Journal of Natural Disaster Science, Volume 24, Number 2, 2002, pp51-59

The September 2000 Torrential Rain Disaster in the Tokai Region:
Investigation of a mountain disaster caused by heavy rain in three prefectures;
Aichi, Gifu and Nagano
Yasuhiro SHIMIZU
Faculty of Science and Technology, Meijo University
Naohiko TOKASHIKI
Faculty of Engineering, University of the Ryukyus
Fujio OKADA
Faculty of Science and Technology, Meijo University(Received for 11. Oct., 2002)

ABSTRACT
Torrential rain on September 11, 2000 in the Tokai region caused floods and many landslides. In this disaster,
most of the soil and rock slope failures that caused closure of roadways occurred concentrically in Gifu
Prefecture. In addition, flooding caused bridges to collapse, and many residential areas were inundated. Slope
failures and damage to trees in the southern mountainous district of Gifu Prefecture were investigated taking into
consideration the geomorphological and geological characteristics of this area. The relationships between slope
failures and torrential rain characteristics as well as possible causes are discussed. Old eroded stubble was foundto be the caused frequent slope failures in a young plantation.

1. INTRODUCTION
A stationary autumn rain front produced by stagnation from
September 11 to 12 2000 of Typhoon 14 caused flooding in the
Tokai region of Japan. In this disaster, about 65,000 houses were
flooded, and about 58,000 persons in this region required refuge
advice.
Sediment disasters such as slope failures with debris flow
were frequent in the broad mountainous region which spans thethree Tokai prefectures of Gifu, Aichi and Nagano. Many roads inthe region became impassable and many settlements were isolated.
In particular, Kamiyahagi Town, Ena District, Gifu Prefecture
received intense rainfall which exceeded 80mm (per hour) at thetime amount and the total rainfall in this region was 590mm.
Severe damage, which included the collapse and washing away ofcivil engineering structures and houses, occurred.
Results of a disaster investigation made in the Yahagi Riverbasin are reported, and the causes of damage to mountain slopes
and civil engineering structures are discussed.
2.

As seen from Fig.1, an area with 500mm total rainfall is centered mainly on Kamiyahagi Town, Gifu Prefecture. This area ofsevere rainfall covers a narrow region from the north of Aichi
Prefecture to the south of Nagano Prefecture. This heavy rain produced a record-breaking amount of rainfall at both observatories.
In particular, the maximum hourly rainfall at the Yarigairi observatory was 80mm, and the total rainfall 595mm between 11 p.m.,
September 11th and 0 a.m., September 12th. The maximum
hourly and total rainfalls at the Kamiyahagi observatory respectively were 65mm and 437mm. The Ministry of Land,
Infrastructure and Transport reported that these amounts set
records in the past 500 years.

RAINFALL CHARACTERISTICS

The observed total rainfall in Yahagi River basin from 8 p.m.,
September 10th to 11 a.m., September 12th and the position of theobservatory of the Ministry of Land, Infrastructure and Transport
near Kamiyahagi Town are shown in Fig.1. Fig.2 gives theobserved times and total rainfall per hour at Kamiyahagi and the
Yarigairi observatories from 0 a.m., September 10th to 6 p.m.
September 12 (Ministry of Construction, Chubu District
Construction Bureau, Toyohashi Construction Work Office, 2000 ).

KEY WORDS: torrential rain, landslide, investigation report, granite zone plantation

Fig. 1

Total rainfall for the Yahagi River system.

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Y. SHIMIZU, N. TOKASHIKI, AND F. OKADA

Photo. 1 Collapsed bridge washed away by the accumulated
driftwood.

Fig. 2

Table 1.

Observed times and total rainfalls at Yarigairi andKamiyahagi observatories. (Ministry ofConstruction, Chubu District Construction Bureau
Toyohashi Construction Work office.［2000］）

Civil engineering structures in the Mt. Ena region
damaged by the Tokai 2000 torrential rain.

Photo. 2

Collapsed road and revetment washed due to scour of theretaining wall base.

3. THE DISASTER
In the area where the total rainfall exceeded 400mm during
this torrential rain, many civil engineering structures were damaged by mountain slope disasters and debris flows. A summary by
the Gifu Prefecture Ena construction office of the damage to civilengineering structures is given in Table 1. Damage was most frequently caused by driftwood striking bridges and driftwood
deposited in housing area. Results of resistance to the flow of
streams due to the piling up of driftwood at the sides of bridges
was observed at most bridge piers in the basin. Examples ofbridge piers and bridge girders washed away are shown in Photo.1.
Photo.2 shows a failed revetment and the asphalt paving on anational highway. The main causes of the washing away of therevetment seem to have been the bulging in the river channel at thedisaster position and local scour of the foundation ground by theflow in the stream.
A large quantity of driftwood was produced by this torrential
rain disaster. The dam control office reported that about 40,000 m3
of driftwood flowed in to Yahagi Dam Reservoir. This was 70
times the average (about 600 m 3) for the previous two years.
Photo.3 shows the pile up of driftwood after the heavy rain.
Simultaneously, much gravel flowed down and accumulated
in the dam’s reservoir. The amount of sedimented silt was about
1,100,000m3 , estimated as more than 12 times the average annual

Photo. 3

Yahagi Dam Reservoir with piled up driftwood.

rate (80,000 m3). The estimated quantity of banking sand thatflowed into Yahagi Dam is shown in Fig.3.
4. FIELD SURVEY
The shapes and cross sections of collapsed rock slopes were
measured as well as the gradient and inclination direction of theslip surface. A non-prism laser range finder, a clinometer, a map
with the scale 1 to 25000, and aerial photographs were used.
Results of the survey are given in Table 2 (Tujimoto, etc. 2001,

THE SEPTEMBER 2000 TORRENTIAL RAIN DISASTER IN THE TOKAI REGION:INVESTIGATION OF
A MOUNTAIN DISASTER CAUSED BY HEAVY RAIN IN THREE PREFECTURES; AICHI, GIFU AND NAGANO

some cross fall in this V characteristic at the maximum gradient of
45 degrees.
In the lower section of Photo.4, there is a landform in whichwidth and height are continuous in steps of several meters, but inthe upper part of the failed slope, it continues as a gradient along
the joint plane and there is a banquette of several centimeters. Thisphenomenon seems to represent the stripping off of the joint plane
to a depth of several meters caused by collapse over a long period
and by the force of the debris flow generated in this disaster.
In Table 2, No.D1, destruction of a forest road is induced. In
this case, the surface layer in the stricken area was mixed with theMasado layer of weathered granite rock and humus soil. Therewas gully erosion in middle of the slope at almost overall length.
Photo.5 shows that this failed cross section was that of slope of the
U type and afforested full face before it failed.
The topographic map for this collapsed ground is shown in
Fig.4. Photographs were used simultaneously to determine thedetail of the collapsed shapes. In the plan seen in this figure, crosssections are shown in which the gradient of differs at points. The
longitudinal map shows the height of each interval, the lowermost

Hight (EL.m)

Shimizu, etc. 2001). This Table shows that the gradients of thecollapsed slopes were steep, the collapsed slope length long, andthe catchment areas were wide.
Granitic bedrock underlies the mountains in this area, androck converted to a surface layer, called Masado is widely distributed. In the neighborhood of the Kamiyahagi Town, 90% of thearea is forested, and the most of the tree plantings are cedar andcypress.
The surface soil was exposed in most of the plantations investigated in which the undergrowth is 20 to 30 years old. The surface soil is composed of Masado or decomposed granite andhumus soils, the thickness varying about 50cm to 3m. Slope collapse mainly occurred at the boundary with the unweathered division. As the thickness of soil layer in the slip area is thin, grove
failure was frequent. A typical example of the characteristic slope
failure found in this survey follows.
The case of a steep landform, No.A11 is shown in Table 2.
The rock mass was exposed on the failed slope of this stricken
area, and a joint Nagareban of about 80 degrees developed on thesurface of the slope. The valley became V-shaped, and there wasDistance (km)Fig. 3

Bank sand quantity curve，the Yahagi Dam inflow division.（Ministry of Land，Infrastructure and Transprt
Chubu District improvement station material［2000］）.

53

Photo. 4

Steep slope failure in Tappara，Kamiyahagi Town（lower part of the slope）.

Table 2. Disaster field inventory findings for the Tokai 2000 torrential rain.

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Y. SHIMIZU, N. TOKASHIKI, AND F. OKADA

height of the forest road being the standard. It shows a 153m difference in elevation, 296m in horizontal distance, 337m in slope
distance, a maximum width of 23m, largest tilt angle of 38°and
largest collapsed depth of 3m. Valley erosion occurred for about
200m in this landslide area at the forest road in its lowermost part,and flooding deposited a layer of gravel in the farming area thatwas several meters high. Fallen trees with diameters of more than
80cm and length of 20m are present.
The existence of the 6.0m wide forest road which is in contact
with the upper most end of the landslide scar is considered to havebeen the inducement for this slope failure. This forest road

Photo. 5

Slope failure at Adera. Kamiyahadi Town，in which a forest
road was an inducement（middle part of the slope）.

Fig. 4

became a sluice as rain water fall from it in to the watershed within
about 200m of the highland. Water corrected at this site where
there is a bend. The catchment area, as increased by the forest
road was about 8.6×103cm3 produced an increase of 34% in thecatchment area of the same landslide scar. This also was an
inducement to erosion because none occurred in the adjacent areathat was without water inflow from forest road.
Another case in which the erosion of stumps was an inducement is shown in Table 2, Nos.2 to 5. A partial investigation ofthis landslide scar has been made because of the occurrence of too
many slope failures. These landslide scars were present in thesouth of Nagano Prefecture. An aerial photograph of one landslide
scar is shown in Fig.5. Much decay has occurred in the plantation
over 10 to 15 years, whereas there has not been much generated in
the natural forest and plantation area over a 20-year. In this landslide area, the catchment area is 9.9×105m2, average gradient of
the failure slope about 42 degrees, and density of failure high at
17/km2. Also, the maximum rainfall in this area was 69mm/h, andtotal rainfall 403mm.
Slope failures have been in close in this region, and many failures have occurred on concave or otherwise flat landforms. A distinguishing characteristic, seen in Photo.6 is the eroded stumps
produced by tree trimming at the crown of the slope.
Clear cutting cypress trees 50 years old took place 15 years
ago, and this region has been reforested in respect to new cypresses. As shown in Photo.6, erosion of the stumps has progressed,part of the underground root system has become hollow, and theresistance of the soil mass been reduced. The match rain water

Plan and longitudinal section of the failed slope in Kamiyahagi Town.

THE SEPTEMBER 2000 TORRENTIAL RAIN DISASTER IN THE TOKAI REGION:INVESTIGATION OF
A MOUNTAIN DISASTER CAUSED BY HEAVY RAIN IN THREE PREFECTURES; AICHI, GIFU AND NAGANO

was flooded underground because the eroded stumps acted like apipe. These phenomena seem to have been the cause of the slope
failures concentrated in this region. Such failure comparatively
easy to generate under geotechnical conditions where there is athin sand layers e.g. weathered residual soil, and rain water easily
permeates the layer.
Fig.5 shows that slope failure mainly occurred on the southwest as compared to the northeast side of the slope. This phenomenon is explained as follows: 1) Though the erosion progressed on
stumps which remained after the clear-cutting, its progress on
southwest side was more rapid than on the northeast one due tosunshine and the temperature gradient at ground level. 2) After the
15 years that have passed since the tree clearing, the slope direction has affected progress of erosion, and the number of slope failures generated differ according to the direction of the slope. Moreover, it is said that “the surface in the area with the erosion stumps
is soft like the thick carpet”.

5.

FAILURE OF WEATHERED RESIDUAL
SOIL IN ODONA, NEBA VILLAGE.

Bamboo grass has grown widely over the top of a failed slope
in this region. The depth of the failed soil is about 0.5m -1m, andthe surface soil at the failed slope is mostly homogeneous, decomposed granite where there is partially weathered rock. The surfaceof the failed slope is a sandy layer composed of weathered residual
soil, as classified by Nishida (1986, 1991) and Deere and Patton(1971) (Fig.7). This is regarded to be the underlayer of the sandy

Fig. 6

Fig. 5

Decay mechanism of slope failure due to erosion.

Trees planted near lna County，Neba Villge，Nagano Prefecture.

Fig. 7
Photo. 6

Top of the failed sloppe in the eroded stump area.

55

Schematic representation of a weathering layer section.
（Deere and Patton 1971）

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Y. SHIMIZU, N. TOKASHIKI, AND F. OKADA

layer as there is partially weathered rock. Because the field survey
was done by visual inspection and the hammer test, further examination is necessary to determine whether the failed surface classifies as a granite soil (zone IV, Masado B) or (zone V, Masado A)
in the weathered soil classification of Kimiya (1975).
Geomorphological observation of the failed slope showed that
it is convex, with a center top where the slope has a concave crosssection. If slope failure is assumed to be limitless in weathered
residual soil, the mechanism of failure caused by rainfall is
explained by three stages (Enoki, 1999), as shown in Fig.8:
1) “Vertical infiltration of rainfall”: Rainfall permeates theslope is surface soil, forming WF drops over time.
2) “Bedrock parallel infiltration process”: The ground watertable is in the surface soil layer after WF reaches the
bedrock, and seepage flow parallel to the bedrock is generated.
3) “Collapse process” : Because this groundwater level is ashigh as downstream, it exceeds the limitation for the
ground water level of some sites downstream, and this partcollapses.
The slope in Neba Village seems to have failed by the abov process.
6.

SOIL SURVEY OF GROUND OF THE FAILED SLOPE
A soil test was done to examine the characteristics of the sur-Fig. 8face soil which seems to be an important factor in slope failure.
Physical constants for the collected surface soil are shown in Table
3. Particle size distribution in the soil sample shows there is only
several percent or less in terms of grain size and a content mostlyof sand and gravel.
The constituent minerals of the surface soil as estimated by Xray analysis are shown in Table 4. The particles consist of large
amount of quartz and feldspar as the primary minerals, and may
also contain gibbsite talc and a few clay minerals. Compaction of
the surface soil therefore is assumed to be loose. An enlargementof the Masado layer is shown in Photo.7.
A direct shear test was made on the same sample. Test resultsfor Odona in Neba Village are given in Fig.9, which it shows arelationship between the shearing stress τ, shearing displacement
D and normal displacement ΔH . In this test, the normal stresses
applied were σN = 24.5, 49.0, 73.5 and 98.0 KPa. In this sample,
displacement reached D = 7mm, and the shear stress τ, increased
linearly. The volume change characteristic during shear showsthat the specimen increases in volume under the normal stressσN=24.5kPa, but decreases little at the other normal stresses.
Lines of rupture, including those of other landslide areas, areshown in Fig.10. Landslide areas where shearing tests were made,are beside Odora Neba Village, Adera and Hongo, Kamiyahagi
Town, as well as Nakato and Inabashi, and Inabu Town. Strength
coefficients, Cd and φd , as well as K15, results of a constant-head

The three slope failure mechanism stages.（Enoki 1999）

Table 3. Geological characteristics of surface soils on the failed slopes.

THE SEPTEMBER 2000 TORRENTIAL RAIN DISASTER IN THE TOKAI REGION:INVESTIGATION OF
A MOUNTAIN DISASTER CAUSED BY HEAVY RAIN IN THREE PREFECTURES; AICHI, GIFU AND NAGANO

57

Table 4. Constituent minerals of the surface layer soil on the failed slopes.

Table 5. Box shear and permeability test results.

Fig. 9Photo. 7

Direct shear test（CD）results, Odona Neba Village

Enlargement of strong wind granite（granite soil）.

permeability test, (Fig.10) are given in Table 5. In this table, theadhesive strength becomes Cd = 6.0-25.4kPa, and internal friction
angle φd =27.5°-47.9°. These findings give values closer to thoseof the soil whereas the strength constant of the general Masado
gives someone higher/lower values in the report of Yamamoto, etal. (2001). These findings also seem to be affected by the mixingof humus soil and the difference in the degree of weathering of theMasado. The permeability test gave a permeability coefficient of
K15 = 2.1-10.1×10-3cm/s.
The geologic map near Kamiyahagi Town in the region investigated is shown in Fig.5. Inagawa granite, Tenryukyo granite,
Mitohashi granite, Busetu granite and Ryoke metamorphic rock are

Fig. 10

Relationship between normal and shearing stress various sites.

58

Y. SHIMIZU, N. TOKASHIKI, AND F. OKADA

distributed in this area (Nakai, 1970). The area (Table 2, No.A3A11) with rainfall more than 400mm in particular abounds inTenryukyo granite. It has a steep landform in which the rock mass
has been exposed, weathering erosion is slower than for the othergranites, and the Masado of the surface layer is thinly extensible.
7.

RELATIONSHIP BETWEEN SLOPE FAILURE
AND THE AMOUNT OF RAINFALL

Fig.12 shows the relationship between the slope gradient andcatchment area of the catchment landform of the area near
Kamiyahagi Town for the occurance of landslides. The unfailed
slope shown was chosen, because of the watersheds of both shores
along several kilometers of the upper and lower stream between
ridge and river, base on the mapping of 1/25000 and an aerial photograph. Although unfailed slopes are frequent even for steep
slopes, there could be many failed slopes when the slope gradient issmall and the catchment area large. We showed that the scale ofthe catchment area has a greater effects on slope failure than thesize of the slope gradient. The failed slope area in Odona, Neba
Village is shown in Fig.13. No wide catchment area can be seen inthis area in Fig.12, the area being about 32×103m2 at maximum.
The slope gradient is 20°-55°, and the failed area also is within asimilar landform condition.

8. SUMMARY
Te torrential rain disaster of September 2000 in the YahagiRiver basin was investigated. Because the heavy rain region covered a highland 500 - 1,000m above sea level, and the steep slopeand catchment area constituted a large catchment landform, theslope failure caused by the torrential rain seriously damaged thenational highway and the downstream part of the river. The survey
results are summarized as follows:
1) Slope failure occurred mainly in the highland and widecatchment area in a catchment landform that had steep
slopes.
2) Failed slopes showed for easy failure, the surface soil consisting of a weathering residual soil and a colluvial deposit.
3) Slope failure mainly occurred in a plantation area where thesurface soil layer was thin, and plantings were the short roottype.
4) Driftwood produced when trees in a planted grove fell, theliver bank erosed, and debris and soil of 40,000m3, flowed
into Yahagi Dam Reservoir, nearly 70 times the averageannual amount.
5) Looseness of the mountain slopes due to the erosion after
tree clearing, leaving stumps, became an inducement forslope failure.
Investigations of the disaster in the mountainous area causedby the Tokai heavy rain are extensive, and further investigations
are being done at present. Research findings on the relationshipbetween stumps and slope failure, however have been few and that
topic is considered a future area of research.
9. ACKNOWLEDGEMENTS

Fig. 11

Fig. 12

Geological map of the Kamiyahagi Town area.

Relationship between the catchment area and slope gradientof the failed slope.

This disaster was investigated under the Ministry of
Education’s “Investigation and research on the September, 2000,
Tokai heavy rain disaster”. Much help was given at Nagoya
University’s graduate school, engineering graduate course, and byProfessor Tetsuro Tsujimoto and members of his laboratory.
Our gratitude is expressed for many materials received from
the Ministry of Land, Infrastructure and Transport; The Chubu
Improvement Office; Tajimi Construction Work Office; Town
Office of Kamiyahagi, Ena District, Gifu Prefecture; Town Officeof Inabu, Kitashitara District, Aichi Prefecture; Ena Forest
Owner’s Cooperative, Shitara Office, Aich Prefecture; Iida

Fig. 13

Relationship between the slope gradient and catchmet area
（Odona Neba Village)

THE SEPTEMBER 2000 TORRENTIAL RAIN DISASTER IN THE TOKAI REGION:INVESTIGATION OF
A MOUNTAIN DISASTER CAUSED BY HEAVY RAIN IN THREE PREFECTURES; AICHI, GIFU AND NAGANO

Construction Office, Nagano Prefecture; and the Nagano
Prefecture Ina Local Office (forest section). Thanks also are owed
to graduate students Miyuki Mizuno and Shingo Miyata who are
activity involved in this field study.
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