Water-Related Disasters and Disaster Risk Management in Metropolitan Areas (Tokyo)

This
article is an abstract of Dr. Koji IKEUCHI’s presentation who is a professor at
the University of Tokyo since October 2016.
Before
that, he worked at the Ministry of Land, Infrastructure, Transport and Tourism
(MLIT) till June 2016. As a civil engineer of the MLIT, He was in charge of
flood control measures and management of natural disasters for many years. He
became the Director General of the Water and Disaster Management Bureau in 2015
and he was the Vice Minister for Engineering Affairs of MLIT in 2016, when he
left.
In
this symposium, He talked about the Water-Related Disasters and Disaster Risk
Management in the Tokyo Metropolitan Area.
The
subject of the discussion is Water-Related Disasters and Disaster Risk
Management in the Tokyo Metropolitan Area. His discussion can be considered as
a case study to deal with the water and flood related disasters, mitigation
measures and risk management, for any metropolitan area of the world, since
Tokyo, Japan is considered one of the world’s most developed city. 
He
started with the outline of the flood control measures in Tokyo and told that
there are four water systems in Tokyo (Arakawa River, Tone River, Tama River
and Tsurumi River). He said that flood control measures in Tokyo can be
classified into three types namely;
1)Measures against large
rivers such as Arakawa, 2) measures against small and medium rivers such as
Kanda River and 3) storm surge and earthquake countermeasure of river in the
lowland areas.   
He
showed the historical background of flood control countermeasures to reduce
flood damage in Kanto area. He also presented the cross section of the downtown
in Tokyo and said that urban areas are located lower than the flooding levels
of the rivers and some part of the land is lower than the average sea level.
Therefore, the downtown of Tokyo is an extremely vulnerable area against
flooding and storm surge. About 1.5 million people are living here so
embankment failure will cause tremendous damage. Further, he also talked about
typhoons and said that every year, heavy rains due to typhoons are causing
flooding damages. Aforementioned issues are critical one and needs to be
addressed.
Talking
about the measures against large rivers such as Arakawa, he introduced the
Arakawa River and said that Arakawa is a class “A” river and managed by the
national government. This river is flowing down the central part of Saitama
Prefecture and the city part of Tokyo and poured into the Tokyo Bay.
Asakusa,
Sumida, Kinshicho and other downtowns in Tokyo were inundated in 1910 due to
Arakawa flood. Catherine typhoon in 1947 broke levee of Arakawa River at many
points, and Saitama and Tokyo lowlands flooded at a large scale. This was the
greatest flood after the Second World War.

Comprehensive
flood control measures of Arakawa were started after the 1910 flood.

These
measures includes; Linearization of river way and construction of cross levee to
control flood flow at the middle part were carried out. Improvement of the
confluence of three branches such as Iruma River and Construction of downstream
Arakawa floodway were also done (Figure 1).

Water-Related Disasters and Disaster Risk Management in Metropolitan Areas (Tokyo)
He
added that the national government constructed a large-scale floodway which is 22
km long and have a width of 500 m, as a fundamental measure to protect the
downtown of Tokyo from flood damage.
This
Arakawa floodway is called now Arakawa River and original Arakawa is now known
as Sumida River.
Construction
of this flood way started in 1911 and completed in 1930. This measure increased
the safety of floods in lowland areas of downstream of Arakawa which resulted
in advancement of urbanization for the development of Tokyo.
He
also told about Arakawa’s flood control measures of 1973. The Peak flow rate of
target flood is 14,800 m3/s with the probability of 1/200. Various
measures were adopted to control the flood such as, construction of Arakawa No.1 retarding reservoir and Urayama Dam for
flood control, construction of embankment and excavation of river to improve the
flood capacity of river, and reconstruction of bridge which was disturbing flowing
flood (Figure 2).

Water-Related Disasters and Disaster Risk Management in Metropolitan Areas (Tokyo)

Arakawa
No.1 retarding reservoir controls Flood and Supply water
to urban area. Speaker shared his experience of working on the same project and
he said that he was the in charge of planning and implementation team of this
project at site, during its construction which started in 1980 and finished in
1996.
Arakawa
No.1 reservoir is an excavated reservoir with the area of 5.8 km2
and the flood capacity of 39 million m3. This reservoir has the
ability to reduce the flood flow rate up to 850 m3/s. For flood control
of Arakawa, overflow dike was also lowered to allow overflow into the reservoir
when the water level exceeds from a certain limit. The surface of this dike is
covered with thick asphalt (about 50 cm) to prevent overflow collapse.
He
conversed that even though the flood control facilities in Arakawa are in
progress but still there are possibilities of flooding of same scale as it was
during the Kathleen typhoon, at the location of city center of Tokyo. He
described the expected damage in case of the Akabane embankment failure in case
of floods. According to him, 110km2 area with papulation of 1.2
million will be inundated and death toll may rise up to 1200. About 147 km
length of subway with 17 routes and 97 stations will be affected. He urged that
publication and dissemination of hazard map is very important, to mitigate the
disaster risks and for effective evacuation plans. MLIT personals are actively
considering this fact and are trying to disseminate hazard maps to residents in
various ways. Information on near real time such as radar rainfall information,
river water level, flood forecast, live image etc. are transmitted through the
homepage or data broadcasting on television. The timeline which is the time
series action plans is very effective for accurate emergency response at the
time of disaster.
The
MLIT promotes the formulation of evacuation timelines and evacuation plans in
municipalities and promotes implementation of evacuation drills based on these.
Other non-structural measures have also been taken such as crisis management and training and strengthening emergency response system in
case of disaster.
Talking
about flood control measures of small and medium rivers in Tokyo, he informed
that these rivers are managed by Tokyo Metropolitan Government.
He
stated that in Kanogawa typhoon disaster of 1958, a lots of small and medium
rivers were flooded and a wide area of downtown in Tokyo got destructed. This
flood damage became the starting point of flood control measures for small and
medium rivers in Tokyo. Since than Tokyo has been promoting river improvements
for 46 rivers of 324 km length. A rain intensity of 50 mm per hour was
considered as a targeting rainfall intensity to adopt measures against floods
of small and medium rivers. He explained the Kanda river flood control measures
to describe the improvements in small and medium rivers against floods. The
population in the catchment area of the river was about 600,000 people and the urbanization
rate was about 60%, in around 1955. But later, urbanization rapidly increased
and as a result, population in 2005 was about 1.65 million people and the
urbanization rate in 2003 reached about 97%. This increase in urbanization has
caused the increase in outflow of the rain to the river in catchment. As urbanization
progressed, the surface of the land covered with asphalt or concrete and the
rain started flowing quickly in the basin and the peak flow rate of the flood
became larger. Previously the rain water used to flow slowly in the basin
because it used to stored in farms and natural land. This phenomenon has caused
frequent flooding in Kanda River. Comprehensive flood control measures has been
taken to tackle this situation namely, measures in rivers, measures in basin
and non-structural
measures. He said that as a measure in the river, River improvement and
Construction of flood control reservoirs are implemented. And as measures in
basin, retainment of the area to suppress urbanization, conversation of
natural land, construction of retarding basin, construction of permeable pavement, infiltration inlet etc. have been taken. Non-structural measures
include establishment of warning and evacuation system, strengthening of the
flood fighting system and risk communication with residents. Figure 3 shows a
schematic diagram of Comprehensive flood control measures.
Water-Related Disasters and Disaster Risk Management in Metropolitan Areas (Tokyo)
He
said that measures have been taken to utilize everything in the catchment area
to store and infiltrate rainwater to suppress flood outflow and its peak. In
river improvement, construction was done to enlarge the cross sectional area of
the river. He added that construction activities were very difficult because of
the very narrow spaces especially in urban areas where houses were densely
located along the river way and it was difficult to widen the river width. They
adopted the division canal system to mitigate flood damage and to increase the
ability of water to flow by using a culvert system under the road. There are parks
constructed with the function of a flood control reservoir. There is a flood control reservoirs in the shape of tunnel
with an extension of 4.5 km and an inside diameter of 12.5 m under the road,
and stores about 540,000 m3 of floods of the Kanda River. Figure 4
shows the above mentioned examples.
Water-Related Disasters and Disaster Risk Management in Metropolitan Areas (Tokyo)
He
said that at the time of Typhoon No. 16 in 2004, there was a rain of
approximately the same size as the Typhoon No. 11 in 1993 which brought about a
lot of flooding damage in the middle Kanda River basin but due to the effect of
this reservoir and the improvement of the river, flooding damage was greatly
reduced.

He
delineated that in recent years, heavy rain downpour exceeding 50 mm/h have
been increased and he explained that 50 mm/h is planned target
rainfall. He presented the fact that most of the flood damages in recent years
are caused by rainfall which exceeds from 50 mm/h. He talked about the flood
damages which occurred in 2005. According to him, 8 rivers were flooded due to the
torrential rains exceeding 100 mm/h and about 6000 houses were inundated. He
spoke about the Effects of measures against floods targeting 50 mm/h and said
that the number of inundation damaged houses in the past 30 years steadily
declined with the progress of river improvement but however, in recent years, due
to the frequent occurrence of rainfall exceeding 50 mm/h flood damage has been
increased. He argued that in order to cope with the frequent occurrence of
torrential rain of recent years, the current plan is insufficient. He
introduced a new flood control policy which was formulated with goal of preventing
flood damages against rain fall of 75 mm/h. He further explained that 65 mm/h
rain fall is dealt by measures in rivers and sewers and in particular
improvement of rivers and construction of flood control reservoirs. For the
part of 10 mm/h rain fall, measures in basin are taken such as permeable
pavement, infiltration inlet and infiltration gutter. He informed that measures
for parts beyond 50 mm/h are accommodated by flood control
reservoirs
and infiltration measures (public spaces such as under the road and parks are
efficiently used). In order to cope with the local concentrated torrential rain
which has been increasing in recent years, several underground flood control
reservoirs are connected by a tunnel and mutual utilization across the
watershed. He presented the example of a regulating reservoir using a tennis
court and local government school ground use, as a flood control storage (figure
5).
Water-Related Disasters and Disaster Risk Management in Metropolitan Areas (Tokyo)
He
said that as a measures in basin Tokyo Metropolitan Government has adopted
permeable pavements while updating the pavement of the sidewalks and nowadays
many sidewalks are permeable pavements in Tokyo. He also showed the infiltration
facilities such as Infiltration inlet and infiltration trench (figure 6).
Water-Related Disasters and Disaster Risk Management in Metropolitan Areas (Tokyo)
He
added that measures such as publication and dissemination of hazard map and development
of a system to provide disaster information to residents are also being
promoted.

In
the summary of measures against small and medium rivers he told that previously,
river improvement were conducted targeting the rain of 50 mm/h and with this
measure, flood damage could be reduced but however, frequent flooding happened due
to the rain exceeding this limit therefore Flood control measures for 75 mm/h
(65 mm/h in the suburbs and 10mm/h in basin) have been formulated. Due to the
fact that it is difficult to further widen the river channels due to congestion in urban areas, construction of
flood control reservoirs, Rainwater storage, Infiltration in basin and
enhancement of warning and evacuation system are being advanced.
While
summarizing the whole discussion he said that Tokyo has greatly developed
through measures against floods, storm surges and earthquakes over the years. However,
it is necessary to continue measures against flood control using all means. In
his conclusion he said, “The flood control measures are investment, not cost. For
sustainable development, it is indispensable to take preventive structural and
non-structural measures. After a disaster, it is important not to recover to
the same state as the former, but to build back better, based on lessons
learned from the disaster.

Saad Iqbal
Hi there, I am Saad Iqbal from Pakistan. I am an enthusiastic blogger, passionate content creator, construction geek, and a creative graphic designer. Connect with me at my social channels.