Floods in Tokyo and Safety Tips and Preparation

Poste date: Tuesday, March 28, 2023

Japan, located in the eastern end of Monsoon Asia, one of the world's rainy regions, has an average annual rainfall of 1718 mm, which is about twice the global average (880 mm).

Moreover, the precipitation in Japan fluctuates greatly from season to season, and is concentrated in the rainy season and typhoon season. For example, the monthly average rainfall in Tokyo is 208.5 mm in September, which is the heaviest rainy month, and 39.6 mm in December, which is the least rainy month of a year.

Rainfall Intensity and Definitions of Terms

Precipitation Intensity (in an hour)	Forecast Terms	Situation
10 - 20mm	Moderate to Heavy Rain (やや強い雨 - Yaya tsuyoi ame)	Rainfall of the amount enough to create puddles on the ground. If it lasts for a long time, stay alert for possible flooding.
20 - 30mm	Heavy Rain (強い雨 - Tsuyoi ame)	Pouring rain to the extent that holding up an umbrella is not helping to protect yourself from rain. Gutters, sewage systems and small rivers will be flooded, while minor landslides occur.
30 - 50mm	Very Heavy Rain (激しい雨 - Hageshii ame)	Bucketing down of rain. The road becomes like a river, and the cliff collapse easily occurs.
50 - 80mm	Extreme Rain (非常に激しい雨 - Hijôni hageshii ame)	Downpour like a waterfall. Severe rain that turns surrounding air white and makes it difficult to see clearly. There is a possibility that the water can pour into underground shopping centers.
80mm and more	Torrential Rain (猛烈な雨 - Môretsuna ame)	Heavy downpour that makes the air stifling. Strict vigilance is required as it can result in a catastrophic disaster

Reference from Japan Meteorological agency

Localized Heavy Rainfall - Guerrilla Rainstorms

Localized heavy rainfall is called "Guerrilla Rainstorms (ゲリラ豪雨 - Gerira-Gôu)" in Japan. It's a localized downpour of short time.

Guerrilla Rainstorms have been increasing in Tokyo in these years. This type of rain is unexpectedly strong and is also very difficult to predict exactly where it occurs. Sewers and small rivers may overflow and it can cause serious damage.

Japanese rivers flow higher than residential area

In Japan, a lot of land where people live is lower than the river level at the time of the flood. High embankments are built along the river however, if these embankments break down, they would immediately cause enormous damage to people.

River level in Tokyo (Edogawa / Arakawa / Sumidagawa Rivers)

*Image translated from "Ministry of Land, Infrastructure, Transport and Tourism"*

River level in London

River level in Paris

River level in New York

Major Rivers in Tokyo and Flood Risk

There are several rivers in Tokyo such as Edogawa, Nakagawa, Arakawa, Kandagawa, Megurogawa and Tamagawa.

According to Ministry of Land, Infrastructure, Transport and Tourism of Japan, the flood inundation areas (in maximum assumed scale) are as indicated in the image (click to enlarge). These are areas where river flooding is expected.

Note: Flooding of tributaries, rainfall exceeding the scale which is the premise of the simulation, flooding due to storm surge and inland water, etc are not calculated in this simulation. Inundation may occur in other areas and the predicted depth may differ from the actual depth.

Koto 5 wards (Sumida-ku, Koto-ku, Adachi-ku, Katsushika-ku and Edogawa-ku) located in the east area in Tokyo are known to be at risk of being flooded by large-scale floods however, other areas in west side of Tokyo are also at risk of flooding as you can see in the simulation.

In fact, due to the Typhoon 19th in 2019, the Tama River was partially flooded around Futakotamagawa (Setagaya-ku), and rainwater and other water flowed back from the manhole in Musashikosugi (Kawasaki City), flooding the area. These are in the west area in Tokyo.

What is Inland Flooding?

Apart from River Flooding and Coastal Flooding, there is another flooding called Inland Flooding. It's caused by heavy rain in a city and it overflows on the ground surface.

When heavy rain falls, it will not be able to to drain the water just by gutters, sewers and drains. Also, if the water level of the main river rises in the area where tributaries join together, the main river water can flow back to the small and medium-sized rivers (tributaries).

The amount of damage caused by Inland Flooding in Japan is about 50% in total floods (river flooding, coastal flooding and inland flooding) however, about 80% of floods in Tokyo are caused by inland floods. In urban areas where embankments are well developed, inland flooding has become a new issue.

Inland Flooding occurred by the Typhoon 19th in 2019 and caused damage around Musashikosugi Station. The increased water of the Tama River flowed back through the sewer and overflowed into the city.

The Metropolitan Area Outer Underground Discharge Channel

The Metropolitan Area Outer Underground Discharge Channel is the world's largest underground river-type spillway and built to prevent flooding. The length is 6.3 km, the diameter is about 10m and the depth is about 50m. It's constructed under the Route 16 where located between Edogawa, Nakagawa and Ōotoshi Furutonegawa in Saitama prefecture.

When small and medium-sized rivers are flooded, part of the water will be drained to the Edo River through this channel. With the completion of the Metropolitan Area Outer Underground Discharge Channel, the number of houses flooded in the surrounding area and damages have been greatly reduced.

You can also visit there to join the walking tour. Please refer to the links below for details.

・ Chocotabi Saitama: Experience Japan’s flood control technology at the Metropolitan Area Outer Underground Discharge Channel

・ Walking Tour Guide (Choose English)

Flood / Landslide Warning Systems and Disaster Prevention Information

The disaster prevention information is specified by 5 levels of warnings so that the residents can easily understand intuitively the actions to be taken. The disaster prevention information is published by the local government and the Japan Meteorological Agency.

When the local government issues an evacuation advisory (warning level 4), or an evacuation preparations or evacuation of elderly people (warning level 3), take action immediately.

When evacuating, you don't have to choose to go to the pre-designated evacuation site, but you can consider evacuating to the upper floor of a sturdy building nearby, to be away from rivers and cliffs. It is important to take action according to the situation.

Evacuation Terms in Japanese

Evacuation Order (避難指示 hinan-shiji): This will be announced when the level of danger is imminent (Alert Level 4). Once this is announced, please evacuate to the safe site.

Warning System by Japan Meteorological Agency

Click the image to see more details from the Cabinet Office.

How to Evacuate

Click the image to see more details from the Cabinet Office.

Hazard Maps in Tokyo

Each ward has its own Hazard Map and some of them have it in English.

Floods in Tokyo and Safety Tips and Preparation

Rainfall Intensity and Definitions of Terms

Localized Heavy Rainfall - Guerrilla Rainstorms

Japanese rivers flow higher than residential area