Numerical Simulation of Long-Runout Landslides
Ryoichi Michihata1, Wataru Sagara1, Ryosuke Tsunaki1
1 SABO ＆ LANDSLIDE Technical Center, Japan
OBJECTIVE
In order to ensure people are safe, it is effective to simulate the traveled area of a landslide as soon as signs are observed on a slope, such as the expansion of cracks. Until now many numerical simulation models for landslide runout estimation have been proposed. In Japan, some long-runout landslides have occurred recently. Various mechanisms of these long- runout landslides have been suggested, such as excess pore pressure by Iverson et al. (2000) and grain collision by Hsü (1975). But there are few comprehensive simulation models involving such mechanisms. So we simulated a long-runout landslide by using one of the common simulation models, Zhang’s model (Zhang et al. (2004)). And we considered how to calibrate parameters for simulating of long-runout landslides.
SIMULATION MODEL
Zhang’s model is an application of the Coulomb mixture model, which assumes that grain- fluid flows behave like mixtures of interacting Newtonian fluids and Coulomb solids. In order to exactly predict the range of sediment, the third-order upwind scheme and the preserving mass conservation method are applied to the model. And we additionally applied Voellmy’s flow resistance which depends on velocity to the model.
There are some important parameters to apply within this model, and they are the friction angle of soil (φint), friction angle of the boundary between the ground and soil (φbed), and the coefficient of pore water pressure (λ).
SIMULATION RESULTS
We simulated the Akamatsu landslide as an example of a long-runout landslide that occurred in Yamagata Prefecture in Japan in the snow-melting season. The deposit range of sediment in the Akamatsu landslide had topographic characteristics such as plane surface and little influence of a river. The sediment volume was about 100,000m3. The relative height between the top of the landslide and the front of the deposit was about 100 m and the horizontal distance was about 400 m (the horizontal distance except the landslide area was about 300 m).
The simulation cases and results are shown below (Fig. 1).
Fig. 1 Simulation cases and results
‐200
‐150
‐100
‐50 0 50 100
0.1 0.2 0.3 0.4 0.5
difference between the actual and calculation of the front deposit position;D
coefficient of pore water pressure; λ
φbed=5°φint=25°
φbed=10°φint=25°
φbed=20°φint=25°
φbed=10°φint=15°
DISCUSSION
The most important parameter is φbed where about a 5°change in the parameter causes about 50 m change in reach distance. Also parameter λ is important, where a change of about 10%
in the parameter causes a change of about 10 m. On the contrary parameter φint has little influence. Considering the actual condition, we assumed that parameter φbed was about 20°, φint was about 25°and λ was about 20–30%, and then the difference between the actual and simulated results of the front deposit position (D) is about -150 m. Parameter λ is fixed in a relatively high range considering infiltration of melting snow into weathering mudstone. With rich water content, the runout mechanism of a landslide tends to change to a flow, and then the Coulomb solids resistance decreases.
Fig. 2 Cross section of simulation results
CONCLUSION
We have recognized that parameter φbed and λ influence the results of simulations. It is important to fix these parameters considering the water content conditions of the slopes and flow mechanism when there is a rich water content. When we assume the flow mechanism, we need to fix the parameter φbed smaller than the actual condition. It is important to fix the parameters while considering the following factors related to water condition: catchment area, channel location, climate, geological structure, weathering conditions, and so on. We plan to perform additional simulations to fix appropriate parameters.
REFERENCES
Iverson, R. M., Reid, M. E., Iverson, N. R., LaHusen, R. G., Logan, M., Mann, J. E., and Brien, D. L. (2000): Acute sensitivity of landslide rates to initial soil porosity, Science, 290, 513-516.
Hsü, K. J. (1975): Catastrophic debris streams, sturzstroms generated by rockfalls, Geological Society of America Bulletin, 86, 129-140.
Zhang, C., Yoshimatsu, H., Iwahori, Y., Abe, S. (2004): Numerical simulation of grain-fluid flow due to slope collapse, Landslides, 41-1, 9-17.
Keywords: Landslide, sediment-related disasters, mitigation, long-runout landslide
