow, thus increasing the risk of damage in downstream regions.
Landslide dam formation and deformation strongly affect water and sediment runoff. When a large-scale landslide dam collapses due to overflow erosion, peak flood discharge may exceed inflow discharge by several times. Such an abrupt flow discharge increase by a dam burst may cause serious damage downstream. We propose a one-dimensional model for river-bed variation and flood runoff consisting of a two-layer model for immature debris flow and a bank erosion model. We applied this model to the Nonoo landslide dam in Japan’s Miyazaki Prefecture, formed by typhoon Nabi in September 2005, and China’s Tangjiashan landslide dam formed in the Wenchuan earthquake in May 2008. The model reproduces the observed flood runoff processes in the two areas. Calculated results suggest that peak flood discharge diminishes when water accumulating behind the landslide dam is small, and excavating the landslide dam crown effectively reduces flood discharge.
Debris flows often cause substantial losses to human life and the economy. Damage can be effectively reduced using numerical simulation models, which can describe the debris flow process and determine possible effects of sabo dams, or erosion and sediment control dams. However, non-experts find it very difficult to run simulations independently, because the systems do not currently have an efficient user interface. We developed a system that produces one-and two-dimensional debris flow simulations and is equipped with a graphical user interface (GUI). The system is based on an integration model and employs onedimensional simulations for gully areas and two-dimensional simulations for alluvial fan areas, and then considers their mutual influence in boundary areas between gullies and alluvial fans. The system was developed with "MS Visual Basic.NET." Data can be input using a mouse and be checked on the monitor, users can see real-time visualized images of the debris flow during the simulation. The interface enables non-expert users to run the debris-flow simulation independently, enabling better solutions for sabo engineering.
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.
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