E3S Web of Conferences 415, 01016 (2023)
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https://doi.org/10.1051/e3sconf/202341501016

A novel approach to measuring pore fluid
concentrations of debris flows in a volcanic torrent

sediment

Shusuke Miyata1*, Michinobu Nonaka2, Seiki Kubo3, Kenshi Fukunaga3, Shohei Ando3, and Masaharu Fujita4
1Disaster Prevention Research Institute, Kyoto University, 436-13 Nakao, Okuhida-onsengo, Takayama, Gifu, Japan
2Hydrotech, Co. Ltd., 876 Kaminoda, Hino, Shiga, Japan3Ohsumi Office of River and Highway, Kyushu Regional Bureau, Ministry of Land, Infrastructure, Transport and Tourism, 1013-1

Shintomi, Kimotsuki, Kagoshima, Japan
4Disaster Prevention Research Institute, Kyoto University, Shimomisu, Yoko-oji, Fushimi, Kyoto, JapanAbstract. Pore fluid in a debris flow is not fully understood, despite its contribution to the fluidity of thedebris flow. To observe sediment concentrations of the pore fluids in debris flows, we established a novel
monitoring system in a volcanic catchment, in southern Japan. An observation culvert, 1.0 m in width, 1.5m in height and 2.0 m in length, was placed along the stream bank. An intake covered by a grating wasequipped on the side of the culvert so that only pore fluid of debris flows was led inside. Measurements ofdielectric permittivities were conducted within the culvert and used for the calculation of sedimentconcentrations of the fluid. The sediment concentrations of the pore fluid were successfully measured fornatural debris flows. The preliminary observation revealed that the volumetric sediment concentration ofpore fluid varied from approximately 5 to 69 %, which were slightly smaller or similar to those of the entire
debris flows. Successive occurrences of debris flow caused decreases in the sediment concentration of thepore fluids with each surge.

1 Introduction
Debris flows typically contain sediments of various
grain sizes (i.e., from clay to boulders). Whereas coarser
particles behave as solid phase in debris flows, waterand finer particles such as clay and silt may compose
pore fluids also as known as interstitial fluids. The finesediments in pore fluids can influence the pore fluidpressure and shear behaviour [1].
To understand the flow mechanism of debris flows,detail measurements of debris flows are required innatural torrents as well as experimental flumes. As
debris flows are severe and destructive phenomena,
noncontact measurements such as ultrasonic sensors,
vibration sensors, and videos are commonly used indebris flow observation stations [2]. Thus, internal
information of natural debris flows is still insufficient.
Field observations of load and pressures at thechannel beds have revealed the pore fluid pressuresconsists of predicted hydrostatic and excessive pressures[3, 4]. The pore fluid pressures higher than the predictedhydrostatic pressures potentially influence the fluidity
and run-out distance of debris flows [5]. The hydrostaticpressure can be estimated by the flow height and theconcentrations of suspended sediment in the pore fluid.
Recent studies revealed measurement results of the pore
fluid pressure in natural torrents [6, 7]. Information of
the sediment concentrations of the pore fluid in a debris
flow can be a breakthrough of understanding flowmechanism of debris flows.

This study aimed to propose a novel approach tomeasuring sediment concentrations of pore fluids ofnatural debris flows. TDR (time domain reflectometry)measurements enabled us to estimate sedimentconcentrations of pore fluids. The measurement system
was installed in a culvert with an intake of the pore fluids,
which was fixed on the bank of a debris flow prone
channel.

2 Application of TDR to sedimentconcentration measurement
A TDR measurement yields the dielectric permittivity ofa target substance. If it is assumed that a debris flow
consists of only water and sediment particles, the pore
fluid is also composed by water and fine sedimentparticles. The dielectric permittivity of the pore fluid canbe calculated using the volumetric mixing model. Thesquare root of the complex dielectric permittivity of theturbid water is calculated using the dielectric
permittivities of the water and the sediment particles, asfollows:𝜀 = 1−𝐶
𝜀 +𝐶 𝜀(1)where Cf is the volumetric sediment concentration ofpore fluid. The dielectric permittivities of water andsediment particles are known parameters (𝜀 = 78.3 at
25 °C and 𝜀 = 5.27 in this study). The 𝜀 value wasreferred from the previous study in which the sediment
samples collected at this study site was applied for a* Corresponding author: miyata.shusuke.2e@kyoto--uc.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, 01016 (2023)
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dielectric
permittivity
test.
concentration is expressed as:
𝐶 =https://doi.org/10.1051/e3sconf/202341501016

Thevolumetric

the intake window to filter coarser particles of debris
flows. Therefore, the pore fluid, mixture of water andfine sediment particles, flowed through the culvert withthe least disturbance to the debris flows.
Pressure-type water level gauges, TDR sensors, anelectromagnetic velocity meter, and a vibration meter
were installed in the observation culvert (Fig. 1e). The
pressure water level gauges were installed at 10, 40, and70 cm above the culvert bed. The TDR sensors wereinstalled at the heights of 10, 30, 50, and 70 cm tomeasure dielectric permittivities and water temperatures.
The TDR sensors were placed to avoid the influences of
the wall and bottom surfaces and other sensors on theresultant permittivities. To validate the flows within theobservation culvert, the flow velocity meter was placed
close to the outlet. The vibration meter was fixed to theceiling of the culvert. These measurements wereconducted every 5 seconds and began in March 2021.
The entire debris flows were observed at the checkdam (Fig. 1b). An ultrasonic velocity and flow stage
meter, CCTV cameras and load cell system have beenoperated since 2012 [5]. When malfunctions of thevelocity meter and/or flow stage meter, the velocity andflow depth were estimated from the video. The load cellsystem with the surface area of 2 m by 4 m is installedat the left bank of the dam crest and measured total
weights of the debris flows at 100 Hz of the samplingrate. The volumetric sediment concentration of an entire
debris flow Cv were obtained by dividing the weight perunit area by the flow depth. Precipitation was observed
by a X-band MP radar.

(2)

Our previous study has demonstrated that thedielectric permittivity measurement is applicable toobtain volumetric suspended sediment concentrations ofstream water higher than 2 % [8]. A comparison
between the dielectric permittivity measurement andbottle samples in a glacier fed stream revealed that the
95 % confidence intervals of the estimated sedimentconcentration were approximately equivalent to theobserved concentrations (i.e., 2SD ≈ 1.0)[9]. As the pore
fluids of debris flows typically contain greater fractionsof sediment particles than those of river flows, thismethod is expected to estimate the sedimentconcentrations of pore fluids.

3 Materials and Methods
The field observations were conducted in the Arimura
River catchment, which is in a volcano, Sakurajima,
southern Japan (Fig. 1). The volcano erupts frequently
and supplies sediment to the headwaters of thecatchment. The average channel slope is 1/4.2 and thecatchment area is 4.22 km2 [4]. The channel has
ephemeral flows only during storms and experiences
about 10 debris flows in a year. Check dams and channel
works have been constructed to mitigate the damage ofthe debris flows. Various equipment such as camera,load cells, ultrasonic stage sensors have been establishedto observe the debris flows at the most upper check dam
shown in Fig. 1b.
We installed an observation culvert about 50 m
upstream of the check dam (Figs. 1b-d). The observation
culvert was fixed at the left bank of the channel, and thegap between the culvert and the bank was filled withconcrete and protected by boulders collected in the field.
The culvert has the outlet with 1.5 m in height and 1.0
in width and equips intake window of 0.8 m by 0.8 m at
the side. A grating of 0.033-m spacing was mounted on

4 Results and Discussion
4.1 Deposits in the culvert
Several debris flows and floods took place after theinstallation of the observation culvert in 2021. The
photos during the evets showed that muddy fluids weredischarged from the observation culvert. SubstantialFig. 1 (a) Map and location of the study study site, schematic illustrations of (b) the study site and (c) observation culvert, andphotos of (d) observation culvert and (e) sensors installed in the culvert.

2

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amounts of deposits probably brought by the pore fluids
were found on the culvert bed after these events. The
grain size distributions of the deposits in the culvert
collected in April and June 2021 reveals that the fluids
led into the observation culvert were the mixtures ofwater and the fine particles mostly less than 10 mm (Fig.
2). Even if the TDR sensor at the bottom was buried inthe deposits, the sensor was not damaged and could
continue measuring after removing the deposits. The
measurement system proposed in this study wasvalidated the robustness and sustainability with theappropriate maintenance works.

values at the heights of 10, 30, and 50 cm (i.e., Cf10, Cf30,and Cf50) were about 60 %, suggesting these TDR

Percentage passing through [%]100

4.2 Sediment concentrations of pore fluid andentire debris flow
Debris flows with the peak discharge of 148 m3/s
were observed on 15 May 2021 (Fig. 3). The volumetricsediment concentration of the entire debris flow at thecheck dam reached 63.7 %. The sedimentconcentrations of the pore fluid were measured
successfully in the culvert during 13:53 – 13:55 and13:57 – 13:59 (Cf30 and Cf50 in Fig. 3), which wereslightly lower or similar with the concentrations of theentire debris flow (Cv). The measured Cf30 and Cf50
ranged from 33.3 to 42.2 % and from 35.0 to 54.6 %,respectively. Before 13:53, the resultant concentration

8060402000.1

110100Grain size [mm]Deposit in the culvert on April 20, 2021
Deposit in the culvert on June 4, 2021
Streambed on April 20, 2021
Streambed on June 4, 2021

Fig. 2 Grain size distributions of deposits collected in theculvert and stream bed after debris flow events

Precipitation[mm/10min]

20
15
10
5Discharge
3[m /s]

13:20

13:30

13:40

13:50

14:00

14:10

13:20

13:30

13:40

13:50

14:00

14:10

150
100
50
0Pressure[cmH2O]Cv [%]

Dam crest

0
13:10
20010080
60
40
20
013:10150
100
50

Cf [%]

Culvert

0P10
P40
P70

10080Cf1060
Cf3040
Cf50
20
0
13:10
13:20

Cf70
Sensor in deposit
13:30

Sensor in fluid

13:40

13:50

14:00

14:10

15 May 2021

Fig. 3 Observation results of precipitation, discharge and volumetric sediment concentration Cv of debris flow, pressures P andsediment concentrations Cf of pore fluid in culvert. Subscription of P and Cf indicate the sensor heights in the unit of cm.

3E3S Web of Conferences 415, 01016 (2023)
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sensors were in the saturated deposit. A small debris
flow at 9:20 brought the fine sediment into the culvert
and buried the sensor at 10 cm height. The pore fluid of
another debris flow at 13:20 probably caused furtherdeposition reaching the sensor at the height of 50 cm(Fig. 3).
The resultant Cf values after removing the periods ofburied TDR sensors ranged from 4.81 to 68.8 % by June
2021. During the observation period, Cf tended todecrease as the sequence of the debris flows. Because noeruptions occurred during this period, the catchment
possibly lacked sediment to be delivered by the debris
flows.

5 Conclusion
We proposed and demonstrated an approach to measure
volumetric sediment concentrations of pore fluids ofnatural debris flows. An observation culvert with anintake enabled the pore fluids led into exclusively. The
sediment concentrations of the fluids in the culvert weremeasured by applying the TDR technique. The sedimentconcentrations of the pore fluids varied greatly andtended to decrease as the series of the debris flows
withing the 4-months period of little sediment supplies
from the volcano. The combination of this measurement
system and a load cell system is expected to contributeunderstanding the flow mechanism of debris flows.

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