Report of Field Trip to Shobara, Hiroshima
Hufeng YANG
(Department of Geoscience, Shimane University, Matsue, Shimane 690-8504, Japan)

Date: Oct.28, 2011 - Oct.29, 2011
Trip Route: Matsue CityShobara CityHiroshima CityMatsue City
Organizer: Japan Landslide Society (Kansai Branch)

Debris Flows at Shobara
At July 16, 2010, massive debris flows occurred shortly after storm rainfall atShobara, Hiroshima. Debris flows destroyed road, houses and vegetation nearby thegulley. Unfortunately, this debris-flow hazard took one life when destroyed one house
located at the foot of slope. The debris flows obtained most of their volume byscouring a mass of topsoil and channel bed, and destroyed extensive vegetation(Fig.1).

Fig. 1

A gulley after debris flow that occurred at Shobara, Hiroshima, Japan

We investigated the source area of one channelized debris flow (Fig. 2). It is atranslational shallow landslide. According to the exposed main scarp, the bedrock is1

weathered andesitic rock. Topsoil layer located on the upper part of slope is thin,about 40cm to 100cm in thickness. This layer is composed of completely weatheredandesitic rock, vegetation root system, and humus layer (Fig. 3). Variously, topsoil
layer located on the lower part of slope is thicker, about 1m to 1.5m in thickness. Thislayer is composed of pumice, volcanic ash, and humus layer (Fig.4). The flow path
has a terrain with 30° to 45° degree slope.
Undoubtedly, intense rainfall is the triggering factor. Before the occurrence ofmassive debris flows, the maximum precipitation is 91mm within one hour (Masahiro,2011). The rapid infiltration of rainfall, causing topsoil layer saturation and atemporary rise in pore-water pressure, is believed to be the mechanism of this shallow
landslide. Because the porous nature of topsoil layer located on the lower part slope,
piping holes are visible under biggish hydraulic gradient (Fig. 5).
Attentively, concentration of runoff from the storm mobilized talus and otherdebris in tributary channels and scoured material from the main channel into a debris
flow, which maybe occur under next heavy rainfall. In addition, the flank of landslide
forms the exposure face of topsoil. The topsoil should be partly failure under othertriggering factors (heavy rainfall or earthquake) along the flank of landslide.

Fig. 2 View of debris flow area from thecrown of landslide

Fig. 3 Topsoil layer located on the upper partof slope

Fig. 4 Topsoil layer located on the lower partof slope

Fig. 5 Piping holes in the topsoil layer locatedon the lower part of slope2

Debris Flows Mitigation and Slope Stabilization
The local government immediately undertook necessary mitigation methods after
the debris-flow hazard.
At the mouth of valley, concrete check dams were built to store debris-flow
sediment (Fig. 6 and Fig.7). Although concrete check dams are expensive to construct,
they go a great way with controlling channelized debris-flow frequency and volume.

Fig. 6

Concrete Check dam with low-flow
center section

Fig. 7

Upstream view of a concrete checkdam

Biotechnical slope protection method was used along the road (Fig. 8). This typeof seeding is the application of seed in water slurry that contains fertilizer, soil binder,and mulch. Seeding with grasses reduces surface erosion, which can under certain
conditions lead to landslides. If not controlled, surface erosion and small, shallow
slope failures can lead to larger problems that cannot be controlled.
Compared with shotcrete, this type of slope protection can reduce theenvironmental consequences of landslide-mitigation measures, and is more visuallypleasing and environmentally friendly. At some rock slope area, these are tools
composed of steel robs or cables that reinforce and tie together a rock face to improve
its stability (Fig. 9).

Fig. 8Biotechnical slope protection

Fig. 9 Anchor & biotechnical slope protection
3

Gabion walls were constructed at small-scale landslide area along the road(Fig.10). The gabion retaining walls were composed of stacked gabions which arewire mesh and boxlike contains filled with cobble-sized rock that are 10 to 20
centimeters size. Gabion walls are inexpensive and are simple and quick to construct.
Because of their coarse fill, they are very permeable and thus provide excellent
drainage.

Fig. 10 Gabion wall along the road4