Proceedings of the 22nd IAHR-APD Congress 2020, Sapporo, Japan

EFFECT OF EXISTENCE OF HOUSES ON INUNDATION AREA OF DEBRIS FLOWHIDETOSHI NAKAMOTO
Department of Civil and Earth Resources Engineering, Kyoto University, Kyoto, Japan, e-mail nakamoto-h@tokencon.co.jp
HIROSHI TAKEBAYASHI
Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan, e-mail takebayashi.hiroshi.6s@kyoto-u.ac.jp
MASAHARU FUJITA
Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan, e-mail fujita.masaharu.5x@kyoto-u.ac.jp

ABSTRACT
It is expected that the number of occurrences of debris flow will increase due to the effects of global warming,in residential area, it is required to understand the flow characteristics of debris flow. In this study, twodimensional debris flow numerical simulation considering destruction process of houses is performed and effectof houses on the inundation area of debris flow is discussed. The simulated debris flow was occurred in Yagi 3
Chome, Asaminamiku, Hiroshima, Japan in 2014 and it destructed a lot of houses. The developed numericalanalysis model can reproduce the horizontal distribution of complete/partial destruction of houses. If the housesin the inundation area are neglected, debris flow spreads widely and forms debris flow alluvial fan. As a result,the difference of inundation area between with house case and without house cases becomes large. This
information must be important for evacuation act, when it is difficult to evacuate to far refuge under heavy rain
conditions.
Keywords: Debris flow, Destruction of house, Numerical analysis, Hazard map, Sediment disaster

1.

INTRODUCTION

In recent years, sediment disasters that houses are destructed by debris flow are occurred frequently. In thefuture, it is expected that the number of occurrences of debris flow will increase due to the effects of global
warming, in residential area, it is required to understand the flow characteristics of debris flow, and to takeeffective measures and evacuation based on it. In this study, focusing on the debris flow that occurred in Yagi
3 Chome, Asaminamiku, Hiroshima , Japan in 2014 (Figure 1) when some of the houses in the residential area
were completely or half destructed, two-dimensional debris flow numerical simulation considering thedestruction process of houses is performed, and the effect of the existence of houses in the inundation area ofdebris flow is discussed.

Figure 1. The debris flow that occurred in Hiroshima in 2014 (by MLIT)

2.

OVERVIEW OF NUMERICAL SIMULATION

2.1 Overview of the model
In this analysis, the constitutive equations of Egashira et al. (Egashira et al. 2004) and the two-dimensional
debris flow analysis model (Takabayashi et al. 2020) considering the turbulent flow region over the laminarflow region are used. The debris flow is generated by the slope failure. The development, transport and1

deposition processes are analyzed with the two-dimensional model. The shape of a house is represented bymultiple analysis grids, and the stress acting on the house is determined by calculating the force acting on eachanalysis grid and is used to determine the destruction/non-destruction of the house. This is because many houses
that were actually destructed by debris flow were half destructed, the house shape was easily changed, and theentire house did not behave like a single rigid body. The stress Fhx per unit width of the flow direction, acting
on the analysis grid of the house, is evaluated by the sum of the static pressure due to debris flow and the fluid
force by kinetic energy acting on the house as follows.

1Fhx  m gh2 cos mhu2(1)
2
where, m is the density of mixture of water and sediment, g is the gravity acceleration, h is the flow depth,  isthe bed slope, and u is the depth integrated flow velocity. The house destruction critical stress per unit width
seems to vary depending on the house structure. In this analysis, 800 kN/m was applied to all wooden houses
as the house destruction critical stress per unit width.
2.2 Analysis conditions
The analysis grid is 2 m square grid. Three types of analysis conditions were used: Case1: analysis considering
the destruction of houses; Case2: analysis treating houses as impervious non-destructive structures; and Case3:
analysis that disregarding the existence of houses. The destruction of houses was assumed to occur only inwooden houses, and not in RC structure houses. A house is represented by multiple analysis grids, and only thepart exceeding the destruction critical is determined to be destructed. Therefore, the half or partially destructedhouse can be evaluated.
3.

RESULTS AND DISCUSSION

Figure 3 shows the horizontal distribution of the amount of change in ground height in the residential area after
the debris flow obtained in Case 1, which was superimposed on the photograph after the disaster. The portions
shown in blue in the figure 3 are the portions where houses are determined to have been destructed. In the housearea A located directly downstream of the mountain stream exit, the analysis results show that all houses werecompletely destructed. In the house area B located further downstream, some houses were completely destructed,and others were half destructed. In the house area C located further downstream, no house was determined tobe completely destructed, but some houses were partially destructed. Therefore, we can see that the calculated
results match the photograph after the disaster.

Figure 3. Reproduction of house destruction (Case 1)

4.

CONCLUSIONSIn this study, focusing on the debris flow that occurred in Yagi 3 Chome, Asaminamiku, Hiroshima in 2014
when some of the houses in the residential area were completely or half destructed, two-dimensional debris flow
numerical simulation considering the destruction process of houses was performed, and the effect of theexistence of houses on the inundation area of debris flow was examined. As a result, this numerical simulation
model seems to be able to reproduce the state of complete or half destruction of houses relatively well, and tosome extent to be able to evaluate the possibility of complete or half destruction of houses due to debris flow.
REFERENCES
Egashira, S., Itoh, T. (2004). Numerical Simulation of Debris Flow. Journal of Japan Society of Computational FluidDynamics, Vol.12-2, pp. 33-43. (in Japanese)
Takebayashi H., Fujita, M. (2020). Numerical Simulation of a Debris Flow on the Basis of a Two-Dimensional
Continuum Body Model. Geosciences, 10, 45.
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