Effect of existence of houses on inundation area of debris flow
Hidetoshi NAKAMOTO(Kyoto University, Japan), Hiroshi TAKEBAYASHI(Kyoto University, Japan), Masaharu FUJITA (Kyoto University, Japan)

１．Introduction(2) Analysis condition

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

The analysis grid is 2 m square grid. Three types of analysis conditions were used(Table 1). The
destruction of houses was assumed to occur only in wooden houses, and not in RC structure houses. Ahouse is represented by multiple analysis grids, and only the part exceeding the destruction critical isdetermined to be destructed. Therefore, the half or partially destructed house can be evaluated.
Table 1. Analysis conditions
Contents

Case No.
Case1

Considering destruction of houses

Case2

Treating houses as impervious non-destructive structures

Case3

Disregarding the existence of the houses

3．Result and discussion

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

Figure 2 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 thedisaster. The portions shown in blue in the figure 2 are the portions where houses are determined tohave been destructed. In the house area A located directly downstream of the mountain stream exit,
the analysis results show that all houses were completely destructed. In the house area B locatedfurther downstream, some houses were completely destructed, and others were half destructed. In
the house area C located further downstream, no house was determined to be completely destructed,
but some houses were partially destructed. Therefore, we can see that the calculated results match
the photograph after the disaster.

2．Overview of numerical simulation(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 laminar flow region are used.
The debris flow is generated by the slope failure. The development, transport and 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 each analysis grid and is used to determine the destruction/non‐destruction of the house. This isbecause many houses that were actually destructed by debris flow were half destructed, the house
shape was easily changed, and the entire 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 evaluatedby the sum of the static pressure due to debris flow and the fluid force by kinetic energy acting on thehouse as follows.
1
2
2Fhx  m gh cos   m hu
2
ρm ： the density of mixture of water and sediment, g ： the gravity acceleration,h ： the flow depth, θ ： the bed slope, u ： 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 perunit width.

Figure 2. Reproduction of house destruction (Case 1)

4．Conclusions
In 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 theeffect of the existence of houses on the inundation area of debris flow was examined. As a result, thisnumerical simulation model seems to be able to reproduce the state of complete or half destruction ofhouses relatively well, and to some 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 ofComputational Fluid Dynamics, 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.