10th Asian Regional Conference of IAEG (2015)

Displacement properties of a coastal landslide during a winter season
Sumio MATSUURA(1), Tatsuya SHIBASAKI(1), Hikaru OSAWA(1),
Hokuto SATO(1) and Issei DOI(1)(1) Disaster Prevention Research Institute, Kyoto University, Japan
E-mail:matsu03@scs.dpri.kyoto-u.ac.jp
Abstract
Weakly consolidated rocks are distributed widely in the area of eastern Hokkaido
facing the Pacific Ocean. Therefore, the occurrence of coastal landslides has resulted in therecession of the shoreline. In this area, the soil layer is frozen due to the low temperatureduring the winter period, and the frozen soil may have some effects on landsliding. For thisreason, coastal landslide displacement was monitored using a wire-type extensometer during
the winter period. Although the cumulative landslide displacement of five months was as
small as 8.6 mm, intermittent displacement was observed throughout the entire period. There
was a close relationship between landslide displacement and tide level and/or sea waves.
Heavy rain may have little effect on landslide displacement when topsoil is frozen.
Keywords: coastal landslide, frozen soil, sea wave, tide level
1.

Introduction

Coastal landslides occur frequently due to seaside
erosion along the coast of the Black Sea, the CaspianSea, and even the Baltic Sea (Edil et al. 1983; Işik etal. 1988; Setayeshirad et al. 2013; Uścinowicz et al.
2014). In Japan, reductions in the amount of sedimentcarried from mountains to the sea by rivers haveresulted in rapid recession of the shoreline. Althoughthese depend on the geological and oceanographic
conditions, slope failure, rock fall, and landslidesoccur in the coastal slope.
Weakly consolidated rocks, such as siltstone andargillaceous rock, are distributed widely in the area ofeastern Hokkaido facing the Pacific Ocean(Yamagishi et al. 1994). Therefore, landslides often
occur by coastal erosion, and this landsliding leads toyearly recession of the shoreline. In this area, the soil
layer is frozen during winter due to the lowtemperatures and the frozen soil may have someeffects on the displacement properties of landslides.
For this reason, the displacement of a coastallandslide was monitored during the winter period.
2.

Fig. 1 Location map of research site.

landslide-prone area (Fig. 2). Small to medium
landslides occur every year, and large-scale landslides
occur occasionally. The investigated landslide is asmall reactivated landslide with a length ofapproximately 70 m and a width of approximately 15
to 20 m (Fig. 3). Its head scarp is located just below asmall ridge of coastal hills, and its terminus is at theOverview of the research site andmonitoring method

We selected a reactivated landslide located in thehillside along Akkeshi Bay, Hokkaido, as the researchsite (Fig. 1). The hillsides along Akkeshi Bay are a1

10th Asian Regional Conference of IAEG (2015)

Fig. 4 An installed wire-type extensometer.

Fig. 2 Ongoing landslides in hillsides ofAkkeshi Bay.

siltstone and mudstone are weakly consolidated andbreak down into sand-sized particles in a shortamount of time. The annual average temperature inthis region is 5.5 °C, and the daily mean temperature
falls below 0 °C from December to March. However,the research site is located at the shoreline, so thelowest temperature is no lower than -15 °C, which isin contrast with inland areas of Hokkaido. The depthof snow during the coldest season is shallow because
little solid precipitation falls during the winter period.
To observe the landslide displacement, weinstalled a wire-type extensometer with a resolutionof 0.1 mm at the head scarp of the research site (Fig.
4). Landslide displacement was monitored every one
hour from December 2013.

Fig. 3 Research landslide as seen from the sea.

3.
shoreline. Therefore, this landslide is subject tofluctuations of tide level and/or wave erosion.
The local geology consists of the Nemuro Group,composed of alternating Late Cretaceous sandstone,siltstone, and mudstone, and it has a homoclinal
structure with a dip of approximately 20° south.
Although the geological age is Cretaceous, theResults and discussion

Fig. 5 shows the landslide displacement of thefour months from December 22, 2013, to April 28,
2014. Although the cumulative displacement of thelandslide during the monitoring period was as small
as 8.6 mm, intermittent landslide displacement wasobserved throughout the period. The maximum daily
10.0

Rainfall

142.0

Displacement

8.0

140.0

6.0

138.0

4.0

136.0

2.0

134.0
22-Dec0.0
1-Jan

11-Jan21-Jan31-Jan10-Feb

20-Feb

2-Mar

12-Mar22-Mar

1-Apr

11-Apr

21-AprFig. 5 Relationship between rainfall and cumulative displacement of landslide during the winter period.

2Rainfall (mm/h)

Cumulative disp. (mm)144.0

10th Asian Regional Conference of IAEG (2015)displacement of 1.2 mm was observed on April 5, andsmall displacement was detected in even the coldest
months of January and February.
Generally, precipitation in Eastern Hokkaido issmall, and only a negligible amount of rain isobserved in the coldest season. Thus, intermittent
slight displacement may be due to oceanographic
conditions. For this reason, we compared the tide dataof Akkeshi fishing port with the observed landslide
displacement and found a close relationship between
the tide data and landslide displacement, as shown inFig. 6. Therefore, there is a high possibility that thetoe of the landslide was destabilized by an increase inpore-water pressure and/or wave erosion.
160
1400.1120
1000.2

800.3

60
400.4

Tide level (cm)Displacement (mm/h)

0.0

scale of the landslide scale becomes smaller. This
landslide movement did not begin during the rain but
instead began one hour after the rain. Because thisresearch site is located in the zone of soil freezing andthe topsoil was frozen to a depth of 40–80 cm for 4 to
6 months, there is a high possibility that rainwater
infiltration was prevented by the frozen soil. Changes
in the timing and thickness of snow cover can cause adramatic reduction of frost depth and change in thesoil water dynamics (Iwata et al. 2010). Therefore,climate change may have a significant impact on thelandslide activities.
The sea was rough during the approach andpassing of the atmospheric depression after April 4(Fig. 8). In addition to the tide level at Akkeshi
fishing port, the average wave height, and thesignificant wave height, the highest wave heightobserved at Kushiro Port was also investigated. Fig. 9shows that the landslide displacements occurred attimes when high waves were generated.
Although detailed investigation is needed because
Kushiro Port is about 40 km from Akkeshi fishingport, the effects of sea waves cannot be excluded.
When landslide displacement is caused by waves, thetotal amount of displacement would increase unlessthe topsoil is frozen. In the future, we intend to add
piezometers and soil temperature gauges to thepresent monitoring system to make a more detailed
analysis.

200.5

09-Jan11-Jan13-Jan15-Jan17-Jan19-JanFig. 6 Relationship between tide level and landslide
displacement in the coldest season.

143.0

10.0

142.0

8.0

2.3mm

141.0

6.0

140.0

4.0

139.0

2.0

138.0
3-Apr

4-Apr

5-Apr

6-Apr

7-Apr

Rainfall (mm/h)

Cumulative displacement(mm)

On the other hand, an atmospheric depression
brought approximately 60 mm of rainfall with amaximum hourly rainfall of 9 mm on April 4.
Landslide displacement was detected immediately
after the end of the rainfall regime, and cumulative
displacement of the three days following the event
reached 2.3 mm. Fig. 7 shows the relationshipbetween the rainfall and the cumulative displacementof the landslide.
When it rains, pore-water pressure increases
because of rainwater infiltration into the slope. The
increase in pore-water pressure causes a decrease ineffective stress, resulting in landslides. Therefore, inthis case, precipitation appears to have induced thelandslide. However, the time lag between rainfall andlandslide displacement tends to become shorter as the0.0
8-Apr

Fig. 8 Weather charts of April 4 and 5, 2014.

Fig. 7 Relationship between rainfall and cumulative
displacement of the landslide.

(http://www.jma.go.jp/jma/indexe.html)

310th Asian Regional Conference of IAEG (2015)

Fig. 9 Relationship between sea wave height and landslide displacement.

4.

infiltration: A paired plot experiment, Water
Resources Research, 46, W09504, DOI:
10.1029/2009WR008070.
Setayeshirad, M. R., Nikudel, M. R. and Uromeihy, A.
(2013): Landslide susceptibility assessment along
the southern coast of the Caspian Sea, Iran,Natural Hazards, 69, 2215–2232.
Uścinowicz, G., Kramarska, R., Kaulbarsz, D., Jurys,L., Frydel, J., Przezdziecki, P. and Jegliński, W.
(2014): Baltic Sea coastal erosion; a case study
from the Jastrzębia Góra region, Geologos 20, 4,59–268.
Yamagishi, H. and Ito, Y. (1994): Relationship of thelandslide distribution to geology in Hokkaido,
Japan, Engineering Geology, 38, 189-203.

Conclusions

We monitored a coastal landslide during a winterseason to clarify the relationships between landslide
displacement
andoceanographicconditions.
Intermittent displacement was observed throughout
the period, and there were close relationships
between landslide displacement and tide level and/or
sea waves. However, the physical mechanism andprocesses of landslide destabilization by theseoceanographic conditions were not elucidated. On theother hand, heavy rains may have little effect onlandslide displacement when the topsoil is frozen. It
is necessary to advance further research of the effectsof frozen soil on landslide stabilization ordestabilization.
Acknowledgment
We thank the Hokkaido Regional Development
Bureau, which provided us with oceanographic datafor Akkeshi fishing port and Kushiro Port.

ReferencesEdil, T. B. and Shultz, M. N. (1983): Landslide
hazard potential determination along a shoreline
segment, Engineering Geology, 19, 159-177.
Işik, N. S., Doyuran, V. and Uluşay, R. (2004):
Assessment of a coastal landslide subjected tobuilding loads at Sinop, Black Sea region, Turkey,and stabilization measures, Engineering Geology,72, 69-88.
Iwata, Y., Hayashi, M., Suzuki, S., Hirota, T. andHasegawa, S. (2010): Effects of snow cover onsoil freezing, water movement, and snowmelt

4