10th Asian Regional Conference of IAEG (2015)

The Geographical features of the Hiroshima landslide disaster triggered by heavy
rainfall on August 20, 2014
Shoji DOSHIDA(1) and Kiminori ARAIBA(1)(1) National Research Institute of Fire and Disaster, Japan
E-mail:sdoshida@fri.go.jp
Abstract
On August 20, 2014, the landslide disaster occurred at 166 or more places in
Asa-minami ward and Asa-kita ward, Hiroshima city, Hiroshima prefecture, Japan by theheavy rain. In this research, we aimed at clarifying the geographical features of landslide byfield survey and analyzing LiDAR data measured before and after the disaster. It issurmised that the features of the landslide were the following phenomena. First, a strongly
weathered rock and soil (about 1m-deepth from the surface) causes shallow landslide byshort-time heavy rain. Next, the collapsed rocks and soils flow and go down along a valley.
At that time, the rocks shaves a valley deeply, joins the sediment of a valley, and becomelarge debris flow. Finally, a lot of debris flows attack the downstream housing area. The
analysis of LiDAR data showed that there are an area which is easy to do erosion, and anarea which is hard to do erosion in a valley. As two of various causes, we consider a gentle
slope and valley junctions.
Keywords: heavy rainfall, shallow landslide, debris flow, LiDAR, geographical features
1.

Introduction

Asa-kita ward, Hiroshima city, adjacent to the area
where landslide disasters occurred frequently,
hourly-maximum precipitation is 101.0 mm,
3-hour-maximum precipitation is 217.5 mm and24-hour-maximum precipitation is 257.0 mm areobserved (Fig.2) (Japan Meteorological Agency,
2014a; Japan Metrological Agency, 2014b). These
precipitations were the heaviest on the records at theobservatory. In this research, We considered thegeographical features of two landslides in the disasterby analyzing the LiDAR (Light Detection andRanging) data acquired before and after the disasterand field survey.

On August 20, 2014, the landslide disasteroccurred in Asa-minami ward and Asa-kita ward,
Hiroshima city, Hiroshima prefecture, Japan at morethan 166 places by the heavy rain. The serious
damage caused by this heavy rain was 74 deadpersons, 44 injured persons, and 3,562 material
damage, etc (Fire and Disaster Management Agency,2014). The landslide distribution map shown in Fig.1
quoted and corrected the data of the Geospatial
Information Authority of Japan (2014), and thegeological map quoted the seamless digital geologicalmap of Japan (1:200.000) (Geological Survey ofJapan, 2014). The geological units which thelandslide disaster occurred are Late Cretaceous
granite (Younger Ryoke Granite), mélange matrix ofMiddle to Late Jurassic accretionary complex andTriassic to Middle Jurassic chert block of Middle toLate Jurassic accretionary complex in Asa-minami
ward, and Late Cretaceous granite and Late
Cretaceous non-alkaline felsic volcanic rocks inAsa-kita ward. At the AMeDAS Miiri observatory,

2.

LiDAR data and Research area

The LiDAR data which analyzed in the researchwere offered from Aero Asahi Corporation, Inc. andOtagawa River Office, Chugoku Regional
Development
Bureau,MinistryofLand,Infrastructure, Transport and Tourism. The data
before the disaster acquired in December, 2009 –March, 2010, and the data after the disaster measured1

10th Asian Regional Conference of IAEG (2015)

another is the landslide in 6-chome, Kabe-higashi,

on August 25, 2014 (5 days after the disaster). The

Fig.1 Index map in this research (geological map and geographical map)
Asa-kita ward which the accident in which thefire-fighting staff are killed during a rescue effort
occurred (Kabe-higashi landslide). Chapter 3 and 4
explains the method of analysis and results in detail.

resolution of both data are 1-m mesh size.
We set the two landslide as the target of theresearch, and clarified the geographical features oflandslides by analyzing LiDAR data. One is thelandslide at the Midorioka municipal housing,
Asa-minami ward (Midorioka landslide) which
damage was the most serious in the disaster, and3.

The geographical features oflandslide - Midorioka landslidg

3.1 Field survey
The topographical map (left figure) andphotographs (right figures) around the landslide at theMidorioka municipal housing, Asa-minami ward(Midorioka landslide) are shown in Fig.3. In Fig.3
left, we illustrate the landslide distribution, valleylines (flow accumulation more than 1,000 m2) and theline of Profile A (Fig.4) with topographic map.
Overlapping of the landslide distribution and valleylines shows that landslides which occurred in thedisaster are mostly distributed on valley lines. Right
photographs show the result of a field survey. The
photo 1 shows the head of landslide. The depth of thelandslide was about 1 m and the landslide wasshallow landslide. The dike (rhyolite) was exposed atFig.2 Histograms of hourly rainfall, cumulative
rainfall and 10-minutes rainfall.

210th Asian Regional Conference of IAEG (2015)(left figure) and the graphs which calculated thefromProfile A is a profile in alignment with the mainvalley line of the Midorioka landslide. We displayed
four parameters in the graphs as geographical featuresof Profile A. The horizontal axis of the graphs show
the distance from ridge. The vertical axes of thegraphs show the elevation after the disaster (graph A),
the difference of elevations (before – after) (graph B),
Slope before the disaster (graph C), and flow
accumulation after the disaster (graph D). Each datais calculated the whole mesh (1-m resolution). The
data of B and C also expresses a 50-m moving
average deviation as the dotted line respectively.

this place. The photo 2 shows the large-scale side
erosion of landslide. The side erosion is considered to
be one of the causes by which the debris flow became
large-scale. The photo 3 shows the huge boulders
distributed with a debris flow sediment. Judging fromthe size of huge boulders, it seems that they weresupplied from not a head of landslide but eroded areaof landslide (Doshida et al., 2014; Doshida andAraiba, 2015).

3.2 Numerical geographical feature analysis
Fig.4 shows the different of elevation map(LiDAR data before the disaster – after the disaster)3

10th Asian Regional Conference of IAEG (2015)

However, overall variation is large and it is difficult
to explain all only from the slope before the disaster.
Next, we compared the difference of elevations(graph B) and flow accumulation (graph D). In theflow accumulation graph, the part where a value
becomes high suddenly show the point where abranch joins a main valley. As a result of comparing,the increase of the erosion was seen in the valley
junction (near the point of 580 m and 950 m).
However, overall variation is large and it is difficult

In B graph, the value is changing a lot. The
whole tendency shows that there are an area which iseasy to do erosion, and an area which is hard to doerosion in Profile A. In order to clarify thegeographical features of the easy to do erosion area,
we compared the difference of elevations (graph B)
and slope before the disaster (graph C). The
comparatively large amount of erosion was observedin the area of 20 degrees or less of slopes, especiallynear 900 – 1100 m (the distance from ridge).

410th Asian Regional Conference of IAEG (2015)

left, we illustrate the landslide distribution, valleylines (flow accumulation more than 1,000 m2) and theline of Profile B (Fig.6) with topographic map. The
photo 4 shows the distant view of the Kabe-higashi
landslide. The landslide occurred near the ridge,
flowed straight along the valley line, and hit two or
more houses directly. The photo 5 shows the erosion
and sediment area, and the photo 6 shows thesediment area of the landslide. With thesephotographs, the sedimentation structure which twoor more times of debris flows reached is seen. The
sediment consisted of the sand and the rock of thegranite, and the decomposed granite. (Doshida et al.,2014; Doshida and Araiba, 2015)

to explain all only from the flow accumulation too.
As mentioned above, the area where a slope before
the disaster is gentle and a valley junction are easy
erosion. However, it seems that there are othercauses.
4.

The geographical feature of landslide –Kabe-higashi landslide

4.1 Field survey
We analyzed the landslide in 6-chome,Kabe-higashi, Asa-kita (Kabe-higashi landslide) as
well as Chapter 3. The topographical map (left figure)and photographs (right figures) around theKabe-higashi landslide are shown in Fig.5. In Fig.5

4.2 Numerical geographical feature analysis

510th Asian Regional Conference of IAEG (2015)

area which is easy to do erosion, and an area which ishard to do erosion in Profile B. We compared thedifference of elevations (graph B) and slope before
the disaster (graph C), the amount of erosion waslarge in the area where a slope becomes gentle (200 –300 m). And we compared the difference ofelevations (graph B) and flow accumulation after thedisaster (graph D), the amount of erosion which wasdecreasing was maintained in near 295 m, and theamount of erosion was increasing in near 410 m. In

Fig.6 shows the different of elevation map(LiDAR data before the disaster – after the disaster)(left figure) and the graphs which calculated thegeographical features in Profile B (right figures).
Profile B is a profile in alignment with the mainvalley line of the Kabe-higashi landslide. We
displayed four parameters in the graphs asgeographical features of Profile B, there are the sameas that of Chapter 3. In B graph, the value is changing
a lot too. The whole tendency shows that there are an6

10th Asian Regional Conference of IAEG (2015)

profile B, the area where a slope before the disaster isgentle and valley junctions are easy to do erosion as
well as the area in profile A. However, it seems that
there are other causes too.
5.

References
Fire and Disaster Management Agency (2014): A
bout activity of fire-fighting and the damage sit
uation in Hiroshima Prefecture by the heavy rai
n from August 19 (The 43rd report) (in Japanese), http://www.fdma.go.jp/bn/2014/ (accessed Nov.
21th, 2014).
Geospatial Information Authority of Japan (2014):
Information about the damage situation by theheavy rain in August, Heisei 26(2014) (in Japanese), http://www.gsi.go.jp/BOUSAI/h26-0816heavyrain-index.html (accessed Nov. 4th, 2014)
Geological Survey of Japan (2014): Seamless Digital
Geological Map of Japan, Jan. 14th.
Japan Meteorological Agency (2014a): A Hiroshi
ma-related weather bulletin, Heavy rain of Hiroshima Prefecture in August 19 to 20, Heisei 26(2014) (in Japanese), http://www.jma.go.jp/jma/menu/h26hiroshima-menu.html (accessed Nov. 4th,2014)
Japan Meteorological Agency (2014b): The pastmeteorological data search (in Japanese), http://www.data.jma.go.jp/obd/stats/etrn/index.php.
Shoji DOSHIDA, Kiminori ARAIBA, Makoto SA
ITO, Daisaku KAWABATA (2014): Landslide ofHiroshima city occurred by the heavy rain on
August 20th, 2014 (in Japanese), Journal of theJapan Landslide Society, No.51, No.6, 256-259.
Shoji DOSHIDA, Kiminori ARAIBA (2015): Thedamage situation and its feature of the debris flow disaster of Hiroshima city by the heavy rai
n on August 20th 2014 (in Japanese), Fire-fighti
ng science and disaster prevention (Shoubou kagaku to Bousai), No.119, 39-45.

Conclusions

It is surmised that the geographical features ofthe landslide in the Hiroshima landslide disaster on
August 20, 2014 were the following phenomena. First,
a strongly weathered rock and soil (about 1m-deepthfrom the surface) causes shallow landslide byshort-time heavy rain. Next, the collapsed rocks andsoils flow and go down along a valley. At that time,
the rocks shaves a valley deeply, joins the sediment ofa valley, and become large debris flow. Finally, a lotof debris flows attack the downstream housing area.
The debris flow damage become large has thegreat influence by the erosion of a valley. Therefore,
we considered the geographical features of erosion
from the geographical analysis used by LiDAR datameasured before and after the disaster. The analysisof LiDAR data showed that there are an area which iseasy to do erosion, and an area which is hard to doerosion in a valley. As two of various causes, weconsider a gentle slope and valley junctions. However,it seems that there are other causes.
Acknowledgements
The LiDAR data which analyzed in this researchwere offered from Aero Asahi Corporation, Inc. andOtagawa River Office, Chugoku Regional
Development
Bureau,MinistryofLand,
Infrastructure, Transport and Tourism.

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