Tokyo MetroGinza LineRESILIENCEUTILIZING THE CITY’SUNDERGROUNDSPACES

Japan employs underground spaces to ready urban areas
for otherwise catastrophic flooding.

TO PREVENT FLOOD DAMAGE

Massive underground spaces, such as
this one, below the ring roads that run
through major residential areas are criticalelements of flood control in Tokyo.

River flow

To the reservoir

Water from a rising river flows into an underground regulating reservoir
instead of spilling over the banks of the levee.

Annual rainfall totals have been increasing
worldwide in recent years, given the growingfrequency of localized torrential rain, typhoons,hurricanes, and other extreme weather phenomena.
Not only does the inundation of buildings andunderground structures represent a danger to people
living in highly developed urban areas in particular,
but it also poses a grave threat to urban functions ingeneral. Therefore, developing measures against flood
damage is a pressing issue. The Japanese government
considers national resilience—including disaster

Left: Shibuya is a
popular spot amonginternational tourists.
Over 3 million people
use the station every day.
Right: New rainwater
retention facilities havebeen constructed below
the Shibuya Stationarea’s undergroundcommercial complexes.

prevention and mitigation, such as flood control—acentral element of its policy, and is moving ahead
with both public and private initiatives.
Tokyo, one of the world’s foremost metropolitan
areas, is making use of underground spaces in the fightagainst flooding. One example is the undergroundregulating reservoir being constructed to prevent
overflow from small and medium-sized rivers. When
heavy rain causes a river’s water level to rise, waterflows from a weir built into a revetment down into
the reservoir, thus reducing the amount of waterflowing downstream. So far, 28 such facilities havebeen constructed, collectively holding a total of 2.56
million m3 of water.
One massive reservoir has been built under a major
ring road that runs around central Tokyo. When atyphoon dropped record rainfall of up to 32 mm perhour on Tokyo in 2019, the reservoir retained about
490,000 m3 of water, or 90% of its total capacity.
Estimates indicate that it lowered water levels
downstream by up to 1.5 m. “We have systematicallydeveloped river facilities such as this reservoir, whichhave helped to prevent massive flood damage,” says
Director ODANAKA, from the Tokyo Metropolitan
Government (TMG)’s Bureau of Construction, withconviction. The reservoir is currently undergoing
expansion. Eventually, it will be able to compensate
for localized heavy rainfall coming down at up to 100mm per hour.
Life-saving flood control countermeasures arealso being implemented in the area around Shibuya
Station, one of Tokyo’s most well-known and popular
sightseeing spots. Since the land around the stationis slightly lower than the surrounding area, theunderground malls used to be prone to flooding.
However, as part of a massive redevelopment
project in the area that is currently underway, a new
rainwater storage facility with a capacity of 4,000 m3

Shibuya River

Underground plaza(East exit)

Rainwater storage
facility

was completed underneath the station’s east exit, andwent into operation in August 2020. It temporarily
retains rainwater if the rainfall exceeds a certain
amount, and the water drains out through sewer
pipes when the weather returns to normal. Director
OKUDA, from the TMG’s Bureau of Sewerage,
said, “Combined with the facility previously built
underneath the station’s west exit, our storage system
can now hold a total of 8,000 m3 of water. Shibuya
has evolved into an even safer urban area.”
Many other efforts to save lives from flooding
are also being made across Japan, including theconstruction of levees and detention basins, as wellas the refinement of heavy rain forecasts. A further
example of Japan’s inundation prevention measures
is the use of dams for hydropower and irrigation tocontrol flooding. To prepare for this year’s Olympic
and Paralympic Games Tokyo 2020 and beyond,Japan is advancing the development of national land
infrastructure that is resilient to natural disasters.
In order to prevent floods
in downstream areas whenheavy rain is predicted,
such as during typhoons,
the national andprefectural governments
have established a system
to discharge a certain
amount of dam water inadvance.