E3S Web of Conferences 415, 03013 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341503013

Initiation and runout characteristics of partially saturated debris
flows in Ohya landslide scar, Japan
Fumitoshi Imaizumi1*, Shunsuke Oya2, and Shoki Takayama1
1Faculty of Agriculture, Shizuoka University, Shizuoka, Japan
2 Nippon Koei Co., Ltd. Fukuoka, JapanAbstract. A partially saturated flow, which has an unsaturated layer in its upper part, has been monitoredin the steep initiation zones of debris flows. Understanding the initiation and runout characteristics ofpartially saturated flows is essential for predicting the timing and magnitude of downstream debris flows.
Monitoring performed using time-lapse cameras and water pressure sensors in the Ohya landslide scar incentral Japan allowed us to obtain data on a series of partially saturated debris flow surges from initiation totermination on July 6, 2020, and July 13, 2021. Debris flow surges were mainly induced by repetitive mass
movement of sediment deposit caused by the overland flow erosion of channel deposits, channel deposit
slides, and water and sediment supply from channel banks and tributaries. The excess pore water pressure
in a partially saturated flow on July 6, 2020, was clearly higher than that on July 13, 2021. Rainfall patterns,
which control the water content in channel deposits, and the flow height likely affect the magnitude of theexcess pore water pressure in partially saturated flows.

1 Introduction
Partially saturated debris flows, which have an
unsaturated layer in their upper part, have beenmonitored in many debris flow torrents worldwide [1-3].
Partially saturated flow is typical in steep debris-flow
initiation zones due to the higher shear stress at thebottom of a sediment mass, which exceeds the shear
strength even without full saturation of the sedimentmass [4]. Many debris flow monitoring activities havebeen undertaken in debris flow transportation zones, inwhich fully saturated flow is the predominant flow type.
Therefore, initiation and runout characteristics ofpartially saturated flows remain unclear.
Debris flows with higher pore water pressure havehigh flow mobility and a long travel distance due to thedecreased flow resistance caused by a reduction in theeffective stress among large particles [5]. Pore waterpressure consists of hydrostatic and excess pore waterpressures [6], and excess pore water pressure occurs
owing to contraction by surrounding large particles
under high shear stress [5], loading of particles to theinterstitial water [7], and Reynolds stress from theturbulence of the interstitial water [8]. Many of thesefindings have been obtained through experiments on
fully saturated debris flows. Excess pore water pressure
in a partially saturated flow is yet to be understood.
The aim of this study was to elucidate the initiation
and runout characteristics of partially saturated debris
flows based on field monitoring of the Ohya landslide
scar in central Japan. Debris flows occur frequently in
the Ohya landslide due to the mobilization of deposits(e.g., channel deposits and talus slopes) during rainfall

events. We monitored debris flows from initiation totermination using time-lapse cameras (TLCs) on July 6,
2020, and July 13, 2021. The pore water pressure in thedebris flows was measured using water-pressure sensors.
The objectives of this study were to (1) clarify theinitiation timing, location, and mechanism of partiallysaturated flows; (2) reveal the runout characteristics ofpartially saturated flows; and (3) discuss the factors
affecting pore water pressure in partially saturated flows.

2 Study site
The Ohya landslide, which has a total volume of 120
million m3, was initiated during an earthquake in 1707
CE. Most of debris flow in Ohya landslide occurs in
Ichinosawa torrent, which flows down from the northernend of the landslide (Fig. 1). The Ichinosawa catchment
has an altitude ranging between 1,270 and 1,905 m a.s.l.
with an area of 0.3 km2, and channel length of 1000 m.
It is divided into upper and lower Ichinosawa, separated
by the “Ohya-ohtaki” waterfall located at 1,450 m a.s.l.
(P20 in Fig. 1). The upper Ichinosawa is characterized
by a deeply incised channel and steep slopes (40–65°).
The channel gradient is mostly steeper than 25° andapproaches the slope gradient of the talus slope (37.3°)
in the uppermost part of the channel. Rockfall and dryravel promoted by freeze-thaw in the winter and early
spring are the predominant sediment infilling processes[11]. A large volume of sediment, ranging from sandparticles to boulders, is stored in the channel bed andtalus cones. The lower Ichinosawa is a debris flow fan,and the channel gradient is in the range of 15–20°.

* Corresponding author: imaizumi@shizuoka.ac.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, 03013 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341503013

Sensors were installed in holes made using a hammer
drill and covered with a mortar. Three additionalpressure sensors were installed at three heights (S3, S4,and S5 at 0.64, 0.91, and 1.17 m from the channel bed,respectively) along the same cross-sectional line in May
2021. The data sampling interval was set at 4 s.
Components of the pore water pressure p are
expressed as follows:(1)‫ ݌‬ൌ ‫݌‬௛ ൅ ‫݌‬௘ ൌ ‫݌‬௪ ൅ ‫݌‬௦ ൅ ‫݌‬௘where ‫݌‬௛ is the static water pressure, ‫݌‬௘ is the excess
pore water pressure, ‫݌‬௪ is the static pressure of cleanwater, and ‫݌‬௦ is the static pressure of suspended finesediments. In the case of clean water without any ‫݌‬௘ ,μ‫݌‬/߲‫ ݖ‬is 9800 (Pa m-1). Assuming that the maximum
volumetric weight of the interstitial water is the same asthat of the sediment (=2,650 kg m-3), the maximum
value of μ‫݌‬௛ /߲‫ ݖ‬is 25,970 Pa m-1. Therefore, excess
pore water pressure certainly exists in debris flows if theμ‫݌‬/߲‫ ݖ‬exceeds 25,970 Pa m-1. In this study, ο‫݌‬/ο‫ݖ‬,
obtained from the difference in the pore water andelevation between the two water pressure sensors, wasused as a substitute for μ‫݌‬/߲‫ݖ‬.

oE138 18’ 45’’

Tokyo

Ohya landslide
N35o18’ 29’

Kyoto

Ichinosawa
P1

P20
Rain gauge
P36

Ohya landslide

Check dams

(a)

500 m

Fig. 1. Map of the Ohya landslide.

Front

Tail

Fully
saturated
Partially saturated
Partially saturated

Partiallysaturated

3 Methodology(b)

Front

Tail

Fig. 2. Schematic diagram of surges including partiallysaturated flow [10]. (a) Surges in which only partiallysaturated flow was identified. (b) Surges in which bothpartially and fully saturated flows were identified.

3.1 Time lapse photography
In April 2020, twenty-seven time-lapse cameras (TLCs,Brinno TLC200Pro and TLC2000) were installed along
the Ichinosawa torrent (P1–P36 in Fig. 1) to capture
debris flows from the initiation zone to the depositionzone. The interval of images captured, which ranged
from 1 to 10 s, differed among the cameras and periods.
The number of TLCs was reduced to twenty-one in April
2021. A video camera was also installed at P20 in 2021.
We set 36 analysis points (P1 to P36) along the main
channel of Ichinosawa with a spacing of approximately
20 m to interpret the arrival timing of debris flow surges
using TLC images. Temporal changes in the flow type(partially and fully saturated flow) during debris flow
events were visually and manually identified fromimages based on the presence of interstitial water on theflow surface [4]. The fully saturated flows are turbulent
and are characterized by a black surface due to highconcentrations of silty sediment in the interstitial water
filling the boulder matrix. In contrast, muddy water is
not identified in the flow surface matrix of the partiallysaturated flows (Fig. 2).

z (m)Sensor1.17
0.91

MortarS5
S4

Bedrock
S30.640.33
0S2
Bedrock

S1

Fig. 3. Schematic diagram of water pressure sensors at P20.

4 Results
4.1 Debris flow on July 6, 2020
The debris flow was triggered by long-lasting rainfall
from July 5 to July 11, 2020 (Fig. 4). The volume ofsediment storage in the upper Ichinosawa, which wasestimated by UAV-SfM (structure from motion using anunmanned aerial vehicle) [10], was 50,438 m3 before thedebris flow event. Debris flow surges first occurred at
the upper end of the monitoring section at 10:24 on July
6. The peak rainfall intensity of the rainfall event was
7.6 mm/10 min recorded at 10:25 (Fig. 4a). The
cumulative rainfall before this rainfall peak was 99.6mm. A total of 65 surges were monitored by TLCs on

3.2 Pore water pressure
Pore water pressure sensors (KELLER AG fürDruckmesstechnik, PR-26Y), which measure pressurein the range from 0 to 98 kPa with an accuracy of±0.25㸣, were installed at two channel bank heights (S1and S2 at 0 and 0.33 m from channel bed, respectively)
with bedrock exposure at P20 in May 2020 (Fig. 3).

2

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10

Horizontal distance from P36 (m) 10-miunte rainfall
intensity (mm)Horizontal distance from P36 (m) 10-miunte rainfallintensity (mm)

(a)
50
Jul. 5

Jul. 7

Jul. 11

Jul. 9700 P1100(c)Sensors
S1 (0 m)S2 (0.33 m)

Pore waterpressure (kPa)

Pore waterpressure (kPa)(b)

No data¨p /¨z (Pa m-1)

Flow type
PS
FS

60000
40000
20000

40

Flow type
PS
PS+FS
FS
Unclear

(b)
15:10

20
10

30000

15:20

15:30

Sensors
S1 (0 m)S4 (0.91 m)
S5 (1.17 m)

(c)

30

40000

No data(d)

Flow type
PSFS

25970

20000

25970

10000

9800

0
10:20

19:58

P20

0
15:00
50

050000

0(d)

17:58

200

2080000

15:58300

Flow typePSPS+FSFS30
100
13:58

400

P2005040

10500

200100(a)600500300

20700 P1600
40030

10:40

11:00

9800

0
15:09

11:20

15:11

15:13

Fig. 5. Monitoring results of debris flow on July 13, 2021.
(a) Rainfall intensity. (b) Runout timing and flow type atanalysis points. Debris flow could not be monitored in graysections because of shade of topography. PS and FS indicate
partially and fully saturated flows, respectively. (c) Porewater pressure at P20. (d) ο࢖/οࢠ calculated from S1 and S4
sensors.

Fig. 4. Monitoring results of debris flow on July 6, 2020. (a)Rainfall intensity. (b) Runout timing and flow type atanalysis points. Debris flow could not be monitored in graysections because of shade of topography. PS and FS indicate
partially and fully saturated flows, respectively. (c) Porewater pressure at P20. (d) ο࢖/οࢠ calculated from S1 and S4sensors.

July 6. Many surges during this event were initiatedabove P1 (Fig. 4b). Debris flow surges that started in themonitoring section of the TLCs (i.e., P1 to P20) weretriggered by the erosion of channel deposits by overland
flow and water and sediment supply from tributaries.
Many of the surges were partially saturated flow,followed by a fully saturated flow (Fig. 2b). The debris
flow passed P20 from 10:34 to 10:53 on July 6. The
largest surge during this rainfall was monitored using aTLC at P20 from 10:47. The estimated peak flow heightof this surge is 2.7 m. The pore water pressure sharply
increased with the arrival of the surge (Fig. 4c). The firsthalf of this surge was a partially saturated flow followed
by a fully saturated flow. The ¨p /¨z in the partiallysaturated flow exceeded the maximum value of μ‫݌‬௛ /߲‫ݖ‬(25,970 Pa m-1), indicating a large excess pore waterpressure in this surge (Fig. 4d); its travel distance was
the longest among the surges during the rainfall event.
The surge proceeded with a gentle channel section
between P21 and P26, with a slope gradient of 15° to20°.

4.2 Debris flow on July 13, 2021
Debris flow was triggered by a short and intensive
rainfall with peak intensity of 23.4 mm/10 min on July13, 2021 (Fig. 5a). The cumulative rainfall before thispeak was 33 mm. The volume of channel storage inupper Ichinosawa before the debris flow event was
59,731 m3. A total of 71 debris flow surges weredetected using TLCs. Two sensors (S2 and S3) failed toobserve this debris flow due to mechanical problems.
Partially saturated debris flow was the predominant flow
type during this event (Fig. 6). Many debris-flow surges
were initiated in the channel section between P1 and P20(Fig. 5b). Three debris flow initiation mechanisms werecaptured by the TLCs: erosion by overland flow
proceeding over unsaturated deposits, sliding of channel
deposits, and sediment and water supply from thechannel bank, talus slope, and tributaries. Ten debris
flow surges passed the P20 and reached the alluvial fan
in lower Ichinosawa. The pore water pressure of thepartially saturated flow at 15:11 was monitored using
3

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D

between July 6, 2020, and July 13, 2021 (50,438 and59,731 m3, respectively), the rainfall pattern was
possibly a more important factor affecting the differencein the initiation and flow characteristics of the two debris
flow events.

E

6 Summary and conclusions

Fig. 6. Video camera images of (a) partially and (b) fully
saturated flows captured at P20 on July 13, 2021. Black lines
indicate sides of flows

To clarify the initiation and runout characteristics ofpartially saturated debris flows, we conducted a field
monitoring experiment in the Ohya landslide scar. Threeinitiation mechanisms of debris flow surges werecaptured by the TLCs: erosion by overland flow
proceeding over unsaturated deposits, sliding of channel
deposits, and sediment and water supply from tributaries.
The excess pore water pressure observed on July 6, 2020,
likely increased the mobility of the partially saturated
debris flows. The rainfall pattern controlled theinitiation points of debris flow surges, the dominance ofpartially saturated flow, and the magnitude of excess
pore water pressure.
Several factors, such as rainfall pattern, flow height,
and volume of erosible sediments, can potentially affectthe mobility of partially saturated debris flows.
Consequently, the accumulation of monitoring data
under various conditions is critical to enhance theunderstanding of partially saturated debris flows.

three water pressure sensors (S1, S4, and S5; Fig. 5c). The
peak flow height was 1.2 m. ¨p /¨z was similar to or
smaller than that of the static water pressure of cleanwater (9,800 Pa m-1; Fig. 5d), indicating that unsaturated
layers existed in the flow. The small ¨p /¨z value also
implies low excess pore water pressure in the flow. We
could not obtain ¨p /¨z in the other surges because apositive water pressure was not observed by S4 owing to
the small flow height.

5 Discussions
Debris flow surges on July 6, 2020, and July 13, 2021,
were initiated by a series of mass movements in theinitiation zone of debris flow (Figs. 4b and 5b). Most of
the debris flow surges on July 6, 2020, occurred above
P1, whereas many debris flow surges occurred between
P1 and P20 on July 13, 2021. Because the cumulative
rainfall on July 13, 2021, was less than that on July 6,2020, a larger catchment area was likely needed for theoccurrence of mass movements on July 13, 2021.
Both partially and fully saturated flows composed
debris flow surges were observed on July 6, 2020,
whereas partially saturated flow was the primary flow
type on July 13, 2021. Debris flow monitoring in othertorrents found that rainfall pattern is an important factor
influencing flow type [3, 11]. The water content inchannel deposits on July 13, 2021, was likely less thanthat on July 6, 2020, due to small antecedent rainfall
before the rainfall peak, resulting in the occurrence ofdry partially saturated debris flow surges.
The monitoring result of pore water pressure on July
6, 2020, implies that the partially saturated flow had ahigh excess pore water pressure (Fig. 4c). Although apartially saturated sediment mass can travel only insteep channel sections (>22.2° in Ichinosawa) in thecontext of static force [4], partially saturated flows havebeen monitored even in gentler channel sections [1, 2].
Decreasing the effective stress owing to the generationof excess pore water pressure likely increases themobility of the unsaturated flow.
The difference in ¨p /¨z between the two debris flow
events implies that the magnitude of excess pore waterpressure varies among debris flow events (Figs. 4d, 5d).
The dry flow condition on July 13, 2021, may haveresulted in a small magnitude of excess pore waterpressure. The difference in flow height between the twodebris flow events also affects the difference in excess
pore water pressure.
The volume of debris flow material is another factor
that controls the debris flow characteristics [4, 12].
Because the volume of sediment storage was similar

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