E3S Web of Conferences 415, 04012 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341504012

Study on debris flow monitoring in Mt. Fuji
Naoki Nishimura1*, Nobuhiro Usuki1, Masaru Touhei2, Takahisa Mizuyama3, and Atsushi Okamoto1
1Asia Air Survey Co., Ltd, Japan
2Ministry of Land, Infrastructure, Transport and Tourism, Fuji Sabo Office, Japan
3Japan Sabo AssociationAbstract. The Osawa River, which has its headwaters at the top of Mt. Fuji, has one of the largest failurein Japan, the Osawa Failure, in its uppermost part of the catchment. The Osawa Failure produces more than
100,000m3 of sediment every year by erosion and collapse, which causes debris flows and slash avalanches.
Since its opening in 1970, the Fuji Sabo Office has been observing the sediment dynamics of the Osawa
River through field surveys, observation equipment and aerial photogrammetry, aerial laser survey, andJapanese pipe hydrophone. In 2021, the largest debris flows since the start of observations in the Osawa
River occurred twice in March and August. In the March 21 debris flow, 416,000m3 of sediment flowed
down from the Osawa Failure, and 482,000m3 was deposited in the alluvial fan. In the August 18 debris
flow, 421,000m3 of sediment was deposited in the alluvial fan. Our observations from CCTV cameras, rain
gauges, and other equipment installed in the Osawa River indicate that the rainfall characteristics during thetwo debris flows were different, and that the conditions for debris flows differed depending on the season.

1 Introduction
The Osawa Failure (Fig 1) is one of the largest collapse
sites in Japan; its dimension is about 2,000m in length,
its maximum width is 500m, and its maximum depth is150m. It is located on the upper western slope of Mt.
Fuji, the highest mountain in Japan (3,776m). At theOsawa Failure, an average of over 100,000m 3 collapses
occur annually, and the sediment deposited on thestreambed causes debris flows and slush avalanches.

Since its opening in 1970, the Fuji Sabo Office has been
conducting aerial photogrammetry and field surveys tomonitor the sediment dynamics at the Osawa River. In
recent years, with the development of aerial surveying
technology, monitoring has been carried out usingvarious observation methods such as airborne LiDAR.
In 2021, large-scale debris flows occurred at Osawa
River on March 21 and August 18. The two debris flows
occurred in different seasons and had different rainfall
characteristics observed at the time of occurrence. We
analysed the characteristics of the debris flow using data
observed by CCTV cameras, rain gauges, Japanese pipehydrophone, load sensors, and other equipment installedin the Osawa River.

2 Study methods
2.1 Analysis of airborne LiDAR data
The Fuji Sabo Office has been continuously conductingairborne LiDAR measurements since 2007 to generate
DEMs with 1m grid. Annual sediment budgets havebeen calculated using the differences of DEMs of twoconsecutive periods. In 2021, five airborne LiDAR
measurements were taken between April and October todetermine the amount of sediment discharged.
Table 1. Timing of debris flows and airborne LiDAR.

Fig. 1. Overview of Osawa River.

20202021
Oct - Mar Apr May - Jul
Aug- OctDF LiD LiD
LiD DF LiD
LiDDF; Debris Flow, LiD; Airborne LiDAR measurement* Corresponding author: nok.nishimura@ajiko.co.jp© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 04012 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341504012

2.2 Installation of observation equipment

water levelaverage load

4.0

In the Osawa River, observation equipment is installedat the Ochudo observation site (elevation 2,300 m),
which is closest to the Osawa Failure, Otaki observationsite (elevation 1,480 m), Iwadoi observation site(elevation 900 m), and Osawagawa-bashi observationsite (elevation 500 m). For example, at the Iwadoi
observation site (Fig. 2), CCTV cameras, ultrasonicwater level gauge, velocity gauge, and load sensor are
installed to observe debris flows.

8.0

3.5
6.0

2.5
2.0

4.0

1.5

1.0

2.0

Average load (t)Wate level (m)

3.0

0.5
0.0

13:00
13:10
13:20
13:30
13:40
13:50
14:00
14:10
14:20
14:30
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15:0015:10
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15:40
15:50
16:00
16:10
16:20
16:30
16:40
16:50
17:00
17:10
17:20
17:30
17:40

0.0

3/21/2021

Fig. 4. Water level and average load (March 21, 2021).

CCTV camera

3.2 Debris flow on August 18, 2021
In the debris flow that occurred on August 18, analysis
of airborne LiDAR measurement data revealed that
241,000m3 of sediment flowed form Osawa Failure. It
was also revealed that approximately 421,000m 3 ofsediment flowed from canyon part into alluvial fan. The
maximum hourly rainfall of 79mm (8/18 06:00-07:00)
and cumulative rainfall of 1,089mm (8/12 15:00-8/18
10:00) were observed at the Ochudo observation site.
We calculated the peak flow rate of the debris flow
based on the water level and velocity readings from theCCTV camera images at the Iwadoi observation, it wasapproximately 255.2m3/s (water level: 2.9m, flow
velocity: 9.2m/s), and the total flow rate during theanalysis period (270 minutes) was approximately
587,000m3. The load sensor measures about 1.5 tons
after the debris flow peak (Fig.5, Fig.6).

velocitygauge
load sensor

Fig. 2. Installation of equipment at Iwadoi observation site.

3 Results and Discussion
3.1 Debris flow on March 21, 2021
In the debris flow that occurred on March 21, analysis
of airborne LiDAR measurement data revealed that
416,000m3 of sediment flowed form Osawa Failure, thelargest single debris flow since observations began in
1970. It was also revealed that approximately
482,000m3 of sediment flowed from canyon part into
alluvial fan. The maximum hourly rainfall of 21mm
(3/21 11:00-12:00) and cumulative rainfall of 97mm
(3/21 03:00-13:00) were observed at the Ochudo
observation site. To determine the status of debris flowat the Iwadoi observation site, we divided the video
taken by CCTV camera into 10-minute segments andread the water level and velocity for one minute of an
average-scale run-off scene in each image frame byframe (1 frame = 0.033 seconds). Based on the readingsof water level and flow velocity, the peak flow rate ofthe debris flow was approximately 290.4m 3/s (waterlevel: 1.9m, flow velocity: 16.9m/s), and the total flowrate during the analysis period (270 minutes) wasapproximately 880,000m3. The load sensor measuresabout 4.0 tons before the debris flow peak (Fig.3, Fig.4).

Fig. 5. Peak of debris flow (August 18, 2021, 10:30).
water levelaverage load

4.0

8.0

3.5

6.0

2.5

Average load (t)Water level (m)

3.0

2.0

4.0

1.5
1.0

2.0

0.5
0.0
6:30
6:40
6:50
7:00
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11:00
11:10

0.0

8/18/2021

Fig. 6. Water level and average load (August 18, 2021).

3.3 Rainfall at the time of debris flow
Debris flow observations that have continued since 1970
have indicated that the amount of rainfall at which debris
flows occur varies with the season in Osawa River.

Fig. 3. Peak of debris flow (March 21, 2021, 14:30).

2

E3S Web of Conferences 415, 04012 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341504012

Therefore, we analysed the relationship between rainfall
data (Ochudo observation site) and the time when debris
flows occurred. To analyse rainfall data during debris
flow events, we divided the year into summer season(Jun-Oct) and the rest of the year (Nov-May).
The relationship between the timing of debris flows andmaximum hourly rainfall (Fig. 7) indicates that debris
flows occur at a maximum hourly rainfall of 50 mm/hr
or higher during the summer season. On the other hand,during the non-summer months, it occurs at 25mm/hr orless, and especially during the severe winter season fromFebruary to April, several occurrences at 20mm/hr orless have been observed.
The relationship between the timing of debris flows andcumulative rainfall (Fig. 8) indicates that debris flows
occur at a cumulative rainfall of about 300mm or morefrom June to October, and especially in September andOctober, it may not occur even when rainfall exceeds
600mm. On the other hand, it has been confirmed thatdebris flows occur from February to April andNovember and December when rainfall is less than200mm.
Hourly rainfall and cumulative rainfall during summer
season debris flows are higher than during non-summer
debris flows, suggesting that the conditions under which
debris flows occur vary with the season.

Ochudo observation site

Maximum hourly rainfall (mm)

300mm

80100

non-occurrence
Debris Flow（summer）
Debris Flow（non-summer）
Slash avalanche

80
6050mm
4025mm2000200

400

600
800
1,000
Cumulative rainfall (mm)

1,200

1,400

Fig. 9. Relationship between maximum hourly rainfall andcumulative rainfall during mudslide occurrence.

4 Conclusions
We conducted detailed and quantitative analysis of thetwo large-scale debris flows that occurred in 2021 by
analysing airborne LiDAR data and observing withCCTV cameras and other equipment. The largest
erosion since 1972 occurred at uppermost part of theOsawa River (Fig. 10), which is presumed to havedestabilized the riverbed and surrounding slopes, andthere is a possibility that active sediment discharge will
occur again.

non-occurrence
Debris flow (summer)
Debris flow (non-summer)
Slash avalanche

100230mm

120

Ochudo observation site

120

Maximum hourly rainfall (mm)is 25mm/hr or less and cumulative rainfall is 230mm orless.

60
40
200JanMarMayJul

Oct

Dec

Fig. 7. Relationship between the timing of debris flows andmaximum hourly rainfall.
Ochudo observation site

1,400

non-occurrence
Debris Flow（summer）
Debris Flow（non-summer）
Slash avalanche

1,000
800

2825

600

2815

2820

Elevation(m)Cumulative rainfall (mm)

1,200

400
200

2810
2805

25m

2800
2795
2790

0JanMarMayJul

Oct2020 Sep2785

Dec2021 Apr2780
20Fig. 8. Relationship between the timing of debris flows andcumulative rainfall.

3040506070Distance(m)8090100Fig. 10. Erosion situation at uppermost part of the catchment(2020 Sep - 2021 Apr, Red Relief Image Map).

As shown in Figure 9, it is inferred that the possibilityof debris flows occurring increases in summer when themaximum hourly rainfall exceeds 50mm/hr andcumulative rainfall 300mm. On the other hand, theresults indicate that debris flows may occur in nonsummer season even when the maximum hourly rainfall

The results also indicated that there were seasonal
differences in the relationship between the occurrence ofdebris flow and rainfall. This is presumably due to theexistence of a frozen layer in the riverbed at high3

E3S Web of Conferences 415, 04012 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341504012

elevations, where debris flows occur, and the depth of
the frozen layer changes with the season. We willcontinue to accumulate data from existing equipment
and study observation methods for Osawa Failure,
where there are many unknowns regarding the actual
conditions of sediment discharge, in order to analyse theactual conditions of sediment discharge in the Osawa
River.

4