IN-01
Small flume experiment on the formation of landslide dam○Hefryan Sukma KHARISMALATRI, Yoshiharu ISHIKAWA, Takashi GOMI, Katsushige SHIRAKITokyo University of Agriculture and Technology
Introduction
Severe rainfall brought by Typhoon Talas in2011 catastrophically damaged Kii Peninsula
including Mie, Nara, and Wakayama Prefectures.
The heavy rainfall induced many sediment-related
disasters in Mie, Nara, and Wakayama Prefectures,
including 33 deep-seated landslides, 30 rock falls,
and 21 stream blockages, with total sedimentamount of approximately 100 million m3 (MLIT,
2011; MLIT, 2013). The collapsed material oflandslide mainly mobilized as 2 type of movement;
flows downstream as debris flow or blocking thestream channel as landslide dam. By data analysis ofdeep-seated landslides in Kii Peninsula caused bytyphoon in 2011, we found that landslide dam werelikely to form in catchments of streams with gradient<10° and inflow angle >60°, while debris flow werelikely in catchments of streams with gradient >10°
and inflow angle <60°. Experimental research isessential to improve and confirm this result.
Experimental research on landslide dam failure
has been largely conducted (e.g. Singh and Scarlatos,1988; Yan et al., 2009; Zhou et al., 2013; Cao et al.,2011). However, experiment on the formation oflandslide dam by deep-seated landslide collapsedmaterial is still lacking. Thus, we conducted small
flume experiment to separate the phenomena ofmobilization and deposition of materials.
(a)

Inflow segment

bucketMain channel(b)soil sample

inflowangle

A, 10 cmBC, 50 cm

D, 70 cm

EFig. 1 Small flume instrument
MethodWe developed a small flume consisted by amain channel and an inflow segment (Fig. 1a). Bothsegments have 10 cm width and 15 cm height byacrylic material. Length and gradient of main
channel is 130 cm and 10°, respectively. A bucket
was placed at the end the flume for capturingtransported soil sample. The gradient of the inflow
segment was 45°. We placed soil sample as sedimentdam in 10 cm upstream from the confluence.
Experiment was applied for two different inflow
angles by 60° and 30°. After the experimental
flushing, percentages of soil deposition weremeasured in 5 sections (Fig. 1b). We used soil
samples collected from Nigoridani, Nara Prefecture
where deep-seated landslide occurred in 2011 due toTyphoon Talas. D50 of the sampled soil was 5 to 6mm. We used 5 classes of water content 0%, 10%,20%, 60%, and 100% for examining the possibilityof landslide dam formation in various water contents.
Result and Discussion
Percentage of soil deposition and pattern ofcollapsed material movement were influenced bywater content. In water content of 0% and 10%, soil
samples were mostly deposited in the section B (theconfluence between inflow and stream segment) for
more than 50% with no deposition on section E (Fig.
2). But in water content 20% to 100%, the soil flowed
further to lower sections. The higher the watercontents the more soil deposition percentage in lower
sections. Nigoridani soil samples have liquid limit of
19.5% checked by fall cone tests and plastic limit of13.6%. The larger the gap between liquid limit andplastic limit, the more water must be incorporated into
the soil to achieve flow (Fleming et al., 1989). In
presence of excess water above the liquid limit, soil
behaved as liquid material and mobilizes as debris
flow (Das, 2014, Fleming et al., 1989). Judging by thesoil deposition percentage at each section, boundary
between landslide dam and debris flow might bedecided as follows: if the soil deposition in section B
is ≥40% then it can be judged as landslide dam,otherwise it can be judged as debris flow. The
comparison of soil deposition at section B to section
E (Fig. 4) shows strong result with R2 >0.8, as well as
the student t-test result (p-value <0.05). Thus thedistinction between landslide dam and debris flow
based on the deposition in section B is adequate.
The possibility of collapsed material from deepseated landslide to form landslide dam depends on
the topographic factors that involve the interaction
between river and slope dynamics (Dal Sasso et al.,2014; Stefanelli et al., 2015). The inflow angle offlume setup was modified to confirm its influence toformation of landslide dam. Generally, by changing
inflow angle from 60° to 30°, the soil deposited
more on lower sections (Fig. 2 and Fig. 3). On watercontent 0% and 10%, smaller inflow angle reduced
the soil deposition in section B to under 50%. On- 378 -

100%

0%

10%

20%

60%

100%

20%

60%

100%

80%
60%
40%
20%
0%
ABCDE

Fig. 2 Percentage of soil deposition for inflow angle 60°
100%
10%

0%

80%
60%
40%
20%
0%

ABCDEFig. 3 Percentage of soil deposition for inflow angle 30°80%60°, 0%- 0.0730 : y = -0.13ln(x)60%R² = 0.82

60°, 20%

Deposition on section E (%)

60%Deposition on section E (%)

et al., 2014). Modification of stream segment
inclination and comparison with soil samples fromother site should be conducted in further experiment.
So that the boundary between landslide dam anddebris flow formation would be more apparent.

80%

60 : y = -0.07ln(x) - 0.07
R² = 0.83

40%60°, 10%60°, 60%60°, 100%40%30°, 0%30°, 10%30°, 20%

20%30°, 60%

20%30°, 100%
0%
0%

20%

40%

60%

80%

Deposition on section B (%)

0%
0%

20%

40%

60%

80%

Deposition on section B (%)

Fig. 4 Comparison of soil deposition at section B and E
water content 20% to 100%, smaller inflow angle
generated smaller soil deposition on section A and B,and more on section C, D, and E.
On large inflow angle, the collision of soil
sample and the stream segment expended a large
amount of energy. The movement of overflowed
collapsed material thereafter was stopped due tolarge energy loss upon the collision, and thus formed
landslide dam. Meanwhile, small inflow angle
generated soil sample to flow easily into the streamsegment and caused the collapsed material to costsmall amount of energy at the collision with
opposite flume. The modification of inflow angle
influenced the deposition in section B significantly,
checked by student t-test with p-value <0.05.
Landslide collapsed material would easily mobilize
as debris flow on stream steeper than 10-20° (HungrConclusion
Result of the experiment confirmed thatlandslide dam formed in inflow angle 60° for watercontent less than the liquid limit. However, landslide
dam was also found in inflow angle 30° with the samewater content. We think that the fluidization of deepseated landslide collapsed material is strongly
influenced by grain size and water. Soil samples withdifferent grain size will generate different result. The
model experiment indicates the trends of phenomena
and general properties. Modification of main channel
gradient will be conducted in further experiments.
ReferenceDas, B.M. (2014) Advanced Soil Mechanics, Fourth
Edition. CRC Press, Taylor & Francis Group.
Fleming, R.W., Ellen, S.D., and Algus, M.A. (1989):
Transformation of dilative and contractive
landslide debris into debris flows. Eng. Geol., 27,pp. 201-223.
Hungr, O., Leroueil, S., Picarelli, L. (2014): The
Varnes classification of landslide type, an update,Landslides, Vol. 11, pp. 167-194.
Stefanelli, T. C., Catani, F. and Casagli, N. (2015):
Geomorphological investigations on landslidedams, Geoenvironmental Disasters, Vol. 2, No. 21.

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