Debris-flow monitoring in Japan

Bolzano 2012 Workshop on Monitoring Bedload and Debris Flows 

Debris-flow monitoring in Japan 

Hiroshi SUWA 

Center for Spatial Information Science, University of Tokyo, 

and Research Center for Disaster Mitigation of Urban Cultural Heritage, 

Ritsumeikan University 

suwa@csis.u-tokyo.ac.jp 

Photo: September 4, 2011 Debris flow disaster at Nachi Katsuura, Wakayama, Japan

More than 70 % of main 

islands of Japan are 

composed of steep mountains 

that are being produced by the 

active tectonics and blessed 

with hazards of earthquakes, 

volcanic activities, rainstorms 

and so on. 

We Japanese suffers serious 

disasters from these hazards. 

Landslides and debris flows 

bring us significant damages 

and great losses of lives every 

year.

Debris-flow disasters in the late 60’s 

A succession of serious debris-flow 

disasters in Japan in the late 60’s 

forced us to promote studies on debris 

flow. 

For instance, the 17 August 1968 

heavy rainstorm induced a debris flow. 

It incorporated two buses on a highway 

and claimed 104 lives of passengers. 

At that time, this author was an 

undergraduate student. 

This disaster forced him join a study 

group of DPRI of Kyoto University 

which just started a debris-flow 

monitoring at Mount Yakedake in 1970. 

The slope of this debris flow that caused the 

17 August 1968 Hida River Bus Accident

Debris-flow monitoring by DPRI, Kyoto Univ. 

Debris-flow monitoring has been conducted at the torrents where 

debris-flow frequency is high. Four slopes are at volcanoes. 

• 1970-now: Kamikamihorizawa torrent of Mount Yakdake, Japan 

(with Matsumoto Sabo Erosion Control Office) (Volcano) 

• 1991-1998: Mizunashigawa torrent of Mount Unzen, Japan 

(Volcano) 

• 1991-1998 Jiang-jia Gou torrent in Yunnan, China (with Chendu 

Institute of Mountain Hazards and Environments) (Sedimentary rock) 

• 1991-1993 Gu-xian Gou torrent in Xīzàng, China (with Lanzhou 

Institute of Glaciology and Geocryology) (Metamorphic rock) 

• 1991-1995 Kali Bebeng torrent of Mount Merapi, Indonesia 

(with Research Institute for Water Resources Development) (Volcano) 

• 2000-2006 Kali Curah Lengkong torrent of Mount Semeru, Indonesia 

(with Dr. Lavigne, F., Prof. Thouret, J-P., and VSI) (Volcano) 

• 2000 Hushe River in Karakorum, Pakistan (Metamorphic rock)

Hushe River, 

Karakorum 

Jiang-jia Gou, 

Yunnan 

Kamikamihorizawa 

torrent, 

Mt. Yakedake 

Gu-xian Gou, 

Xizang 

Bebeng R. 

Mt Merapi, 

Java Island 

Mizunashigawa 

torrent, Mt. Unzen 

Curah Lengkong 

R. Mt. Semeru, 

Java Island

Monitoring at Mount Yakedake 

Link to video 1

Topics from the synthesis of the monitoring 

1)Rheological type 

2)Pulsation 

3)Mass and boulder focusing 

4)Ground tremor and sound 

5) Test of moderation of debris-flow 

discharge

RHEOLOGICAL TYPE 

A stony type-debris flow is 

dominant at the torrents of 

active volcanoes, 

while the turbulent type at 

Fushe River (metamorphic 

rock), 

and the viscous type at 

Jiang-jia Gou torrent 

(sedimentary rock).


Astonytype-debris flow is 

dominant at the torrents of 

active volcanoes, 



rock), 


Jiangjia-gou torrent 

(sedimentary rock). 

= f (rock type, water availability, 

geomorphic aspects of torrent, etc) 

(Turbulent) 

Relative height 

(Viscous) 

Relative height=flow depth/median diameter of solid particle 

(Stony) 

After Takahashi (2000)

PULSATION 

Debris flows are monitored 

as a single episode of 

surging in minor cases. 

They, in major cases, appear 

as a series of pulsation of 

multiple surges. 


PULSATION 

Pulsation every 2–10 hours in the 

case of debris flows from glacier 

outburst flood in the Hushe River, 

Karakorum (Suwa, 2003) 

64 surges in140 

minutes during the 

13 August 1993 

debris flows at 

Jiang-jia Gou 

torrent (Suwa et 

al., 1999) 

9 surges in 15 minutes during the 

17 July 1997 debris flows at Mount 

Yakedake.

PULSATION 

We have to pay attention to 

the pulsation especially in the 

midst of rescue works !!! 

64 surges in140 

minutes during the 

13 August 1993 

debris flows at 

Jiang-jia Gou 

torrent (Suwa et 

al., 1999) 

9 surges in 15 minutes during the 

17 July 1997 debris flows at Mount 

Yakedake.

PULSATION 

Debris flows are monitored 

as a single episode of 

surging in minor cases. 

They appear as a series of 

pulsation of multiple surges 

whose intermission ranges 

from 1 minute to several 

hours according to pulsation 

mechanism. 

1. Pulsation of rainfall 

intensity (Yakedake, etc.) 

2. Successive breach of 

landslide dams 

3. Break down of force 

balance between shear 

strength and shear stress in 

the bed slurry (Jiang-jia Gou) 

4. Instability of the flow 

regime (Yakedake, Jiang-jia 

Gou, etc.) 

5. Pulsation of meltwater 

discharge from glaciers 

(Fushe River)

MASS AND BOULDER 

FOCUSING 

Mass focusing toward the surge 

front: Flow instability 

(for All debris flows) 

Boulder focusing toward the 

surge front: A combination of a 

few factors 

(for Boulder rich flows). 

1. Upward migration of larger 

boulders by dispersive pressure. 


boulders by kinetic sieving. 

3. Higher terminal velocity of 

larger boulders (Suwa, 1988). 

4. Exposure of larger boulders to 

higher velocities in the upper 

layers in the flow, etc.

MASS AND BOULDER 

FOCUSING 

Mass focusing toward the surge 

front: Flow instability 

(for All debris flows) 

Boulder focusing toward the 

surge front: A combination of a 

few factors 

(for Boulder rich flows). 


boulders by dispersive pressure. 


boulders by kinetic sieving. 

3. Higher terminal velocity of 

larger boulders (Suwa, 1988). 

4. Exposure of larger boulders to 

higher velocities in the upper 

layers in the flow, etc. 

Excessively boulder-rich frontal 

part of debris flow is called as 

boulder dam. 

Mass and boulder focusing to the 

flow front is a major factor for 

debris-flow super-elevation. 

The super-elevation often 

unexpectedly enlarges the 

hazardous area over elevated 

grounds on the terraces.

GROUND TREMOR AND 

ACOUSTIC SOUND 

Due to the focusing, debris flows 

radiates elastic waves and acoustic 

sounds especially in the case with 

boulder dam. 

Evacuation is possible from the 

hazards detecting sounds and/or 

ground tremor. 

Discharge estimation is capable 

from ground-tremor monitoring.

GROUND TREMOR AND 

ACOUSTIC SOUND 

Efficiency of energy conversion from 

potential energy to elastic-wave 

energy is the magnitude of 10 -3 . 

Very low compared with efficiency 

10 -1 for earthquakes. 

Energy radiation from the 17 July 1997 Yakedake debris flows 

After Suwa et al. (2003)

Sand reservoir is the best 

for complete trap of debris 

flows in the case the space 

for reservoir is obtainable. 

MODERATION TESTS OF DEBRIS-FLOW DISCHARGE 

Unzen, Japan 

Sarno, Italy 

Sand reservoir


Sand reservoir is the best 

for complete trap of debris 

flows in the case the space 

for reservoir is obtainable. 

Otherwise….. 

Tests for moderating debris-flow 

discharge are necessary and 

were tried at the monitoring site 

of Mount Yakedake. 

1. Grid net debris-flow breaker 

2. Flat-board debris-flow breaker 

3. Ring-net debris-flow breaker


Grid net debris-flow breaker 

is designed to check debris flows. 

And partly successful. It captured 

small size flows, however the net 

was broken by large flows.


Flat-board debris-flow breaker 

is designed to check the boulder 

dam. And partly successful. 

The mechanism is as follows. 

The boulders in the dam lose their 

buoyancy as soon as the dam 

arrives at and gets on the board. 

Due to the change in the fluid 

pressure distribution of the flow, 

the change from hydrostatic 

pressure regime into quasiatmospheric 

one. 

Then the boulders suddenly sink 

and interlock each other to bring 

about large forces of internal friction, 

and the dam has to cease moving.


Ring-net debris-flow breaker 

is designed to completely check the 

boulder dam with its flexible and 

shock-absorbing structure. And the 

test was almost fully successful. 

For instance, the July 12, 2005 

debris flow was checked. 

The breaker captured 2090 m 3 of 

deposits, which was 54% of the 

total 3880-m 3 volume. 



The July 12, 2005 debris flow was 

checked. 

The breaker captured 2090 m 3 of 

deposits, which was 54% of the 

total 3880-m 3 volume. 

However, the video analysis 

showed the sediments of 2090 m 3 

were the sum of the deposits 

brought about both by the debrisflow 

surges and by the successive 

hyper-concentrated stream flow.

Concluding remarks so far are 

given as follows. 

1) Stony type debris flows are major at the active volcanoes, while a 

turbulent type at Fushe River, and a viscous type at Jiangj-ia Gou. 

2) Although some debris flows are monitored as a single episode, they 

often appear as a series of multiple surging pulsation, whose 

intermission time being depending to their pulsation mechanism. 

3) Focusing of mass towards the flow fronts are common, and focusing 

of large boulders towards the front is marked in boulder-rich type. 

4) Due to these focusing, debris flows radiate ground tremors and 

acoustic sounds. Those signals are useful for early warning as well as 

for estimating debris-flow discharge. 

5) Moderation of debris-flow hydrographs is achievable using new 

types of debris-flow breakers. However complete check by use of sand 

reservoirs are preferable. 

These outcomes are expected to give us implication for effective 

hazard-mitigation strategies against debris flows.

And I would like to draw your attention to the 

following point: 

Water availability controls debris-flow motion and deposition.


The stony type debris flow 

is dominant at the torrents 

of active volcanoes, 



rock), 


Jiangjia-gou torrent 

(sedimentary rock). 

= f (rock type, water availability, 

geomorphic aspects of torrent, etc) 

(Turbulent) 

Relative height 

(Viscous) 

Relative height=flow depth/median diameter of solid particle 

(Stony) 

After Takahashi (2000)

RAINFALL CONTROL of BOULDER DAMS 

Hydrologic analysis revealed that 

debris flows can be categorized into 

three types (Okano et al., 2012). 

Type I is large and have a boulder dam 

filled with slurry matrix. 

Type II is small and has a boulder 

dam scarcely filled with matrix. 

Type III is small and has a boulder 

dam filled with matrix. 

Namely, rainfall controls not only 

magnitude of debris flow but also 

boulder-dam structure.

RAINFALL CONTROL 

of BOULDER DAMS 

Rainfall of high intensity in a 

duration as short as 10 minutes 

induces fast and large storm 

runoff to the headwaters and 

source reaches of debris flow. 

While large rainfalls in a 

duration as long as 24 hours 

raise water content in the 

bottom deposits along the 

debris-flow growth reaches and 

generate substantial runoff from 

the tributaries. 

The grouping to 3 types can be 

understood based on water 

availability for debris flows in the 

source and growth reaches of 

the debris flow (Okano et al., 

2012)

DEPOSITION (1) 

Debris flows decelerate with decrease in slope 

angle in the downstream reaches and the fan, and 

halt there to deposit coarse clastic materials. 

Photos show a debris flow in motion (A), deposits 

artificially trapped by a ring-net debris-flow breaker 

(B), and a debris-flow lobe in the fan (C). 

The pictures show that the main part of boulder 

dams consists of openwork structure. 

It should be marked that the debris flows of Type II 

leaves this openwork structure. 

However attention may be paid to a scenario that 

this openwork structure might be caused also by 

escape of slurry matrix at the moment of debris-flow 

termination as Hooke (1967) suggested.

DEPOSITION (2) 

Suwa and Yamakoshi (1999) 

showed the regulated distribution 

of debris-flow lobes in the fan 

where swollen lobes locate in the 

upper fan and the flat lobes in the 

lower fan. 

This distribution now indicates that 

the debris-flows of types I and III 

can travel longer distance in the 

fan and leaves flat lobes with 

matrix, while the type II travels 

shorter distance and leaves 

swollen lobes without matrix. 

The longer travel distance may be 

ascribed to the higher mobility of 

the flows of types I and III. 

We may say that the data indicate 

the rainfall control of debris-flow 

deposition.

Here the following additional remarks are given 

6. Three types of debris flows are found: Large flows with boulder dam 

with matrix (Type I), small flows with boulder dam without matrix (Type 

II), and small flows with boulder dam with matrix (Type III). 

7. Rainfall controls this difference through water availability for debris 

flows at the source and the growth reaches. 

8. Debris flows terminate in the fan leaving 2 types of debris-flow 

lobes: swollen lobe and flat lobe. Main source of the flat lobe is 

attributed to Types I and III, while the swollen lobe to Type II. 

9. It must be important to understand this concept of volcanic debris 

flows from their initiation to termination for mitigation of debris-flow 

hazards.

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Debris-flow monitoring in Japan

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