10th Asian Regional Conference of IAEG (2015)

A Hydrogeological Study for the Hiroshima August 2014 DisasterTomoaki KAYAKI, Seisuke MIYAZAKI, Kazuyuki MUKAI, Mitsunobu TERAMOTO and Sung Gi HU(Study Group for the Groundwater Environment of the Fan)
Abstract
Hydrogeological study has been carried out to unravel the main factor of the Hiroshima
August 2014 disaster at Asa district, Hiroshima city, caused by sudden heavy rain. The study
was done using topographic analysis, measuring the drought specific discharge, searching forthe points of spring water and its volume, and observations of the rainfall runoff response.
Along with the geologic structure and topographic features, the drought specific discharge,
which is closely related to the amount of base flow is assessed to reveal the main factor of thedisaster.
Since it is difficult to forecast disasters caused by heavy rainfall even with the mostadvanced technology, it is necessary to improve disaster prevention, especially at locations
prone to slope failure and collapse. This requires extensive geological surveys and careful
investigation of hydrogeological conditions, particularly the measurement of base flow inmountainous regions during the drought season.
Keywords: hydrogeology, weathering, drought specific discharge, water budget, storage-ability.

1.

Introduction

blocks (Divides), where the disaster mainly occurred.
While doing so we focused our concentration on the
two areas namely, Midori-i area (No.4 to No.6 divides)
and adjoining Yagi area (No.7 to No.11 divides) tocompare the hydrogeological characteristics and waterbalance of divides in each discharge basin. Debris flow
did not occur in the former area, while the latter was
severely affected. Depending on the data, we would
like to propose a disaster prevention plan and also aplan suggesting the ways of study and examination forthe sudden disasters in future.

On 19 and 20 August 2014, a local heavy rain
was recorded in the Asa district of Hiroshima city witha one-day precipitation of 535.4mm that included aconcentration of 217.5mm over a three-hour period in20th August midnight. The heavy rain caused greatand serious damage in the Asa district, especially in
Yagi area, where the disaster related to debris flow
affected hundreds of houses, 74 people died and 44
persons were injured. The mountain range (500m～
600m msl.) strikes NE-SW direction in the Asa district
and several tens of steep and narrow mountain streams
are flowing from north to south or northwest to
southeast directions in this district.
The present authors carried out hydrogeologicalstudies in between October 2014 and March 2015 (20working days total) and measured the discharge during
the drought season in November at the affected Asa
district in order to determine what factors triggered thedisaster and to assess the probability of similar eventsin the future.
2.

3.

Hydrogeology(1) Topography
The study area is situated in the southern end of
the foothill of Chugoku mountain range, which
extends to NE-SW direction. The mean height of themountain around Midori-i area (No.4-No.6) is about300-400m (msl) and that of Yagi area (No.7-No.11) is
500-600m (msl). The elevation differences from thestart to the end of mountain streams are about 300 and
500m, respectively. The mean inclination of the stream
is 1/4～1/6 for the former, while that is 1/2～1/3 forthe latter. Though the width of the river valley inMidori-i area is about 40m, watercourse is only several
ten centimeters to 1m wide. Thus the valley is a
U-shaped and the mountain is having gentle and stableslope. The terraced paddy fields are predominant inthis valley and it is continued from the end of theStudy area

The hydrogeological studies and measurementsof discharges were done at the southeastern side of themountain range stretching NE-SW direction from Mt.
Gongen-yama (397m msl.) to Watashi-ba in this
district (Fig.1). The study area is divided into 16 basin

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10th Asian Regional Conference of IAEG (2015)

Fig.1 Study area, sixteen divides and specific discharge values
stream to the head. While in the Yagi area, width of thevalley is nearly same as that in the Midori-i zone, but
the watercourse shows 1-2m in width. The valley ofYagi area is V-shaped and streams are flowing on steep
surface with many small and short falls. Weatheredsoil, over 10m in thickness covers the top of the valley
walls. Alluvial cones are formed at the river mouth
everywhere in the studied divides.

boundary between granitoid and sedimentary rocks(divides No.10 and No.11). No fault or sheared zones
are observed in the granitoid area (divides No.1No.10). The characteristics of the joint system andcracks (Photos 1 and 2) are different between thevalley less-affected (divide No.4) and that severely
affected (divide No.9) by debris flow calamity. In
divide No.4, which was less-affected, showed asporadic occurrence of joints and cracks in the field,
but the joint system is large compared to other divides
and degrees of adhesion is not good. Therefore weconcluded that divide No.4 shows both highpermeability and high discharge. On the other hand in
divide No.9, though the three dimensional and small
scale joints systems are comparatively well-developed,
they are stuck firmly to each other. So theimpermeable bed is forming in this divide.

(2) Geology
Granitoid is distributed around the divides No.1
～No10, while the sedimentary rocks (changed tohornfels by contact metamorphism of granite) aremainly occupied in the middle to upstream in thedivides No.11～No.16 area as roof pendant in this
district. The thick colluvial soils cover the valley walls
from top to bottom in every divide, except in the No.9
divide, which is covered by remarkably thicker
colluvial soils. Granitoid (mainly consist of
hornblende bearing biotite granodiorite) is solid andhard, and comparatively fresh. The weathering effects
increase with height, but in a minor way. The
weathering effects are broader and extend more deeply
in the granitoid of Midori-i area (No.4 divide, Photo.1)
than in Yagi-sawa area (No.9 divide, Photo.2).
So-called Masa is not observed in this district
except around the top of Mt. Gongen-yama and along
its ridge line. A few but comparatively large quartzporphyry dykes trending NW-SE are intruded into the

4. Characteristics of the slope failure and hydrogeologic structures(1) Yagi area (divides No.7～No.11)
Generally, two-types of collapses are observed individes No.7 to No.11 in the Asa area. One was along
the mountain slope (5 sites), where we observed debris
with 1m to 1.5m in thickness and nearly 500m3 ofcolluvial soils on the valley walls at a height of 30 to
50m from the river bed. The other was at the upstream
head of the rivers (eleven sites; nine in granitoid area2

10th Asian Regional Conference of IAEG (2015)

Fig.2 Geology of divides No.2 to No.12
and two in sedimentary rock area), where the collapse
occurred at the southwest side of the mountain slope.
However, the amounts of debris in the latter case arecomparatively small (1m to 3m thick) and show only
about 200 to 800m3 colluvial soils (Fig.2).
In divide No.9, four occurrences of collapse in
the vicinity of the river head (375m to 480m msl.) of
Yagi-sawa. The slope failure (1.5m to 3m in thicknessof surface soil) occurred along the deteriorated andsoftened cracks in bed rock caused by weathering,where colluvial soils and severely weathered part ofhost rocks collapsed in to shallow hollows on the slope.
Those four sites extend NW-SE and are possibly
controlled by geotectonic movement. Along the left
tributary of Yagi-sawa, two places of slope failures
were occurred at 270m to 260m (msl.). Both cases
occurred at the outcrops of deteriorated granitoid andat a surface moistened by spring water.

(2) Midori-i area (divides No.4-No.6)
In Midori-i area, we observed only threeoccurrences of collapse in the downstream area, but
the proportions are comparatively small.

Photo.1 Strongly-weathered granitoid in divide No.4

Photo.2 Characteristics of joint system and cracks individe No.9

5. Characteristics of the water budget in the
study area
The debris flow disasters occurred at all theabove- mentioned divides except No.4 to No.6 (Fig.1).
Hydrological indices in 16 drainage areas are as
follows (Table 1). Catchment area ranges between 0.03
～0.29km2, while the coefficient of catchment shapewith 0.14～1.06 showing comparatively slender form.
The river density is between 0.006 and 0.013,
irrespective of the area affected and un-affected bydebris flow. It is to be noted that the mean river
inclination is 0.38 at Yagi-sawa (divides No.7 toNo.10), the debris flow affected area. In comparison,3

10th Asian Regional Conference of IAEG (2015)

Table 1 Topographic and hydrological indices of watershed in debris flow affected and unaffected area

Sedimentaryrocks

Granitoid

Divides
Occur
Relative Mean
Total
Length of Mean
NumberNo.
Bed or not of Area ofheight inclination Length Drainage
Drainage
the river width ofof therock debris dividesof the of riverof the density
frequency(stream) the divide
Drainage
flow
river
floor
Stream
A
L
H
H/L
ΣLn
ΣLn/A
ΣN
ΣN/A
2R＝A/L （2R/L)[1/m2]
[km2］
［m］
［m］
［m］
［m］
［1/m］
No.1
0.079
621
127 0.20
185
0.30
643
0.00823
3.8E-05
No.2
Occur
0.166
1,080
154 0.14
235
0.22
1,582
0.009515
9.0E-05
No.3
0.105
507
208 0.41
129
0.26
1,129
0.01079
8.5E-05
No.4
0.288
1,150
250 0.22
268
0.23
3,169
0.011027
9.4E-05
No.5
not
0.034
266
129 0.49
87
0.33
317
0.0092
3
8.7E-05
No.6
0.133
505
264 0.52
161
0.32
1,487
0.0111
10
7.5E-05
No.7
0.075
507
148 0.29
172
0.34
811
0.01087
9.3E-05
No.8
0.026
333
77 0.23
114
0.34
333
0.01301
3.9E-05
Occur
No.9
0.165
765
216 0.28
263
0.34
1,176
0.00717
4.2E-05
No.10
0.048
387
124 0.32
185
0.48
387
0.00811
2.1E-05
No.11
0.130
712
183 0.26
337
0.47
712
0.0055
1
7.7E-06
No.12
0.110
322
342 1.06
169
0.52
731
0.00669
8.2E-05
No.13
Occur
0.215
481
446 0.93
256
0.53
1,284
0.00607
3.3E-05
No.14
0.162
613
264 0.43
302
0.49
1,399
0.00876
3.7E-05
No.15
0.184
631
292 0.46
266
0.42
1,019
0.00555
2.7E-05
No.16
0.194
868
224 0.26
412
0.47
1,327
0.006811
5.7E-05

at Midori-i area not affected by debris flow the
inclination is 0.29 (divides No.4 to No.6), as shown in
Table 1. Based on this data, we examined hydrogeological factors such as the mechanisms of rainfall
penetration, runoff and discharge during the droughtseason in the Asa district.

flow is related to the drought specific discharges. In
compared to drought discharge in divide No.4 andNo.9, discharge of the former is constantly increase by
the downstream, while the latter do not increase(Fig.3).
The base flow in the drought season represents
the groundwater discharges from basins and isindicative of permeability and/or storage-ability. In
heavy rain conditions, the storage-ability in the Yagi
area shows very low values (0.58～0.70mm/d.) and
was affected by debris flow. However, the Midori-iarea, which was not affected by debris flow, shows
considerably higher values (0.87～1.27mm/d.). Hence,
it implies that the recharge-ability of the base rock in
the Yagi area is comparatively small than that ofnon-debris flow the Midori-i area.
Collating all these data it seems that theoccurrence of debris flow depends on the difference of
permeability or storage-ability of the geologicalbasement body of the mountains.

(1) Characteristics of drought specific discharge in
the Asa district
Measurements for all divides were done in
January 11, 2015 and the results of both discharge anddrought specific discharge are shown in Table 2 andFig.1. As shown in Table 2, the mean discharge and its
volume at granitoid dominant Yagi area (No.7 toNo.10, debris flow affected area) are showing to 7.1ℓ/sec/km2 and 0.61mm/d. On the other hand, discharge
and its volume of debris flow un- affected area,
especially divide No.4 yield values of 14.7 ℓ/sec/km2
and 1.27mm/d. This is a remarkable difference indischarge and volume between these two areas.
Especially No.4 to No.6 (12.6ℓ/sec/km2 and 1.09mm/d.
in mean value) divides are showing twice values incompared to the mean values of all but three of the 13
divides (0.51mm/d.), where the debris flow had
un-affected. This implies that occurrence of debris
Elevation (m)500

No.4流域

No.4

300

×
300

200

200
100

100

50

0

0

200

400

600

800

1000

(m)

Discharge rate (L/min)

500

No.9流域

○

No.9

○

400

×

× : Interflow○ : Spring

300

100

200

50
100
50
0

200

400

600

800

1000

(m)

0

Discharge rate
(L/min)×

Discharge rate
(L/min)Elevation (m)400(2) Comparison of water budget for both divides
No.4 and No.9
Analysis using the tank model method is carriedout to verify the river regime and to compare theDischarge rate (L/min)Fig.3 Topographic characteristics of the watershed in debris flow affected and un-affected areas410th Asian Regional Conference of IAEG (2015)

Table 2 Measurement of discharge
Divides No.

No.1
No.2
No.3
No.4
No.5
No.6
No.7
No.8
No.9
No.10
No.11
No.12
No.13
No.14
No.15
No.16
Total

Bed
rock

Occur or not ofdebris flow

Granitoid

Occur
not

Sedimentary
rocks

Occur

Occur

Occur of debris flow (Granitoid)
Not occur of debris flow (Granitoid)
Occur of debris flow (Sedimentary rocks)

Area of
Water
Electric
Discharge
Specific discharge
dividestemperature
conductivity
A
Q
Tw
EC
Q/A
[L/min][μS/cm][L/sec/km2][mm/day]
[km2］
[℃]
0.079
28.8
7.0
68
6.1
0.53
0.166
73.2
7.7
79
7.3
0.63
0.105
27.6
9.4
54
4.4
0.38
0.288
253.2
6.8
73
14.7
1.27
0.034
20.9
6.2
69
10.1
0.87
0.133
104.2
7.5
63
13.0
1.12
0.075
30.0
7.5
64
6.7
0.58
0.026
10.6
8.5
65
6.9
0.59
0.165
80.4
10.5
61
8.1
0.70
0.048
19.2
7.8
63
6.7
0.58
0.130
62.4
6.3
73
8.0
0.69
0.110
52.2
7.3
69
7.9
0.68
0.215
51.6
9.7
88
4.0
0.35
0.162
43.8
8.8
65
4.5
0.39
0.184
60.6
5.9
77
5.5
0.47
0.194
88.8
5.0
73
7.6
0.66
2.114
1,007.5
121.5
10.5
0.664
0.456
0.994

269.8
378.3
359.4

characteristics of the water budget in the two basins
No.4 and No.9. Ten measurements of discharge aredone from October 2014 to January 2015. The
hydrographs represent precipitation per day in
Hiroshima city and discharges in two studied basins(Fig.4).
Precipitation data at Hiroshima city is used due tonon-availability of precipitation data from Yagi area.
Therefore, the correct discharge of debris flow
occurred has not been reproduced in the figure.
However, as shown in Fig.4, discharge in No.9
divide shows a distinct high in comparison to that inNo.4 divide. Hence, it can be intuitively estimated that
non-precipitation continued at No.9 is continued, thedischarges would have been decreased.
According to the calculated budget in a year
100

0
100

Precipitation

Maximum precipitation ≒ 110mm/d

No.9

Maximum discharge ≒ 60mm/d

10Discharge(mm/day)

Caluclation
Measurement

1

0.1
100

（Debris flowed area）
No.4

0.59
1.20
0.52

based on reproduced discharges, the volumes of
surface runoff Qs in No.9 and No.4 divides areapproximately 872mm/y. and 392mm/y., respectively,
and hence the former is obviously showing high values
in compared to the latter.
Trial calculation based on peak discharge(240mm/d., August 20th 2014 by the Meteorological
Agency of Japan) in a day are carried out for bothdebris flow affected divides and un-affected divides.
There is clear difference in the runoff between No.9
and No.4 divides as the discharge at the time of debris
flow is estimated as 155mm/d. in the former and
77mm/d. in the latter.
Actual precipitation was over 200mm/3h.
indicating that the debris flow occurred instantly at
areas with less permeability and storage-ability along

50

Discharge(mm/day)

Precipitation(mm/day)

150

6.8
13.8
6.0

Maximum discharge ≒ 30mm/d

10Caluclation
Measurement

1（Non debris flowed area）

0.1'14-Jan '14-Feb '14-Mar '14-Apr '14-May '14-Jun '14-Jul '14-Aug '14-Sep '14-Oct '14-Nov '14-Dec '15-Jan '15-Feb '15-Mar

Fig.4 Discharge hydrographs for both debris flow affected divide (No.9) and un-affected divide (No.4)

5

10th Asian Regional Conference of IAEG (2015)

with huge runoff by the oversaturated groundwater on
the surface in basement rock body.

fact is demonstrated by the hydrogeological study
and/or land use as in divide No.4.

6.

7.

Conclusion

Characteristics of the Hiroshima August 2014
disaster can be summarized as follows. The studied
divides in the Yagi area, especially in divide No.9
formed as impermeable body due to the topographicalconditions, viz., formation of V-shaped valley, steep
inclination of river floor helping the stream flow in
non-stagnation on the river floor, as well as geologicalconditions viz., lack of open cracks, shear zone,
impermeable geological layer in the studied hard,
massive granitoid with cracks being firmly closed andthick colluvial sediments on valley walls. We conclude
that the abnormally heavy rain could not penetrate thegranitoid mass, thus causing the water to rise and toflow down the steep river floor at tremendous speed
while eroding out unstable soil on the valley walls.
Thus, the debris flow occurred as an eventual outcome(Fig.5). These facts are supported by the measurementof drought specific discharge which is not increased in
the impermeable body distributing area in the valley.
On the other hand, at divide No.4 in the Midori-i
area with minimal damage the topographical factors
pointed that gradual inclination of river floor, gentle
mountain ridge and slope forming U-shaped valley
with stable slope of thick weathering products in thisarea. While, altered clay or seams along the shear in
granitoid mass can be seen, joint system or cracks are
frequently open, many spring-out points are observedat places of mountain slopes. We conclude that thisarea can easily store water following a sudden heavy
rain on the mountain and so no debris flow occurred inthis area. This is also supported by the data result on
drought specific discharge in this area.
It is recognized that the drought specificdischarges are quite different between debris flow
affected and un-affected areas by measuring of runoff.
The debris flow occurred at less permeable and lessstorage-able area, in other words, it occurred at highdischarge area during heavy rains. Specifically,
difference of storage-ability of the mountain has agreat influence on the occurrence of debris flow. This

Future study plan for disaster prevention

While we can assume that there will be anincrease in disasters from debris flow due to sudden
heavy rainfall, it is difficult to predict when and where
the disasters will occur. It is advisable to define thepotential sites according to the vulnerability of debris
flow-related disasters. There is a great need for further
studies on topography, hydrogeology and hydrology,
especially the measurement of specific discharges
during drought seasons. Thus, it will be easy to build
up the disaster prevention plan, facility, proportions
and arrangements of measures at the selected local
places based on hydrogeological study, particularly,
careful measurements of base flow in drought season
are needed on a wider area.
REFERENCES

Editorial Committee of CHUGOKU, Part 7 ofRegional Geology of Japan (Eds.) (1987). Regional
Geology of Japan Part 7 CHUGOKU. Kyoritsu
Shuppan . 290p.
Sugawara M. (1972) A Method for Runoff Analysis,
Kyoritsu Shuppan. 257p.
Suzuki T. (2000). Introduction to Map Reading for
Civil Engineers Volume 3 Terrace, Hills, andMountains. Kokonshoin. 942p.
Yamamoto S. (1992). Groundwater Hydolorogy.
Kyoritsu Shuppan . 228p.

Talus dep.
WeatheredGr.

Before flooding

Uprising of the stream flow level due to the heavy rain
could not penetrate into the basement rock body

Debris flow with flooding

Fig.5 The illustration showing the flooding processes in Yagi-sawa area (Divide No.9)

6