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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1073
DOI: 10.4408/IJEGE.2011-03.B-116
EFFECT OF TWO SUCCESIVE CHECK DAMS
ON DEBRIS FLOW DEPOSITION
f
aRouk
MARICAR
(*)
, H
aRuyuki
HASHIMOTO
(*)
, s
Hinya
IKEMATSU
(*)
& t
omoHiRo
MIYOSHI
(*)
(*)
Department of Civil Engineering, Kyushu University - Nishi-Ku, Fukuoka 819-0395, Japan
E-mail: hasimoto@civil.kyushu-u.ac.jp
b
ovolin
& m
izuno
, 2000; b
usnelli
, s
tellinG
&
l
aRCHeR
, 2001; m
izuyama
, k
obasHi
& m
izuno
, 1995;
m
izuyama
, o
da
, n
isHikawa
, m
oRita
& k
asai
, 2000;
o
sti
, i
toH
& e
GasHiRa
, 2007; w
u
& C
HanG
, 2003). It
is generally believed that check dams can reduce sedi-
ment transport to downstream river reaches and stabi-
lize river beds. Check dams must act to lower the peak
sediment discharge and to decrease the total volume
of sediment outflow to the downstream area.
There are two-types of check dams; one is a
closed type and the other is an open type. The former
type is a traditional structural measure for control-
ling debris flow. However, this type of check dams
has to be empty in order to trap large amounts of
sediment during a debris flow event. The latter type
can be subdivided into slit-check dams and beam-
check dams. Concrete slit-check dam is known as
the slit type, and steel-pipe open dam is known
as the beam type. An open type allows finer sedi-
ments to pass through at lower discharge and coarser
sediments to be trapped at higher discharge such as
debris flow. However designing their appropriate
opening becomes a problem.
The Hofu City in Yamaguchi Prefecture, Japan
had heavy rain on July 21, 2009. The accumulated
rainfall was 240.5mm and the largest hourly rainfall
was 63.5mm/hour. This rainfall caused many shal-
low landslides on the mountainous areas of this city.
Most of these landslides changed into debris flows and
moved downstream in the mountainous rivers.
ABSTRACT
This paper describes the effect of two successive
check dams on the debris flow event which occurred on
21 July, 2009 in Hofu City, Yamaguchi Prefecture, Ja-
pan. The debris flow event caused sediment deposition
in the check dams in the Tsurugi and Hachimandani
River. In the former river with two successive closed-
check dams, driftwood did not accumulate in the check
dams but in the central region of the river bend. In the
latter river with two successive open-check dams,
driftwood accumulated in the opening of the check
dams so that the accumulation obstructed the sediment
transport to the downstream direction. The upstream
check dams in these rivers have sediment deposition
profile of around 2°, whereas the downstream ones
have deposition profile of around 1.3°. The ratio of
sediment deposition volume in the downstream check
dams to that in the upstream ones is 0.3 : 1. The total
amount of sediments trapped by the two successive
check dams can be estimated at around 9,500 m
3
in
each river. Specific sediment runoff volume from the
mountainous areas is q
s
= 6,800 (m
3
/km
2
).
K
ey
words
: two successive check dams, debris flow, sediment
deposition, closed-check dam, open-check dam
INTRODUCTION
The control of debris flows by check dams has
been investigated by experimental and numerical
studies and field surveys (a
Rmanini
& l
aRCHeR
, 2001;
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F. MARICAR, H. HASHIMOTO, S. IkEMATSU & T. MIYOSHI
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
two successive closed-check dams have been con-
structed at a distance of about 200 m, while in the
Hachimandani River two successive open-check dams
have been constructed at a distance of about 400 m.
OUTLINE OF THE DEBRIS FLOW DISASTER
Hofu City area had heavy rain on July 21, 2009.
The situation of the rainfall is shown in Figure 2.
This figure also shows the time-varying water level
of the small river in Hofu City. The accumulated
rainfall was 240.5mm from 4 a.m. to 1 p.m. The
hourly rainfall had two times of peak of 65 mm from
8 to 9 a.m and 49.5 mm from 11 to 12 a.m. This rain-
fall caused flood flows in every river in Hofu City
area. The flood water level of the small rivers had
also two times of peak, as shown in Figure 2.
This rainfall condition resulted in many shal-
low landslides in the mountain region. Most of
the landslides changed into debris flows and then
moved down the mountain rivers. In their down-
stream areas, there are villages, a nursing home
for elderly people, and roads. In Yamaguchi Pre-
fecture the impact of the debris flows resulted in
17 deaths, 33 houses destroyed, 95 houses partially
damaged, 708 houses inundated upstairs and 3,862
houses buried of sediments (Fig.3). Some of the
witness of this event shows that most of the debris
flows occurred at around 12 a.m. Therefore, we can
consider that the second peak of the hourly rain-
fall triggered most of the landslides. The landslide
sediments moved into the second peak flood flows.
These transported a significant amount of sediment
to the downstream river reaches.
There are two-types of check dams in these
rivers; one is a concrete closed-check dam and the
other is a steel-pipe open-check dam. Especially,
the two successive closed-check dams were in-
stalled at the Tsurugi River and the two successive
open-check dams were installed at Hachimandani
River before this debris flow event. After the debris
flow event, significant sediment deposition was
observed in these check dams.
In order to consider the measures against debris
flows in the mountainous areas, it is necessary to
know the effect of check dams against the debris
flow event. This requires an investigation into the
effect of the check dams on the sediment deposition
from debris flows.
The purpose of the present study is to estimate the
effect of the check dams against debris flows from the
comparison of river bed profiles before and after the
debris flow event.
STUDY AREA
The study area is located in Hofu City,
Yamaguchi Prefecture, Japan (Fig. 1). Geology of
its mountain region is mainly composed of fresh and
weathered granite. As a result, this region is vulner-
able to landslides and debris flows. The government
of Yamaguchi Prefecture has made effort to prevent
landslides and debris flows. Therefore, 23 check
dams have been installed to control river bed ero-
sion and bed sediment runoff in the mountain region
of Hofu City. There are two-types of check dams in
these rivers, i.e. a closed type and an open type.
The Tsurugi River and the Hachimandani River
are selected as the study areas. In the Tsurugi River,
Fig. 1 - Location of the study area
Fig. 2 - Rainfall at the Hofu station & water level of the
Mate River in the downtown of Hofu City
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EFFECT OF TWO SUCCESIVE CHECK DAMS ON DEBRIS FLOW DEPOSITION
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
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the contour map of the river bed elevation. This
river reach has a curvature and an inflow of an-
other debris flow from the left-hand side. It can be
seen that the river bed has higher part in the outer
region and lower part in the inner region of the
river bend. Driftwood did not arrive at the check
dam but stopped in the central region of the river
at x=50 m. Here x is defined as distance measured
along the centre line in the upstream direction
from the check dam.
Figure 8 illustrates the cross-sectional profile of
DEBRIS FLOW DEPOSITION BY CHECK
DAMS IN THE TSURUGI RIVER
Figure 4 depicts the plan view of the Tsurugi River
catchment. Figure 5 shows the longitudinal profile of
the river. The distance from the upstream end (Position
B) to the downstream end (Position A) is 2,700 m and
its catchment area is about 2 km
2
. River slope is 3.5
o
at
position A in the vicinity of the national road and 14.2°
at position B in the vicinity of the landslide location. On
the other hand, river slope is 2.7° in the vicin-
ity of the two successive check dams. An aerial
photo shows that there are about 90 shallow
landslides in this catchment area.
On November 25 to 27, 2009 and on De-
cember 7 to 8, 2009, we visited the Tsurugi
River and then found that a significant amount
of sediment was trapped in the check dams. We
surveyed the sediment deposition areas of check dams.
We also took sediment samples from the river bed.
Sediment grain size analysis was carried out by sieve
method at the Hydro Laboratory.
THE UPSTREAM CHECk DAM IN THE TSURU-
GI RIVER
Figure 6 is a photo of the situation of sediment
deposition caused by the upstream check dam in
the Tsurugi River. The sediment deposition formed
new river bed configuration behind the check
dam after the debris flow event. Figure 7 shows
Fig. 3 - Saba River basin
Fig. 4 - Catchment of the Tsurugi River
Fig. 5 - Longitudinal profile of the Tsurugi River
Fig. 6 - Sediment deposition caused by the upstream check dam
on the Tsurugi River (View from upstream to down-
stream direction on November27, 2009)
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at each section yields a longitudinal river bed profile
behind the check dam, as shown in figure 9. In this
figure, we omitted the laser measurement in 2009, be-
cause the field measurement approximated to the laser
measurement after the debris flow event.
The longitudinal river bed slope after the debris-
flow event can be estimated as 1.9° from the field meas-
urement in 2009, while the slope of the river bed before
the event is 3.3° from the laser measurement in 2005.
In addition, Figure 10 shows a change in width
of sediment deposition zone with distance x from
the check dam. The width of the sediment deposition
zone changes with distance ranging from 37 m to 16
m, whereas the width of the lower part of the river
bed ranges from 7.5 m to 3 m and its depth from 1.5
m to 2 m. Using the measurements of river bed before
and after the debris flow event, we can estimate the
volume of deposited sediment as 7,400 m
3
THE DOwNSTREAM CHECk DAM IN THE TSU-
RUGI RIVER
Figure 11 is a photo of sediment deposition at the
downstream check dam in the Tsurugi River. Figure
12 shows the contour map of the river bed elevation
behind the check dam. There is a lateral inflow up-
stream immediately from the check dam. The other
the river bed at x=50m. The solid line in this figure
denotes the field measurements after the debris flow
event. For comparison, the laser measurements be-
fore and after the debris flow event are also shown
by the dashed lines in this figure. The laser meas-
urements were made by the Ministry of Land, Infra-
structure, Transport and Tourism on April 14 to 16,
2005 and on August 17 to 19, 2009. It is confirmed
that the field measurement coincides with the laser
measurement. The right-hand side within the river
bed after the event has depression at around x=50
m. The formation of this depression represents seri-
ous sediment deposition in the outer region (left-
hand side) and less sediment deposition in the inner
region (right-hand side) of the river bend during the
event. From such a figure of the field measurements
we obtain the maximum and minimum elevation of
the river bed at each section.
Using the maximum and minimum bed elevation
Fig. 8 - Cross section of the river bed profile in the up-
stream check dam (x= 50 m)
Fig. 9 - Longitudinal profile at the upstream check dam in
the Tsurugi River
Fig. 10 - width of sediment deposition and the lower
part of channel in the storage region of the up-
stream check dam
Fig. 7 - Contour map of river bed configuration behind the
upstream check dam in the Tsurugi River
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EFFECT OF TWO SUCCESIVE CHECK DAMS ON DEBRIS FLOW DEPOSITION
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1077
debris flow moved into the main river from the left-
hand side. This inflow resulted in significant sediment
deposition and driftwood accumulation.
Using the maximum and minimum bed elevation
at each cross section yields a longitudinal river bed pro-
file behind the check dam, as shown in figure 13. This
longitudinal profile becomes convex upward. The bed
slope is steeper in the region such that 0 < x < 50m
than in the other region. This is attributed to the lateral
inflow of the other debris flow. However, no effect of
the lateral inflow can be seen in the river region at x >
50 m. The longitudinal slope of the sediment deposition
profile is 3.4° in the region with the effect of lateral in-
flow. The longitudinal slope of the sediment deposition
profile is 1.4° in the region without the effect of lateral
inflow, while the slope of river bed based on the laser
measurements in 2005 indicates 4.5°. This approximat-
ed to the river bed slope before the debris flow event.
Figure 14 shows the change in width of sediment
deposition zone with distance x from the check dam.
The width of sediment deposition zone decreases from
34 m to 14 m. On the other hand, the width of the
lower part of the bed decreases from 6.4 m to 2 m and
its depth is 1.0 m. The volume of sediment trapped in
the check dam can be estimated as 2,200 m
3
.
DEBRIS FLOW DEPOSITION BY THE CHECK
DAMS IN THE HACHIMANDANI RIVER
Figure 15 depicts the plan view of the Hachiman-
dani River catchment. Figure 16 shows the longitudi-
nal profile of the river. The distance from the upstream
Fig. 14 - width of sediment deposition and the
lower part of channel in the storage
region of the downstream check dam
Fig. 11 - Sediment deposition caused by the downstream check dam on
the Tsurugi River (View from downstream to upstream direction
on December 8, 2009)
Fig. 12 - Contour map of river bed configuration behind the
downstream check dam in the Tsurugi River
Fig. 13 - Longitudinal profiles at the downstream check
dam in the Tsurugi River
Fig. 15 - Catchment of the Hachimandani River
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F. MARICAR, H. HASHIMOTO, S. IkEMATSU & T. MIYOSHI
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end (Position B) to the downstream end (Position A)
is 2,500 m, and the catchment area is about 1.7 km
2
.
River slope is 2.7° at the confluence with the Saba
River (position A) and 23.7° at position B in the vicin-
ity of the landslide location. On the other hand, the
river slope is 5.6° in the vicinity of the two succes-
sive check dams. An aerial photo shows that there are
about 10 shallow landslides in this catchment area.
On November 25 to 27, 2009 and on Decem-
ber 7 to 8, 2009, we visited the Hachimandani River
and then found that a significant amount of
sediment was trapped in the check dams. We
measured river bed elevation in the sediment
deposition areas of the check dams. We also
took sediment samples from the river bed.
Sediment grain size analysis was carried out
by sieve method at the Hydro Laboratory.
THE UPSTREAM CHECk DAM IN THE
HACHIMANDANI RIVER
Figure 17 is a photo of situation of sedi-
ment deposition caused by the upstream check
dam on the Hachimandani River. Figure 18
shows the contour map of the river bed config-
uration behind the check dam. Figure 19 are the
photos of the open-type check dam. We found
that driftwood had accumulated in the opening
of the check dam. However we did not find
larger sizes of sediment such as boulders in the
opening of the check dam or in the sediment
deposition area. The accumulation of drift-
wood obstructed the sediment transport in the
downstream direction. Figure 20 shows grain
size distribution of deposited sediment behind
the check dam. We find the 50% diameter d
50
= 1.38 mm on the average. Since there are no
boulders in the sediment deposition area, d
50
=
1.38 mm is typical of the deposited sediment.
Using the maximum and minimum bed
elevation at each cross section yields a lon-
gitudinal river bed profile behind the check
dam, as shown in figure 21. Figure 22 shows
the change in width of sediment deposition
zone with distance x from the check dam.
The longitudinal slope of sediment deposi-
tion zone before and after the debris flow
event is 6.3° and 2.0°, respectively. The
width of the sediment deposition zone varies
Fig. 16 - Longitudinal profile of the Hachimandani River
Fig. 17 - Sediment deposition caused by the upstream check dam on the
Hachimandani River (View from downstream to upstream di-
rection on December 8, 2009)
Fig. 18 - Contour map of river bed configuration behind the upstream
check dam in the Hachimandani River
Fig. 19 - Sediment and driftwood at the upstream check dam in the
Hachimandani River
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1079
THE DOwNSTREAM CHECk DAM IN THE HA-
CHIMANDANI RIVER
Figure 23 is the photo of sediment deposi-
tion caused by the downstream check dam on the
Hachimandani River. Figure 24 show the photos
of the open-type check dam. Figure 25 shows
the contour map of the river bed configuration
behind the check dam. We found the driftwood
accumulated in the slit part of the check dam.
However we did not find larger size of sediment
such as boulders in the opening of check dam or
in the sediment deposition area. The accumu-
lation of the driftwood obstructed the sediment
transport to the downstream direction. Figure
26 shows grain size distribution of deposited
sediment behind the check dam. We find the 50%
diameter d
50
= 0.99 mm on the average. Since there
are no boulders in the sediment deposition area, d
50
=
0.99 mm is typical of deposited sediment.
from 14 m to 46 m. The width of lower part of the river
bed is from 1.5 m to 4 m and its depth is from 0.5 m
to 1.5 m. The volume of the trapped sediment can be
estimated as 7,300 m
3
.
Fig. 22 - width of sediment deposition and the lower part
of channel in the storage region of the upstream
check dam
Fig. 20 - Grain size distribution of sediment at the upstream
check dam in the Hachimandani River
Fig. 21 - Longitudinal profiles at the upstream check dam
in the Hachimandani River
Fig. 23 - Sediment deposition caused by the downstream check dam
on the Hachimandani River (View from upstream to down-
stream direction on November25, 2009)
Fig. 24 - Sediment and driftwood at the downstream check dam
in the Hachimandani River on November 25, 2009
Fig. 25 - Contour map of river bed configuration behind the
downstream check dam in the Hachimandani River
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F. MARICAR, H. HASHIMOTO, S. IkEMATSU & T. MIYOSHI
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Using the maximum and minimum bed elevation
at each cross section yields a longitudinal river bed
profile behind the check dam, as shown in figure 27.
Figure 28 shows the change in width of sediment depo-
sition zone. The longitudinal slope of sediment deposi-
tion zone before and after the debris flow event is 7.1°
and 1.3°, respectively. The width of the sediment depo-
sition zone varies from 19m to 35m. The width of low-
er part of the river bed is from 2m to 4m and its depth is
from 0.5m to 1.0m. Using the results, we can estimate
the volume of the trapped sediment as 2,200 m
3
.
DISCUSSION
Characteristics of the sediment deposition in four
check dams are summarized in Table 1.
The curvature of river course produced space-vari-
ation in river bed at the upstream check dam in the Tsu-
rugi River. The lateral inflow of the other debris flows
caused an increase in bed elevation due to significant
deposition at the check dam areas in the Tsurugi River.
The vegetation zone flattened river bed elevation in
the Hachimandani River. The Hachimandani River has
less space-variation in river bed than the Tsurugi River,
because the Hachimandani River has no curvature and
lateral inflow near the check dams.
Although the Tsurugi and Hachimandani River
differ significantly in their river bed slope before the
debris flow event, sediment deposition profiles after
the event have their almost same longitudinal slope in
the upstream and downstream check dam, respectively.
The upstream check dam areas have deposition
slope of around 2°, and the downstream ones have
deposition slope of around 1.3°. The upstream check
dam areas have steeper longitudinal profiles of sedi-
ment deposition than the downstream ones.
The downstream check dam area reveals more con-
vex profile than the upstream one in the Tsurugi River.
Driftwood accumulated above the river bed sur-
face near the region of river bend and lateral inflow
in the Tsurugi River, whereas it accumulated in the
opening of the open-check dams in the Hachimandani
River. The accumulation of driftwood in the open-
ing of the check dams obstructed the transport of the
whole sizes of sediment to the downstream direction.
Grain size analysis of the sediment samples shows
d
50
≈ 1.4 mm for the upstream check dams and d
50
1.0 mm for the downstream check dams. However,
boulders could not be found in the sediment deposi-
tion areas of check dams of the Hachimandani River
but could be found in those of the Tsurugi River.
The volume of sediment trapped by the upstream
check dam in the Tsurugi River is almost same as that by
the upstream check dam in the Hachimandani River. The
volume of sediment trapped by the downstream check
dam in the Tsurugi River is same as that by the down-
stream check dam in the Hachimandani River. The ratio
of sediment volume by the downstream check dams to
that by the upstream ones is 0.3:1. The total amount of
sediments trapped by the two successive check dams is
about 9,500 m
3
in each river. This result reduced the sedi-
ment outflow from the mountainous areas to the residen-
tial areas. It is concluded that more severe disasters have
been avoided by the two successive check dams.
Fig. 26 - Grain size distribution of sediment at downstream
check dam in the Hachimandani River
Fig. 27 - Longitudinal profile at the downstream check dam
in the Hachimandani River
Fig. 28 - width of sediment deposition and the lower part
of channel in the storage region of the down-
stream check dam
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EFFECT OF TWO SUCCESIVE CHECK DAMS ON DEBRIS FLOW DEPOSITION
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1081
2. Although the Tsurugi and Hachimandani River
differ significantly in the river bed slope before
the debris flow event, sediment deposition profiles
after the event have their almost same longitudinal
slope in the upstream and downstream check dam,
respectively. The upstream check dam areas have
deposition slope of around 2°, whereas the down-
stream ones have deposition slope of around 1.3°.
3. Grain size analysis of sediment from the sediment
deposition areas of the upstream check dams
shows d
50
≈ 1.4 mm, while that of the downstream
check dams shows d
50
≈ 1.0 mm. However, boul-
ders could not be found in the sediment deposition
areas of check dams of Hachimandani River but
could be found in those of the Tsurugi River.
4. The volume of sediment trapped by the upstream
check dams in the Tsurugi and Hachimandani
River is about 7,300 m
3
. The volume of sediment
trapped by the downstream check dams in the
Tsurugi and Hachimandani River is 2,200 m
3
.
The ratio of sediment volume by the downstre-
am check dams to that by the upstream ones is
0.3 : 1. The total amount of sediments trapped
by the two successive check dams is about 9,500
m
3
in each river. These result in the same specific
sediment runoff volume q
s
= 6,800 (m
3
/km
2
) for
the Tsurugi and Hachimandani River.
For the discussion we introduce the concept of
specific sediment runoff volume q
s
per one debris flow
event (a
sHida
et alii, 1983):
q
s
= (total amount of sediment outflow from river ba-
sin by one debris flow event) / (river basin area)
Here m
3
and km
2
are used as unit for the amount of
sediment outflow and the area of river basin, respectively.
Total amount of sediment outflow by the debris
flow event in the Tsurugi and Hachimandani River ba-
sin can be approximated by the volume of sediment
trapped by the two successive check dams. These river
basin areas have same value of 1.4 km
2
at the down-
stream check dams. Therefore, we can obtain the same
specific sediment runoff volume q
s
= 6,800 (m
3
/km
2
)
for the Tsurugi and Hachimandani River. This specific
sediment runoff volume is slightly smaller than the
other events (a
sHida
et alii, 1983).
CONCLUSION
The results obtained in this study are as follows:
1. In the Hachimandani River, driftwood accumu-
lated in the opening of the open-check dams so
that the accumulation obstructed the sediment
transport to the downstream direction. In the
Tsurugi River, on the other hand, driftwood did
not accumulate at the closed-check dam but in
the central region of the river bend.
Tab. 1 - Summary of the investigation
into the check dams in the Tsu-
rugi and Hachimandani River
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toH
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ACKNOWLEDGEMENTS
The laser measurement data of land elevation
in Hofu City have been supplied by the Ministry of
Land, Infrastructure, Transport and Tourism. Dr. Ki-
ichirou Ogawa (Asia Air Survey Co., ltd.) gave us the
river profile data. The authors would like to appreciate
their supply of the data on this research.
Statistics