E3S Web of Conferences 415, 03015 (2023)
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https://doi.org/10.1051/e3sconf/202341503015

Continuous debris flow monitoring using DFLP and LVP on
Sakura-jima Island
Takahiro Itoh1*, Tadaaki Iwao2, Seiki Kubo3, Takaya Tsurumoto3 Satoshi Tagata4, Misa Furuya4, and Takahisa
Mizuyama5
1Center for Advanced Research, R & D Center, Nippon Koei Co., Ltd., 2304 Inarihara, Tsukuba, Ibaraki, Japan
2Osumi Office of River and National Highway, Kyushu Regional Development Bureau, Ministry of Land (MLIT), 1013-1 Shintomi

Kimotsuki-cho, Kimotsuki, Kagoshima, Japan (Now in Niigata Prefectural Office, Japan)
3Osumi Office of River and National Highway, Kyushu Regional Development Bureau, Ministry of Land (MLIT), 1013-1 Shintomi

Kimotsuki-cho, Kimotsuki, Kagoshima, Japan
4Fukuoka Office, Nippon Koei Co., Ltd., 1-2-12 Higashihie, Hakata-ku, Fukuoka, Japan
5Professor emeritus, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, Japan

Abstract. Since 2010, numerous debris flows have occurred on Sakura-jima Island due to rainfall events
that occurred after extensive ash deposition associated with volcanic activity. The study area included theNojiri and Arimura rivers in the southwestern and southeastern parts of Sakura-jima, respectively. Debris
flow monitoring systems consisting of loadcell and pressure sensor (DFLP) were installed to evaluatecharacteristics of moving weight and so on during debris flows. The systems were installed at the Arimura
River No. 3 sabo dam in June in 2012 and at the Nojiri River No. 1 sabo dam in 2014. In addition, LVP(Load, Vibration and Pressure) sensors consisting of loadcells and accelerometers for measuring vibrationand pressure were installed on the riverbed for debris flow detection. Modified LVP systems were installedat the Nojiri River No. 7 sabo dam in February in 2015, and at the Arimura River No. 3 sabo dam in Octoberin 2016. Interesting characteristics of debris flows were obtained by the DFLP and LVP systems. Thefindings showed that the sediment concentrations of both the coarse and the suspended and liquid phases
could be estimated by the DFLP systems, and several patterns of debris flows were observed by the LVPsystems.

1 Introduction
Numerous debris flows have occurred on Sakurajima Island in southwestern Japan due to rainfall events
that followed extensive ash deposition due to thevolcanic eruptions that have occurred on the island since2010. Debris monitoring using a variety of sensors has
been carried out at the sabo dams along the Nojiri Riverand Arimura River. The monitoring stations andcatchments have the following characteristics. TheNojiri River drains a catchment measuring 2.99 km2, hasa bed slope of 4.5% and a flow width of 13.2 m at theNojiri No. 1 sabo dam. The Arimura River drains acatchment measuring 1.35 km2, has a bed slope of 19%
and a flow width of 20.5 m at the Arimura No. 3 sabodam. In addition, the average bed slope of the storage
area of the Nojiri No. 1 and Arimura No. 3 sabo dams is
4.5% and 6.5%, respectively.
A DFLP is continuous measurement systemcomprising loadcells and pressure sensors [1]. A
modified DFLP [2] was installed at Arimura No. 3 sabo
dam in 2012. In the Nojiri river, three newly developed
DFLP systems employing an iron plate (1 m2) formeasuring moving weight were installed at the No. 1sabo dam in 2014 [3]. The sensors, which were evenly

spaced in the transverse direction, were maintained
during monitoring because of impacts with boulders due
to debris flows. In addition, maintenance was required
for the loadcells on three occasions (26th April, 9th Juneand 9th September) and the pressure sensor (9th Juneand 9th September) at the Nojiri No. 1 sabo dam in 2021
An LVP, that is a sensor for load, vibration andpressure, system and wires were installed on the river
bed for debris flow detection. The system consisted ofpressure meter, loadcells and an accelerometer formeasuring vibration [2, 3]. The sensor has been
maintained mainly for the detection of debris flows on
an event-to-event basis [2, 3].
In the present study, temporal changes in the specificweight and sediment concentration of coarse/finecomponents of the debris flows were investigated using
DFLP systems installed in the Nojiri and Arimura
Rivers. Data obtained by a LVP system also showedtemporal changes in flow depth, load and vibration
during debris flows. As a result, threshold values wereproposed for debris flow detection. Those could be usedin near future looking ahead application of online tools
for debris flow detection using a LVP system.

* Corresponding author: a6556@n-koei.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, 03015 (2023)
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https://doi.org/10.1051/e3sconf/202341503015

2 Estimates of sediment concentration
by DFLPTable 1. Sediment concentration of debris flows calculated by aDFLP at Arimura No. 3 sabo dam.

A variety of sensors have been employed to measure
the occurrence and runoff characteristics of debris flows
along the Arimura and Nojiri Rivers, including raingauges, X-band multi-parameter (MP) radar, ultrasonicwater level meters, wire sensors, falling ash gauges,
acceleration vibrographs (only in the Arimura River)and closed-circuit television (CCTV) cameras [3, 4]. On
Sakura-jima Island, the number of debris flows arecounted by the number of wires that become
disconnected from wire sensors during events. Also, todetermine the magnitude of debris flows, wires areinstalled vertically on the bed surface at distances of 60,
120 and 180 cm. DFLP data are recorded at a sampling
rate of 100 Hz using a pulse measurement unit (NR6000,Keyence Corp., Osaka, Japan). Sediment concentration
is calculated using time-averaged data at 1 min intervals,
as described previously [2-4].
Tables 1 and 2 show the estimated sedimentconcentration of debris flows measured using a DFLP at
Arimura No. 3 sabo dam and Nojiri No. 1 sabo dam,
respectively. The parameters in the tables are as follows:
Wire: number of disconnected wires, c: time-averaged
volumetric sediment concentration near the peak stage,
C: coarse sediment concentration, F: fine sedimentconcentration, R: accumulated rainfall depth before
debris flow (mm), r10: maximum rainfall depth during
10 min before debris flow (mm), Qp: peak discharge(m3/s). In Table 2, the asterisk indicates peak dischargeof debris flow for data collected at Nojirigawa No. 1No.
12No.
1

Y/M/D
2016/9/20

Wire
22

2017/5/13

13

2017/7/4

142017/8/15

152018/5/2

262018/12/3

272019/3/3

182019/3/10

192019/9/6

1102020/3/27

1112020/6/11

2122020/7/24

113

2021/5/15

3142021/6/4

2

Averagec0.402 (C: 0.0679,F: 0.334)
0.545 (C: 0.293,F: 0.252)
0.323 (C: 0.222,F: 0.101)
0.303 (C: 0.174,F: 0.129)
0.306 (C: 0.164,F: 0.143)
0.247 (C: 0.155,F: 0.0915)
0.386 (C: 0.00, F:0.386)
0.356 (C: 0.286,F: 0.148)
0.346 (C: 0.00, F:0.346)
0.510 (C: 0.246,F: 0.264)
0.478 (C: 0.477,F: 0.01)
0.254 (C: 0.253,F: 0.01)
0.314 (C: 0.314,F: 0.00)
0.322 (C: 0.322,F: 0.00)
0.364 (C: 0.213,F: 0.151)

Table 2. Sediment concentration of debris flow calculated by DFLP at Nojiri No. 1 sabo dam.
Wirec
Averaged
LeftCenterRight
2016/7/11
20.288 (C: 0.245,0.257 (C: 0.211, F:0.285 (C: 0.217,0.323 (C: 0.306, F:F: 0.0398)
0.0346)
F: 0.0680)
0.0168)2017/6/20
2―――
Outliers
Y/M/D

3

2017/6/24

24

2018/5/2

35

2018/8/24

36

2018/12/3

37

2019/3/3

28

2019/3/6

2

9

2019/10/2

2

10

2019/10/23

2

11

2019/11/24

2

12

2020/6/11

3

13

2020/6/25

2142021/5/15

3

Average

Outliers
0.539 (C: 0.488,
F: 0.0510)
0.347 (C: 0.346,F: 0.001)
0.440 (C: 0.435,
F: 0.005)
0.129 (C: 0.0493,F: 0.0793)
0.309 (C: 0.108,
F: 0.201 )0.382 (C: 0.0480,F: 0.335)
0.173 (C: 0.0197,F: 0.153)
0.239 (C: 0.178,F: 0.0615)
0.214 (C: 0.0610,F: 0.153)
0.313 (C: 0.115,F: 0.199)
0.388 (C: 0.175,F: 0.213)
0.314 (C: 0.188,F: 0.125)―――

0.369 (C: 0.230, F:0.139)
0.159 (C: 0.159, F:0.00)
0.223 (C: 0.222, F:0.00118)
0.0157 (C: 0.0137,F: 0.00200)
0.122 (C: 0.122, F:0.00)
0.0757 (C: 0.0757,F: 0.00)
0.259 (C: 0.0410,F: 0.218)
0.310 (C: 0.310, F:0.00)
0.220 (C: 6.00×10-5, F: 0.220)
0.473 (C: 0.150, F:0.323)
0.435 (C: 0.363, F:0.0726)
0.206 (C: 0.157, F:0.0480)

0.766 (C: 0.753,F: 0.0122)
0.518 (C: 0.514,F: 0.00453)
0.622 (C: 0.609,F: 0.0132)
0.149 (C: 0.0307,F: 0.118)
0.379 (C: 0.00, F:0.379)
0.688 (C: 0.00, F:0.688)
0.221 (C: 0.00, F:0.221)
0.142 (C: 0.00, F:0.142)
0.289 (C: 0.0512,F: 0.237)
0.367 (C: 0.153,F: 0.214)
0.00 (C: 0.00, F:0.00)
0.370 (C: 0.194,F: 0.176)

0.482 (C: 0.482, F:0.00)
0.364 (C: 0.364, F:0.00)
0.475 (C: 0.475, F:0.0000106)
0.221 (C: 0.104, F:0.118)
0.426 (C: 0.203, F:0.223)
0.383 (C: 0.0669,F: 0.316)
0.0378 (C: 0.0179,F: 0.0199)
0.226 (C: 0.224, F:0.0422)
0.132 (C: 0.132, F:0.00)
0.100 (C: 0.0409,F: 0.0592)
0.728 (C: 0.163, F:0.565)
0. 324 (C: 0.214,F: 011)

2

R, r1074.0(-)
30.0(-)
11.0(-)
5.0(-)
18.0(4.0)
5.0(2.0)
11.0(5.0)
18.0
(4.0)
36.0
(12.0)
11.0(2.00)
8.00(4.00)
42.0(12.0)
34.0(18.0)
135(16.0)

Qp
132
103
106
44.0
137
124
42.4
78.0
98.5
51.6
180
99.0
178
272

R, r10Qp

14.5(-)
40.0(6.00)
46.0(8.00)
19.0(7.00)
5.00(4.00)
12.0(9.00)
5.00(2.00)
23.0(7.00)
6.00(6.00)
7.00(2.00)
3.00(1.00)
4.00(1.00)
6.00(6.00)
9.00(8.00)

31.9
28.4
112
128
62.3
93.5
213
152
265
46.7
60.9
107
56.1
221*

E3S Web of Conferences 415, 03015 (2023)
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https://doi.org/10.1051/e3sconf/202341503015(a) Debris flow events on 3rd to 4th June, 2021
Fig. 1. Several patterns of debris flow events observed by LVP.

(b) Debris flow events on 12th August, 2021

karyu, which is a channel with a trapezium shaped flowarea.
The depth-averaged sediment concentration can becalculated using Takahashi’s formula [5, 6]; the values
were 0.0643 (6.5% bed slope) at Arimura No. 3 and0.0437 (4.5% bed slope) at Nojiri No. 1 sabo dam,assuming that the specific weight of the bed sediment is
2.65. The coarse sediment concentration can becompared with the values and estimates shown in thetables. The sediment concentration observed at Nojiri
No. 1 sabo dam was occasionally close to that estimatedusing Takahashi’s formula [6]. The monitored value wasgreater than that estimated by Takahashi’s formula at theArimura No. 3 sabo dam because the bed slope atArimura No. 3 sabo dam changes from 18.9% to 7.1%
and the sediment concentration could be affected byseveral factors such as nonstationary of debris flow. In
addition, there are few clear correlations between thesediment concentration of the combined coarse
sediment, suspended sediment and liquid phases and thepeak discharge, rainfall intensity and accumulated
rainfall depth. More monitoring data are required toclarify the reason for this.

3 Detection of debris flow occurrence
by LVPLVP systems installed on the river bed are mainly
used to detect continuous debris flows. The sensor cancapture information on passing debris flows [3, 4],especially when used in conjunction with wires.
Modified LVP sensors were installed at Nojiri No. 7
sabo dam in February, 2015. The sensors and a stainlesssteel plate required maintenance in February, 2019 andMarch, 2022, respectively. In comparison withdisconnected wires, the LVP can detect almost all
occurrences of debris flow events, except debris flows
associated with transverse channel shifting (one event).
The LVP data were recorded at a 10 Hz sampling rateusing an data logger (CR1000, Campbell Scientific Inc.,Logan, USA). The measured data were averaged at oneminute intervals as shown in Fig. 1.
At Arimura No. 3 sabo dam, maintenance of sensors
was performed on several occasions in December, 2017,electric issues were resolved in May, 2021, and a new
LVP was installed in February, 2022 to replace theFig. 2. Relation between flow depth and load at time of debris flow occurrence measured using LVP system and disconnectedwires.

3

E3S Web of Conferences 415, 03015 (2023)
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Fig. 3. Relation between flow depth and acceleration due to vibration at time of debris flow inferred by disconnected wires.

rainfall intensity on debris flow characteristics,
devastation in the sediment yielding area after debris
flow occurrence, and the magnitude of such events
remain to be clarified using DFLP monitoring data.

original system which was installed in October, 2016.
Monitoring has continued, focusing on correlations withDFLP data.
Figures 1(a) and 1(b) show typical debris flow data;
the occurrence of wires becoming disconnected is alsoshown. In the figure, ‘Load’, ‘Vb’ and ‘WL’ refer to theload, acceleration due to vibration, and flow depth for
the LVP measurements. Two surges of debris flows
were detected by the wires and the LVP. If a debris flow
occurs, vibration and flow depth are registered by thesensor, and typical debris flows can be identified by themeasured load. Deposited sediment that is presentbefore the debris flow is eroded during such events (Fig.
1(a)). On the other hand, sediment deposition that occurs
can remain on the bed during and after debris flow
events (Fig. 1(b)).
An LVP detects the flow depth, load andacceleration of vibration, in order, and the data can beused to distinguish the kind of debris flow, i.e., whether
it is associated with storny, muddy and other types ofdebris flows. Figure 2 shows the relation between flow
depth and load at the time a wire is disconnected. Inaddition, the relation between flow depth andacceleration of vibration at the time a wire becomes
disconnected has been evaluated since 2016. Figure 3
shows the relation between flow depth and acceleration
due to vibration at the time a wire is disconnected. Theremay be a threshold value for debris flow occurrence andfor the magnitude of debris flow. As shown in Figs. 2and 3, the threshold is measured to determine whether
the load is 100-150 kgf/m2 or 200-1200 mV. For debris
flows over 60 cm in depth, the threshold can be 6001,250 kgf/m2 or 2,800-3,000 mV.

The authors thank the Osumi Office of Rivers and National
Highways, Kyushu Regional Development Bureau, and theMinistry of Land in Japan (MLIT) for monitoring data anduseful advice regarding the present research.

References
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2.

3.

4.

5.

4 Conclusions
On Sakura-jima, which is still volcanically active,
the sediment concentration in two rivers was estimatedcontinuously by a DFLP system, and debris flows weredetected using wires in conjunction with an LVP system.
Online tools could be used for real-time detection ofdebris flow occurrences using the LVP. The effects of6.

4

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