Hirokazu Ikeda

Utsunomiya University Academic Information Repository

REPRESENTATIVE TWO FLOOD DISASTERS IN NORTHERN 
KANTO REGION IN RECENT TWO DECADES 
Hirokazu Ikeda1* 
1 Dept. Civil Engineering and Regional Design, Utsunomiya University 
 Yoto 7-1-2, Utsunomiya City, Tochigi Pref., 321-8585 JAPAN 
*ikeda@cc.utsunomiya-u.ac.jp
Abstract 
Representative two flood disasters in northern Kanto region in recent two decades are reviewed. One 
is “Kanto-Tohoku Heavy Rain in September 2015” which is caused by training torrential rain band. 
Along Kinugawa River, one of the large rivers in Japan, river bank was broken in Ibaraki Prefecture 
(next to Tochigi Prefecture), which resulted in inundation over 40 square kilometers. This wide-
spread inundation area was not only due to the torrential rain, but also due to geomorphic change 
through historical river improvement in Edo-era, about four hundred years ago. Oyama city in 
Tochigi Prefecture suffered both of internal and external water flooding due to the 
geomorphological characteristics formed up to one hundred years ago. The other flood disaster is 
“Nasu Flooding” which occurred in 1998 in Nasu area, northern part of Tochigi Prefecture. Typhoon 
No.4 in 1998 stimulated rain front, and this area suffered extraordinarily heavy precipitation up to 
670mm daily, 90mm hourly. The big features of this flood disaster were geomorphic change of river 
channels and bridge damages due to driftwood. Similar flood disasters to these two disasters often 
occur in recent years in Japan. We have to learn from the former two disasters for upcoming flood 
disasters in future. 
Keywords: flood, river geomorphology, torrential rain band, internal water flooding, driftwood 
1. INTRODUCTION
Recently, we have severe flood disasters in Japan almost every year. These flood disasters may have 
some similar patterns or features when they brought serious damages. In northern Kanto Region, 
where Utsunomiya University is located (see Fig.1), we suffered two great flood disasters in these 
two decades. One is Kanto-Tohoku Heavy Rain in September 2015, and the other is Nasu Flooding 
(so-called in Tochigi Prefecture) in 1998. These two disasters have similar features to recent severe 
flooding in Japan. This paper reviews these two flood disasters and compare them with those in 
recent years in Japan in order to take advantage to learn from them for upcoming flood disasters in 
future. 
2. KANTO-TOHOKU HEAVY RAIN in SEPTEMBER 2015
Kanto-Tohoku Heavy Rain in September 2015 was caused by training torrential rain band, which 
brought extraordinarily heavy rain on 9 to 10 September 2015 in Tochigi and Ibaraki Prefecture. 
Almost every observation station recorded maximum precipitation in record history (see Fig.3). 
Many places were damaged by flooding due to this heavy rain. 
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Among them, along Kinugawa River, one of the large rivers in Japan (see Fig.2), river bank was 
collapsed at one location and overflow occurred at some locations in Ibaraki Prefecture (next to 
Tochigi Prefecture), which resulted in inundation over 40 square kilometers. It took 10 days to get 
the almost all water out of the inundation area. Very many residents were isolated; about 3,000 
people were rescued onto boats and 1,340 people onto helicopters. 5,300 houses were demolished 
and partially-destroyed with 150 houses flooded above floor level and 3,400 below floor level.  
Fig.1 Northern Kanto Region 
(blue colored area) 
Fig.2 Location of rivers in northern Kanto Region 
referred in this paper. 
Gunma
 Pref.
Ibaraki
 Pref.
Tochigi
 Pref.
Fig.3 Precipitation contour in Kanto Region at 
0:00 on September 10, 2017.  
Red:<100mm/hr, Orange:<50mm/hr, 
Yellow:<20mm/hr,Green:<10mm/hr  
Photo.1 Arial Photo near the bank burst of 
Kinugawa river (Ibaraki Pref.)  
Kinugawa 
Tokyo 
Bay Pacific Ocean 
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This wide-spread inundation area was not only due to extraordinarily large volume of water from 
the bank burst and overflow delivered by the torrential rain, but also due to river geomorphology 
and historical river improvement. Photo.2 is an aerial photograph of inundation area near the largest 
overflow point, which shows the inundation area spread wide and far toward upstream area from the 
overflow point. This is because of the topography around the river shown in Fig.4. Natural 
embankment is formed along the river, and the lower floodplain is developed from the upstream area 
of the overflow point. Meanwhile, Kinugawa river is one of large branches of Tonegawa river 
system (see Fig.2). Tonegawa river flows east into the Pacific Ocean in the present era. However, 
before Edo-Era, about four hundred years ago, Tonegawa river flowed south into the Tokyo Bay, 
and Kinugawa river, merging Kokaigawa river, flowed east to the Pacific Ocean (see Fig.5). During 
Edo-Era, a historical big project of river improvement was performed by Ieyasu Tokugawa, the first 
shogun of the Edo Bakufu (Samurai Government). They created a new river channel by land cutting, 
and jointed several rivers to the east, and then Tonegawa river becomes what it is today. In this 
process, Kinugawa river and Kokaigawa river were separated, and merged into Tonegawa river 
Kinugawa 
Photo.2 Aerial Photo near one of overflow 
points of Kinugawa river (Ibaraki Pref.) 
A red block arrow denotes the overflow 
point. 
Fig.4 Landform classification map for flood control 
near the bank burst and overflow point 
Fig.5 Historical river improvement of Tonegawa river system 
by Ieyasu Tokugawa about four hundred years ago 
Tokyo Bay 
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separatedly. In the Kanto Tohoku Heavy Rain, the inundated water from the bank burst and 
overflow was spread up to this separated channel of Kinugawa and Kokaigawa. Therefore, we have 
to know about river morphology in order to make countermeasures against flood disasters. 
Oyama city in Tochigi Prefecture (see Fig.6) was also seriously damaged (see Fig.9).  The 
elevation difference along Omoigawa river, the main river in this city, up to 7m (see Fig.7). This 
lower area had been formed by the former river channel of Omoigawa River up to one hundred 
years ago (see Fig.8). The heavy torrential rain converged into this lower area behind river bank of 
Omoigawa River, then, internal water flooding occurred, beginning from around the former river 
channel No.1 in Fig.8, and through the former channels No.2, 3, and 4, the rain water came together 
into this area. In addition, the backwater from Omoigawa River to a branch river overflowed into 
this area, so-called external water flooding. Both of internal and external water flooding complicated 
damage pattern in this area, which is very typical in recent years in Japan. 
Fig.7  Elevation map of central area of Oyama
Elevation(m) 
Fig.9 Flood depth distribution in Central 
area of Oyama City due to Kanto-
Tohoku Heavy Rain in September 2015 
Fig.6 Central area of Oyama city and 
related river
小山・栃木排水路
Fig.8 Landform classification map for 
flood control of Central area of Oyama 
City. (Legends are same as Fig.4)  
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3. NASU FLOODING IN 1998
The other flood disaster is “Nasu Flooding” 
which occurred in 1998 in Nasu area, northern 
part of Tochigi Prefecture. Typhoon No.4 in 
1998 stimulated autumn rain front, which continuously developed cumulonimbus over Nasu area, 
then, this area suffered extraordinarily heavy precipitation up to 670mm daily, 90mm hourly, more 
than 1200mm in total which was two-thirds of average yearly rainfall. Fig.10 shows comparison of 
total rain fall between Nasu Flooding and Kanto-Tohoku Heavy Rain. It is clarified that Nasu flooding 
suffered much more rain fall than Kanto-Tohoku Heavy Rain along a very long rain front. Due to this 
heavy rain, in Northern part of Tochigi Prefecture and southern part of Fukushima Prefecture, 22 
people died and 506 houses were demolished and partially-destroyed with 3,328 houses flooded 
above floor level and 11,518 below floor level. 
The big features of this flood disaster were geomorphic change of river channels and bridge 
damages due to driftwood. The unprecedented heavy rain made the water flow in steep rivers more 
rapid and run linearly, which resulted in erosion of river bank, farmland and residential area. Photo.3 
shows the river side erosion along Yosasagawa river near Yosasabshi bridge of National Root 249. 
Photo.4 indicates that the linearized river flow penetrated the farmland, collided on a cliff side, and 
erode the riverside land. This type of river channel transition occurred in many places. 
Photo.3 Erosion of riverside area near Yosasa-
bashi Br. of National Root 294. 
(a) Nasu Flooding during 26-31th August
1998
(b) Kanto-Tohoku Heavy Rain during 9-11th
September 2015
Nasu St. 
 1254mm 
Nikko Dorobu 
 564mm 
Nikko IKari 
 627mm 
Nikko Imaichi 
 647.5mm 
Kanuma 
 526mm 
Fig.10 Comparison of total rain fall between Nasu Flooding and Kanto-Tohoku heavy rain. 
Photo.4 Geomorphic change of river channel 
during Nasu Flooding near Yosasa-bashi Br. 
of National Root 294. 
(a) Before flood.
Blue arrows show the 
former channel line 
(b) After flood.
Red arrows show the 
new channel line 
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Meanwhile, a large amount of driftwood ran into rivers, jamming between bridge piers or 
between water surface and bridge body, then the rapid river flow washed out some bridges or 
intruded onto the side area of bridges to erode surrounding land. In Photo.3 and 5, it is shown that a 
lot of drift woods were jammed in the upstream of Yosasabashi bridge, and the river create another 
route of flowing water, which resulted in the riverside erosion. 
4. RECENT FLOOD DISASTERS IN JAPAN
We have severe flood disasters almost every year. In 2017, Japanese people suffered two serious 
flood disasters. One is “Northern Kyushu Heavy Rain in July 2017” which was caused by training 
torrential rain band similar to Kanto Tohoku Heavy rain in September 2015. The precipitation was 
up to129.5mm hourly, 545.5mm daily, and 616mm in total. Photo.6 shows that a great many 
driftwoods expanded flood damage and cause river channel change similar to Nasu flooding. 
The other is caused by Typhoon No.21 in October.  This typhoon stimulated the autumn rain 
front which resulted in heavy rain more than 500mm in total for one week over Japan islands. This 
weather situation is similar to that of Nasu flooding. And the rain front and the typhoon itself caused 
serious damages throughout Japan.  
5. CONCLUSION
Recently, flood damage becomes more severe and more often in Japan. Especially, we have to take 
care about training torrential rain band and typhoon in late fall. Infrastructure development and 
improvement are indispensable, but, hard infrastructure requires a big budget and a long time, which 
cannot be made in time for upcoming disasters. Therefore we have to develop non-structural 
measures combining the existing infrastructure. To this end, for effective measures, we have to take 
advantage of lessons learned from the former two disasters in northern Kanto region for upcoming 
flood disasters in future. 
REFERENCES 
Mano, A. et al. : Torrential rain disaster over southern Tohoku and northern Kanto in 1998, Project 
Report for Grants-in-Aid for Scientific Research of Japan, No.10400011, 1998. (in Japanese) 
Yamada, T. et al. : Report on natural disaster caused by Kanto Tohoku Heavy Rain in September 
2015, Japan Society of Civil Engineers, 2016. (in Japanese) 
Photo.5 Driftwoods jammed at Yosasa-bashi Br. of 
National Root 294 due to Nasu Flooding in 1998 
Root294
Yosasa-bashi 
Flow
Photo.6 Driftwoods jammed at a bridge in Asakura 
City of Fukuoka Prefecture in Northern Kyushu 
Heavy Rain in July 2017. 
Flow
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REPRESENTATIVE TWO FLOOD DISASTERS IN NORTHERN KANTO REGION IN RECENT TWO DECADES

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