How to prepare for flooding and landslides during Japan’s rainy season

The danger of landslides and flooding in Japan cannot be underestimated, especially during the rainy season. It is also no less a risk factor than earthquakes when assessing the merits and demerits of living in a certain neighborhood or buying or renting a particular property.

Just earlier this month, following torrential rainfall, a landslide was triggered which swept through the Izusan neighborhood of Atami, Shizuoka prefecture, on July 3rd. Nineteen people died during the landslide and nine went missing. Atami is situated on the edge of the prefecture’s mountain ranges, and is known for its steep valleys and hills. Preceding the landslide, the city received 310 millimeters (12.4 inches) of rainfall in a 48-hour period.  The landslide destroyed a number of houses and left roads covered with debris. (Source: Wikipedia)

Investigations into the cause of the landslide are ongoing, but authorities are investigating suggestions by experts that improperly accumulated soil brought in for development purposes may have been partially responsible for the landslide. (Source: Wikipedia)

In the last few years, devastating landslide and flooding disasters have struck Japan on an annual basis, including:

  • Heavy flooding in southwestern Japan in 2018
    • 225 people were confirmed dead across 15 prefectures with a further 13 people reported missing. More than 8 million people were advised or urged to evacuate across 23 prefectures. (Wikipedia)
  • Typhoon Hagibis in 2019
    • Japan’s Fire and Disaster Management Agency stated that at least 98 people have been confirmed dead, 7 people missing, with 346 people injured by the storm. More than 270,000 households lost power across the country. (Wikipedia)
  • Kyushu floods in 2020
    • These were caused by a series of floods in July 2020, when record-breaking heavy rain hit the prefectures of Kumamoto and Kagoshima on the island of Kyushu.
    • As a result of the flooding, 77 people were confirmed dead and about 15,335 buildings were destroyed, damaged or flooded. (Wikipedia)

How much of a danger are landslides and flooding risk in Japan?

According to the MLIT, about 1.23 million, or 23.1% of households in Japan live in areas where there is a risk of a landslide or flood-related disaster occurring.

Of these, about 3% of households live in areas designated as “sediment-related caution areas,” about 4.6% live in “tsunami inundation areas,” and about 19.1% live in “flooding (inundation) areas”.

On August 20, 2014, following torrential rain in which a month’s worth of rainwater fell in a single day, several landslides were triggered near a mountain near the city of Hiroshima, with Asakita ward being the most severely affected. Seventy-four people were reported dead (Wikipedia: 2014 Hiroshima Landslides).

A 2015 study found that long-term land-use changes, notably the expansion of residential areas onto mountainous areas where past debris flow (resulting from a previous devastating flood in 1999) made this area especially vulnerable to sediment-related disasters. The affected neighborhood is very close to downtown Hiroshima, which further multiplied the impact on local residents.

In response to the Hiroshima landslide disaster, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) conducted a five-year survey of the country’s “landslide disaster warning areas” (土砂災害警戒区域) which was published in March 2020. This survey is published on the homepages of each of Japan’s prefectures and is the basis on which certain areas are designated as:

  • Landslide Disaster Warning Areas (土砂災害警戒区域) or
  • Landslide Special Caution Warning Areas (土砂災害特別警戒区域)

For areas that are designated as Landslide Disaster Warning Areas, local governments are tasked with improving the local warning and evacuation system and to inform the public of landslide risks through landslide hazard maps.

In addition, real estate agents are also obligated to explain landslide and other natural disaster risks to people when they buy and sell land or buildings. This is one of the points covered in the Explanation of Important Matters, the official disclosure document provided to buyers prior to closing a transaction.

Hazard Maps

What can you do to protect yourself and your home against the potential dangers of flooding and landslides?

The first step is to inform yourself of whether and how much your home and neighborhood are at risk of these natural disasters.

Unfortunately, are often not published in English

Flooding and landslide hazard maps are published by local governments to provide residents with the following information:

  • Which areas are expected to flood during heavy rain
  • Estimated level of flooding and degree of danger when local rivers overflow
  • Evacuation sites, routes, and other disaster-response information

The link to Japan’s national hazard map website is here. This site aggregates links to hazard maps published by local governments. You can also go to the official website of your city or ward and search for “ハザードマップ” (hazard map).

Unfortunately, most local governments do not publish hazard maps in English.

Here is an example of an English hazard map, published by Toki City, in Gifu prefecture.

Toki City flood hazard map. Image: Toki City

In general, in hazard maps, dark blue denotes areas most likely to flood. However, the color legend of the is not uniform across maps published by different municipalities. If you can’t read Japanese, you may want to ask a friend to help you decipher the map for your municipality.

The legend explains the color codes for the different depths of water expected during flooding. In general, blue designates the ares with the highest level of expected flooding. Image: Toki City

Preparing for flooding – English information

Tokyo’s Edogawa ward has published what is probably the best flood-related hazard map information packet available in English.

You can download a PDF copy here: Edogawa City Hazard Map Pamphlet (English PDF)

Edogawa ward is especially prone to flooding because 70% of the land in the ward lies below sea level. When there are high levels of rainfall in Saitama, Gunma, and Tochigi prefectures, much of the water flows into the Arakawa and Edogawa rivers, which run through the center the ward and empty into Tokyo Bay.

The information published by Edogawa ward paints a dire picture of could potentially happen in the event of a major flood in the area. In the worst case scenario, flooding could continue for two weeks in case the Arakawa and Edogawa Rivers overflow or if there is high-tide flooding.

Some of the information published in Edogawa’s hazard map packet provides specific information to residents of that ward, but a lot of the information is very detailed and widely applicable to every area of Japan, so it’s worth reading.

Here is a short summary of how to prepare for a major flooding disaster, based on the guidelines published by Edogawa Ward:

Know where to get information ahead of time

  • Japan Meteorological Agency (JMA) website
    • Publishes official advisories and warnings on natural disasters (earthquakes, tsunami, heavy rain, flooding, landslides)
  • Official website of your city or ward
  • Official social media accounts of your city or ward
  • NHK
  • NHK radio
  • Smartphone weather apps

Evacuation warning levels

Color codes on the official JMA website tell you what action you should take:

  1. White: Stay on alert for disasters
  2. Yellow: Check evacuation procedures
  3. Red: Elderly people must evacuate
  4. Light Purple: All residents must evacuate
  5. Dark Purple: People must take measures to protect lives

NHK and other media will also broadcast these advisories and warnings as they are updated. Officials strongly urge people to evacuate early if such an evacuation advisory has been issued for your area. Flooding and landslides can occur with very little warning. If an advisory is issued for your area, please take immediate action.

5-Step Warning System

Japan’s 5-level warning system for natural disaster and weather-related emergencies. Source: NHK

Long-term preparation for possible evacuation due to heavy flooding

  • Use a hazard map to understand what might happen to your home or neighborhood in the event of a heavy flood.
  • Confirm evacuation points and routes.
  • Confirm who does what (with your family members or close neighbors)
  • Prepare a Go Bag (evacuation bag)
  • Confirm emergency contacts beforehand

Preparing your home

If you live in a single-family home there are steps you can take to mitigate flood damage. These include:

(1) Raising the foundation of the home higher than the expected water level.

(2) Putting sandbags or water stoppage plates around entrances and the garage.

(3) Strengthening the joint foundation so that the entire building does not rise due to flooding.

(4) Installing outdoor air conditioner units as high as possible off the ground.

Some of these measures require significant additional constructions costs, which of course, have to be weighed against the estimated flood risk for the location of your home.

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Lead image: Torrential rain and flooding in Kyoto, 2018, iStock 997424604