1. No category

Summary of key note speech for 6th International Conference on Debris Flow Hazards Mitigation
Landslide Dam Erosion due to Overtopping
Yoshifumi Satofuka1
1College
of Science and Engineering, Ritsumeikan, University, Kyoto, Japan
INTRODUCTION
The study of landslide dam deformation and the prediction of flooding because of landslide dam
deformation are essential for reducing floods due to landslide dams. Previous studies have revealed the
mechanism for the formation of landslide dams. In the past, the collapse of several dams has caused
significant damage in Japan (Mizuyama, 2011). Recently, in Japan, the 2004 Chuetsu earthquake, the
2008 Iwate-Miyagi Nairiku earthquake, and the 2011 Typhoon Talas caused many landslide dams.
Previous studies of landslide dam failures have
identified three types of dam deformation: erosion
due to overtopping, instantaneous slip failure, and
progressive failure. Costa et al. (1988) revealed that
most landslide dam failures were caused by erosion
due to overtopping. Therefore, we focused on
landslide dam deformation caused by overtopping.
Previous studies have performed landslide dam
failure experiments as flume experiments in the
laboratory (Takahashi and Nakagawa, 1993 ; Oda
et al., 2006). However, flume experiments are
performed typically under ideal conditions because
Fig. 1 Landslide dam created by Typhoon
the laboratory flume is rectangular and the
Talas at the Akadani area, Gojo City, Nara
experiments are scale-limited. Therefore, we
Prefecture
performed landslide dam failure experiments in a
mountainous area and obtained data under near-real
conditions. In the field experiments, we aimed to understand the characteristics of outflow discharge
and deformation.
In addition, we developed a numerical model to simulate landslide dam erosion caused by
overtopping flow. The model simulates overtopping erosion in two dimensions. To improve
predictions of outflow discharge, we incorporated the inertial debris flow model proposed by
Takahashi et al. (2002), the side bank erosion model, and the slope collapse model. The inertial debris
flow model is able to handle stony debris flow, turbulent muddy debris flow, immature debris flow,
and intermediate debris flow.
FIELD EXPERIMENTS AND NUMERICAL ANALYSIS
To observe landslide dam deformation and the characteristics of outflow discharge due to
overtopping flow, we performed field experiments. For the field experiments, we constructed
5-m-wide landslide dams on a mountainous stream bed. In Cases 1, 2, and 3, the dam heights were
1.2m, 1.2m, and 1.4m, respectively. The dam in Case 2 had the shape of trapezoid, whereas in the
other two cases, it was a triangle.
During overtopping flow, materials are washed away because of various types of sediment
transport. Therefore, the inertial debris flow model was used. To check the validity of our calculations,
we compared the results of the calculations with the experimental results.
RESULTS AND DISCUSSIONS
For Case 1, the field experiment site and the calculated results at peak outflow are shown in Fig. 2
and Fig.3, respectively. In addition, the field experiments site and the calculation results at the end of
1
the deformation are shown in Fig. 4 and Fig. 5, respectively. As shown in Fig. 3 and Fig. 5, the
calculated results largely reproduce the widening process of overtopping flow.
Fig. 2 Photograph of the field experiment for
Case 1 at peak outflow
Fig. 3 Calculated results for Case 1 at peak
outflow
Fig. 4 Photograph of the field experiment for
Case 1 at the end of the deformation
Fig. 5 Calculated results for Case 1 at the end
of the deformation
!"#
3
Out flow discharge(m /s)
Fig. 6 shows experimental and calculated
Exp.(Case1)
Cal.(Case1)
!"
outflow discharge after overtopping for the three
Exp.(Case2)
Cal.(Case2)
cases. The experimental results showed that the
!$#
Exp.(Case3)
Cal.(Case3)
dam height affected the peak outflow discharge. As
!$
seen from the Fig. 6, the calculated and observed
results are approximately equivalent. However, for
! #
Case 3, the observed peak outflow in the field
!
experiment was higher than that produced by the
#
$
$#
"
"#
%
Time(s)
simulation. The reason for the higher peak outflow
Fig. 6 Observed and calculated results of
observed in Case 3 is the longitudinal slope failure
outflow discharge owing to dam deformation
in the middle of the overtopping erosion process.
(Cases 1, 2, and 3)
Cross-sectional slope collapses, such as side-bank
collapse, are considered in the numerical model; however, such longitudinal slope failures are not
considered. Therefore, the outflow discharge was not accurately reproduced in Case 3.
CONCLUSIONS
In this study, we incorporated the inertial debris flow model, the side bank erosion model, and the
slope collapse model in a numerical model to simulate overtopping erosion in landslide dams. To
check the validity of the proposed model, we compared calculation and experimental results for three
cases. The proposed model largely reproduced the observed time of change in the outflow discharge
for Cases 1 and 2. However, for Case 3, the observed peak outflow in the field experiment was higher
than that in the simulation. The high peak outflow in Case 3 is attributed to the longitudinal slope
failure that occurred in the middle of the overtopping erosion and the increasing peak outflow
discharge. Therefore, incorporating the longitudinal slope collapse model in the proposed model is
necessary for improving the prediction of outflow discharge.
2