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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
605
DOI: 10.4408/IJEGE.2011-03.B-066
FORTY YEARS OF DEBRIS-FLOW MONITORING AT
KAMIKAMIHORIZAWA CREEK, MOUNT YAKEDAKE, JAPAN
H
iRosHi
suwa
(*)
, k
azuyuki
OKANO
(**)
& t
adaHiRo
KANNO
(***)
(*)
Research Center for Disaster Mitigation of Urban Cultural Heritage, Ritsumeikan University, and Center for Spatial Informa-
tion Science, University of Tokyo, Kashiwabecho 15, Shimogamo Sakyoku, Kyoto 606-0815, Japan
(**)
Asia Air Survey Co. Ltd, Osaka 530-6029, Japan
(**)
Matsumoto Sabo Office, Ministry of Land, Infrastructure, Transport and Tourism, Nagano 390-0803, Japan
the flow radiates elastic waves whose energy is from
the potential energy of the flow. The energy conver-
sion efficiency from the potential energy to elastic-
wave energy is a magnitude of 10-3 much smaller
than the efficiency for earthquake at 10-1 from the
strain energy to the elastic-wave energy. Debris flows
terminate in the fan leaving two types of debris-flow
lobes: swollen lobes and flat lobes. Main source of
the flat lobes is attributed to the Types I and III, while
the swollen lobe to the Type II. It would be important
to understand this concept for volcanic debris flows
from its initiation to termination for the mitigation of
debris-flow hazards.
K
ey
words
: debris flow, Mount Yakedake, volcanic eruption,
boulder dam, rainstorm control
INTRODUCTION
In the late 1960s, several severe disasters were
caused by debris flows in Japan (e.g., i
wasaki
, 1968;
s
aito
, 1973). The disasters were resulted from an ir-
relevant preparedness and inadequate countermeas-
ures against the hazards due to a lack of debris-flow
data which were not available at that time. Field sur-
veys had been executed every time after each disas-
ter. However the surveys had supplied no actual data
of debris-flow motion but only the data of the results
of erosion, deposition and damages brought about
by debris flows. Therefore monitoring of actual de-
bris flows was required to obtain data for designing
ABSTRACT
Kamikamihorizawa Creek on the slopes of Mount
Yakedake, Nagano Prefecture, was selected as a moni-
toring site for debris flows considering a high frequen-
cy of debris flow and instrumented with monitoring
equipments in 1970: eight years after the last phreatic
explosion of this volcano. The monitoring system was
improved by adding speedometers, stage meters, seis-
mometers and so on, in addition to the off-line moni-
toring surveys on the interaction between debris flows,
hillslope hydrology and slope morphology. During
the last 40 years, data were obtained from 91 debris-
flow events that contained more than 200 episodes of
debris-flow surges. Studies from the data supplied a
general concept of the debris-flows and their geomor-
phic effects at volcanic slopes as follows. Debris flows
are triggered by a large intensity of rainfall in a short
duration as much as 10 minute. Threshold of rainfall
intensity for debris flows increases with time after the
end of volcanic eruption, while it drastically decreases
with the eruption. Three types of debris flows were
found: Large flows with boulder dam without open-
work structure (Type I), small flows with boulder dam
with openwork structure (Type II), and small flows
with boulder dam without openwork structure (Type
III). Rainfall conditions were found to have control-
led the difference between these types through water
availability to debris flows at the source and growth
reaches of debris flows. Mass and boulder focusing
to the flow front are marked, and due to this focusing
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H. SuwA
, k. OkANO & T. kANNO
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
STUDY SLOPES AND METHODS
Monitoring slopes are shown in Figures 1 and 2.
Debris flows initiate in the gully bottom near the point
indicated by the arrow “Confluence” (Figure 1) with an
intense storm runoff. Weather observation equipments
including rain gauges were installed on the slope near
the source of debris flows as shown in Figure 2B. De-
bris flows travel down the gully running through the
middle reach observation site as shown in Figure 2C,
where visual data of motion, surface velocity, flow
depth etc were monitored with 6 video cameras, an
electromagnetic Doppler speedometer and an ultra-
sonic distance meter respectively. An online system
controlled with multiple wire sensors synchronized the
operation of monitoring equipments with a travel of
debris flow (o
kuda
et alii, 1980a).
This creek was selected because of the high fre-
quency of debris flows it experiences, more than ten
per year in the 1960s. The high frequency was due to
the activation in gully erosion after the last phreatic
eruption of Mount Yakedake in 1962, in which the
explosion was from new fissures that opened at the
headwaters of this creek. The volcano consists of lava
domes of andesite surrounded by numerous sets of
pyroclastic-flow deposits and debris-flow deposits.
The region has a temperate humid climate with an-
nual precipitation of about 2500 mm; nearly two-thirds
falls as rain while the rest occurs as snow. The water-
sheds and the fans are moderately vegetated except the
slopes of gully walls where soil removal continues.
new effective countermeasures and preparedness
against debris flows. Considering this situation, a
study group of debris flows in Disaster Prevention
Research Institute, Kyoto University and Matsumoto
Sabo Construction Office of Ministry of Construc-
tion started a debris-flow monitoring at Kamika-
mihorizawa Creek on the eastern slope of Mount
Yakedake in 1970 (s
uwa
et alii, 1973).
The authors were acquainted afterwards with the
fact that monitoring of debris flows had been con-
ducted and successful earlier in the Guxiang-gou Gul-
ly in Tibet during 1964-1965 (m
a
, 1994) and in the
Jiangjia-gou Gully, Yunnnang, China since 1973 (e.g.,
l
i
et alii, 1983; d
avies
et alii, 1991), followed by the
monitoring of debris flows at Mt Thomas, New Zea-
land (P
ieRson
, 1980), Muddy River of Mount St. Hel-
ens (P
ieRson
, 1986), again at the Guxiang-gou (s
uwa
et alii, 1994), Bebeng River of Mount Merapi (e.g.,
s
uwa
& s
umaRyono
, 1996; l
aviGne
et alii, 2000), Cu-
rah Lengkong River of Mount Semeru (e.g., l
aviGne
& s
uwa
, 2004; t
HouRet
et alii, 2007), the slopes of
Mount Pinatubo (e.g., m
aRCial
et alii, 1996), Mo-
scardo Torrent (e.g., a
Rattano
et alii, 1997; m
aRCHi
et
alii, 2002) , Acquabona Creek (G
enevois
et alii, 2000)
and so on. Majority of these studies have put their
large efforts on understanding the general characters
of debris-flow travel processes. On the other hand, a
synthetic and systematic monitoring of debris-flow ini-
tiation, travel motion, inundation and deposition in the
fan, and the interaction between debris flows and topo-
graphic changes of the basin have been intended in the
observations at Kamikamihorizawa Creek. It would be
worthwhile to synthetically summarize the character-
istics of debris flows obtained from the monitoring at
this creek in the followings.
Fig. 1 - Debris-flow monitoring sites at Mount Yakedake
Fig. 2 - View of monitoring slopes. (A) Eastern slope of
Mount Yakedake. (B) Headwaters of kamikami-
horizawa Creek. (C) View upwards at the middle
reach observation site. (D) Monitoring view field
of a video camera at the fan
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FORTY YEARS OF DEBRIS-FLOW MONITORING AT KAMIKAMIHORIZAWA CREEK, MOUNT YAKEDAKE, JAPAN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
607
cates that the lower limit intensities for debris flows af-
ter 1985 are somewhat higher than those before 1985.
This means debris flows are not as likely to occur as
they were in the past.
A similar trend might exist in the longer tempo-
ral trend in the hydrologic response of hillslopes to
rainstorms after a volcanic eruption. y
amakosHi
et
alii (2001) described the changes in hillslope hydrol-
ogy caused by vegetation recovery. The changes occur
largely in the first few years after volcanic eruptions
as well as more gradually over a period of several
decades (y
amakosHi
& s
uwa
, 2000). These temporal
changes decrease the storm-runoff ratio of hillslopes
resulting from a combined effect of the increase in in-
filtration coefficient and roughness parameter, and the
decrease in contributing area for runoff generation.
MOTION OF DEBRIS FLOW
Over the last 40 years, data were obtained from
91 debris-flow events that contained more than 200
episodes of debris-flow surges, which mean that major
portion of debris-flow events contain multiple surges
(e.g., s
uwa
& o
kuda
, 1988; s
uwa
et alii, 1993)
HYDROGRAPH
Observations have revealed that debris flows are
hydraulically unsteady and non-uniform. Figure 5
shows an example of the hydrographs of the 21 July
1985 debris flow, where the flow consists of an abrupt
rise of flow depth at the front and a recession limb be-
hind. However, the surface velocity at the front was
markedly smaller than the velocities in the middle and
at the back. This low velocity is ascribed to large inter-
nal friction caused by the higher frequency of mutual
collision and interlocking of boulders at the frontal part
The slopes are neither farmed nor inhabited because
the area belongs to the Central Japan Alps National
Park and the Kamikochi National Forest.
RAINFALL CONDITION FOR DEBRIS
FLOW
Even if the duration of rainfall is short, high rain-
storm intensities can trigger debris flows in volcanic
torrents. Figure 3 shows an example of the hydrologic
response at the headwaters of Kamikamihorizawa
Creek that led to debris flows (s
uwa
, 1989). The dia-
gram shows that the high rainstorm intensity raises the
subsurface perched-water stage in the deposits. The
peaks in the perched-water stage coincide with the in-
creases in surface runoff that trigger debris flows. This
temporal coincidence of debris flow and surface-runoff
peak indicates that debris flows are not initiated by
landslides, but by a drastic incorporation of gully bot-
tom deposits due to appearance of a rapid storm runoff.
Threshold of rainfall intensity for debris flows
increase with time after the end of volcanic eruption,
while it drastically decreases with volcanic eruption
(s
uwa
& y
amakosHi
, 1997). Temporal changes are
found in the rainfall intensity for debris flows over
years. Figure 4 uses solid circles to show the peak in-
tensities of hourly rainfall that triggered debris flows,
and open circles for those that triggered no debris flow
for all rainfall events since 1975. The diagram indi-
Fig. 3 - Debris-flow initiation, 5-min rainfall, surface run-
off, and subsurface perched-water stage. Modified
after Suwa (1989)
Fig. 4 - Relationship between debris-flow occurrence and
the peak of hourly rainfall
Fig. 5 - Hydrograph of the July 21, 1985 debris flow
showing the gravel content and size parameters.
Modified after Suwa (1988)
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H. SuwA
, k. OkANO & T. kANNO
608
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
i.e., if the flow mode is hyper-concentrated stream
flows. Under these conditions, taller or larger boul-
ders are exposed to higher velocities, depending on the
height above the bottom. The actual focusing of large
boulders may be caused by a combination of these ma-
jor factors, and results in the formation of boulder-rich
frontal part which is often called as boulder dam.
MOBILITY
Multiple wire-sensors were installed at 30 loca-
tions from the source reaches at 1950 m a.s.l. through
to the distal fan at 1500 m a.s.l. They recorded data
related to the spatial change in frontal velocities of
debris flows. Integration of these data indicated that
frontal velocities as large as 6–16 m/s appeared at the
upper reaches with slope angles of 14–26°. However,
they decreased to less than 4.5 m/s at the middle reach
observation site with the angle of 7°, and decelerated
to less than 1 m/s in the terminal reaches with the angle
of 2-6° (o
kuda
et alii, 1980).
The data have indicated that large-scale debris
flows with high mobility tend to have higher frontal
velocities. The mobility of debris flows may be evalu-
ated by the velocity coefficient, defined as the ratio of
the frontal velocity V
f
to shear velocity u* which is
approximately equal to
in which θ is the channel floor slope angle, h is the flow
depth, and g is gravity acceleration. This coefficient is
proportional to f
–1/2
where f is drag coefficient. Figure
7 shows the relationship between velocity coefficient
and relative depth of debris flows, where relative depth
is defined as the ratio of flow depth to the median di-
that causes a marked difference in the vertical velocity
gradients at the front and rear. Overall, the data indi-
cate a common feature where peaks in hydraulic pa-
rameters appear in order of flow depth, discharge, and
then surface velocity (s
uwa
, 1988).
The temporal change in discharge shows a signifi-
cant focusing of mass on the surge front. This is clear
in Figure 6, which shows the longitudinal cross-sec-
tion of the first surge of the 21 July 1985 debris flow
based on the same data as in Figure 5. The horizontal
axis, the distance from the surge front, corresponds to
the time integral of surface velocity.
BOULDER DAM
The focusing of large boulders towards the surge
front is a common feature in boulder-rich debris flows,
and is the result of a combination of a few factors.
t
akaHasHi
(1980) attributed the faster transportation
of larger boulders towards the front to the combina-
tion of two processes in the flow: an inverse grading
process resulting from the upward migration of larger
boulders due to a dispersive pressure, and the upward
increase in the flow velocity that is common in open-
channel flows. Namely, larger boulders tend to migrate
upwards to be exposed to higher velocities, and are
transported more quickly to the front. s
uwa
(1988)
added another remarkable factor for this boulder fo-
cusing as follows. The terminal velocity of boulders on
steep slopes is larger than the mean velocity of the sur-
rounding slurry, and the larger boulders attain higher
terminal velocities to arrive earlier at the front. In ad-
dition, larger boulders are assumed to be transported
faster even if the inverse grading effect does not work,
because the larger boulders are exposed to higher ve-
locities if the transportation mode is near to bed load,
Fig. 6 - Shape and structure of the July 21, 1985 debris
flow illustrated after the data shown in Fig. 5.
Modified after S
uwA
(1989)
(1)
Fig. 7 - Velocity coefficient Vf/u* versus relative depth h/
d50. Modified after s
uwa
et alii (1997)
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FORTY YEARS OF DEBRIS-FLOW MONITORING AT KAMIKAMIHORIZAWA CREEK, MOUNT YAKEDAKE, JAPAN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
609
during the approaches and in the 40-60-Hz range as the
flows pass by (s
uwa
& o
kuda
, 1985). This may due
to collision of the larger boulders in the boulder dam
that contributes strong wave radiation with longer pe-
riodicity, while collision of smaller gravels contributes
weaker wave radiation with shorter periodicity.
A clear positive correlation with correlation co-
efficient of 0.89 was found between the peak of the
flow discharge and the peak of the tremor acceleration
amplitude. This strong correlation suggests that esti-
mation of the peak discharge is possible by monitor-
ing the acceleration amplitude. There is also a positive
correlation with coefficient of 0.98 between the total
volume of the surges and the time integral of the ac-
celeration amplitude of the tremors. This strong cor-
relation suggests that estimation of debris-flow volume
is possible using the time integral of the acceleration
amplitude (s
uwa
et alii, 2000). A small fraction on the
order of 10
-3
of the total kinetic energy consumed as
internal and boundary friction is radiated as elastic-
wave energy by the passage of debris-flow surges. The
conversion efficiency in debris flows from the loss of
kinetic energy to elastic-wave energy through friction
is estimated at about 10
-3
, which is much smaller than
efficiencies on the order of 10
-1
found in the case of
earthquakes (s
uwa
et alii, 2003).
RAINSTORM CONTROL
Boulder dams have a number of different features.
In some cases, all the interstices between the boulders
are filled with slurry matrix, while in the others, the
interstices remain completely empty. Namely the dam
consists of an openwork structure. o
kano
et alii (2009)
analyzed the rainstorm control on such different fea-
tures to find that the large and longer-duration rainfalls,
for instance, in the previous 24 hr increase the water
content in the source deposits, causing debris flows
with large boulder dams in which the inter-boulder
spaces are filled with slurry. However, the combination
of small rainfalls in the previous 24 hr and large rain-
fall intensities with durations as short as 10 min cause
low water content in the source deposits, resulting in
debris flows with small boulder dams in which the
inter-boulder spaces are completely empty as arranged
in Figure 9. Namely they characterized three types of
debris flows: Large flows havng boulder dam without
openwork structure (Type I), small flows having boul-
der dam with openwork structure (Type II), and small
ameter of the solid particles in the flow. The figure
shows that the mobility of the Kamikamihorizawa
debris flows is much smaller than that of the Jiangjia
debris flows in China (s
uwa
et alii, 1997).
GROUND TREMOR
Travel of boulder-rich debris flow is accompanied
by ground tremors and sounds like long-lasting thun-
der, the intensity of which increases as the debris-flow
surges approach the monitoring sites. Figure 8 shows
the temporal change in the acceleration amplitude of
ground tremors along with a hydrograph of a debris
flow. The time series data clearly show the approach
and passage of the surges. The seismic signal can be
monitored before the arrival of the surge at the moni-
toring site, showing that debris-flow warning using
seismometers would be effective for the mitigation of
hazards. Attempts have been made to develop an intel-
ligent system to warn about debris flows for evacua-
tion using seismic signal detection (e.g., k
uRiHaRa
et
alii, 2007). On the other hand, seismic signal detection
has been applied to trigger video camera operation at
many monitoring sites of debris flows (e.g., s
uwa
&
s
umaRyono
, 1996), while it should be stated that the
intensity of the ground tremor was too small to trigger
the same sets of video cameras at Jiangjia creek where
debris flows do not radiate very strong elastic wave
due to complete absence of boulder dams.
Studies of the debris-flow seismicity resulted in
several new insights. The intensity of the ground trem-
or increases as the boulder dam approaches the moni-
toring site and decreases as they passes, as shown in
Figure 8. The dominant frequency, i.e., the peak spec-
trum of the ground tremor, is in the 20-30-Hz range
Fig. 8 - Temporal changes in the acceleration amplitude
of ground tremor and the discharge of the July 17,
1997 debris flow. Modified after Suwa et al. (2003)
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, k. OkANO & T. kANNO
610
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
flows having boulder dam without openwork structure
(Type III). Rainfall conditions were found to have
controlled the difference of these types through water
availability to debris flows at the source and growth
reaches of debris flows.
DEPOSITION
Debris flows decelerate with a decrease in the slope
of downstream reaches or of the fan, and halt there to
deposit coarse clastic materials. Figure 10 shows the
debris flows in motion (A), deposits artificially trapped
by a ring-net debris-flow breaker (B) near the middle
reach observation site, and a debris-flow lobe in the
fan (C). The pictures show that the main part of boul-
der dams consists of openwork structure. It should be
marked that the Type II out of the three types of debris
flows leaves this openwork structure in its deposits.
The result may be caused also by escape of slurry ma-
trix at the moment of debris-flow termination. H
ooke
(1967) found the openwork structures consisting of
boulders on the alluvial fans in an arid region of Cali-
fornia and named them sieve deposits differentiating it
from debris flow deposits. However the sieve deposits
may be equivalent to the boulder-dam deposits with
openwork structure, because s
uwa
& o
kuda
(1983)
reported very similar openwork structures of fresh
debris-flow deposits and named them as swollen lobe
of debris-flow deposits. The swollen lobes have steep
fronts, referred to snouts (J
oHnson
, 1970).
Afterwards s
uwa
& y
amakosHi
(1999) reported
the regulated distribution of debris-flow lobes in the
fan where swollen lobes locate in the upper parts in
the fan and the flat ones locate in the lower parts. The
distribution now indicates that the debris-flows types
I and II as indicated in Figure 9 can travel longer dis-
tances in the fan and leave flat lobes without openwork
structure, on the other hand the flows type II would
travel shorter distances and leave the swollen lobes
with openwork structure as found in Figure 11.
Debris flow discharge at each event does not cover
whole surface of the fan, but cover only a narrow belt
in a radial direction of the fan. Debris flows keep their
travel courses in this narrow belt during a period of
several years or more as shown in Figure 11. Termi-
nation point would migrate upwards in the belt. The
orientation of the travel course would abruptly migrate
when the termination point reaches at the fan head
(s
uwa
& y
amakosHi
, 1999).
CONCLUDING REMARKS
Studies from the data from the 40 years of de-
bris-flow monitoring at Kamikamihorizawa supplied
a general concept of the debris-flows and their geo-
morphic effects at volcanic slopes as follows. Debris
flows would occur with heavy rainfall. Threshold of
rainfall intensity for debris flows increase with time
after the end of volcanic eruption, while it drastically
Fig. 9 - Rainstorm control of debris flows at kamikami-
horizawa Creek, Mount Yakedake
Fig. 10 - The 6 August 1976 debris flow in motion (A), the
18 July 2004 debris flow trapped by the ring-net
breaker (B) both at the middle reach observation
site, and the terminal part of the 18 July 2004 de-
bris flow on the fan (C)
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FORTY YEARS OF DEBRIS-FLOW MONITORING AT KAMIKAMIHORIZAWA CREEK, MOUNT YAKEDAKE, JAPAN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
611
reaches of debris flows. Mass and boulder focusing to
the frontal part of the flow is marked, and due to this
focusing the flow radiates elastic wave whose energy
is from the potential energy of the flow, and the energy
conversion efficiency is calculated as a magnitude of
10
-3
much smaller than the efficiency of earthquake
at the magnitude of 10
-1
from strain energy to elastic-
wave energy. Debris flows terminate in the fan leaving
two types of debris-flow lobes: swollen lobe and flat
lobe. Main source of the flat lobe is attributed to the
Types I and III, while the swollen lobe to the Type II. It
would be important to understand this concept of vol-
canic debris flows from its initiation to termination for
mitigation of debris-flow hazards.
ACKNOWLEDGMENTS
The authors thank many students of Kyoto Univer-
sity and other universities who supported the debris-
flow monitoring work in the field. The debris-flow
monitoring at Kamikamihorizawa Creek has been con-
ducted as a joint study program between the Disaster
Prevention Research Institute of Kyoto University and
the Matsumoto Sabo Office of the Japanese Ministry
of Land, Infrastructure, Transport and Tourism. A por-
tion of this study has been supported by many grants
for scientific research from the Japanese Ministry of
Education, Culture, Sports, Science and Technology,
including the recent grant-in-aid No. 19510188
decreases with volcanic eruption. Three types of de-
bris flows were found: Large flows with boulder dam
without openwork structure (Type I), small flows with
boulder dam with openwork structure (Type II), and
small flows with boulder dam without openwork struc-
ture (Type III). Rainfall conditions were found to have
controlled the difference in these types through water
availability for debris flows at the source and growth
Fig. 11 - Superposition of debris-flow lobes on the kami-
maihorizawa fan during 1978–1997, modified af-
ter S
uwA
& y
AmAKoShi
, 1999. Routes (a), (b), and
(c) denote the three main orientations in which the
previous debris flows were conveyed
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a
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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
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Conf. on Debris-Flow Hazards Mitigation: Mechanics, Prediction, and Assessment, w
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