The deadly mudslides in Hiroshima and other parts of western Japan last month caused by torrential rains have raised concerns about how vulnerable Japan is to such natural disasters, especially given severe weather events due to climate change.

While a combination of factors were responsible for the high death toll in Hiroshima, the national and prefectural governments have been seeking ways to ensure flash floods and other natural disasters cause minimal damage.

Why was Hiroshima hit so hard by the mudslides?

The geographical makeup of the prefecture is such that the central government has designated it as particularly vulnerable to landslides and floods. In 1999, 32 people were killed or went missing in mudslides similar to the ones last month.

After that disaster, the central government asked all prefectures to conduct inspections of areas where floods, landslides and mudslides posed a risk. In Hiroshima’s case, that meant surveying more than 6,000 different locations the city had thought to be in danger of land or mudslides, places often in hilly and remote areas where access roads are limited. Less than 2,000 of these locations, however, have been designated as special caution zones where building is restricted.

What is it about the topography that makes mudslides and landslides more likely in Hiroshima or in other parts of Japan?

Katsutoshi Ueno, a professor of geotechnical engineering at Tokushima University, points out that Hiroshima’s soil contains lots of decomposed granite, which causes the land to become loose, especially when hit with large amounts of rain.

This loose granite is also a problem in many other prefectures with hills and narrow valleys or gorges.

When people build houses or other buildings in those areas, the dirt is often loosened further, placing them further at risk.

Why weren’t officials notified in advance the heavy rain could pose a danger?

The problem is that the rains that triggered the mudslides were extremely localized. In meteorological terms, what happened over the area of Hiroshima where the worst floods occurred is known as “back building,” whereby thunderclouds pile up and the result is an intense rainfall in a very small area. In one Hiroshima ward, about 217 mm fell in three hours, a record.

Meteorological officials in Hiroshima did issue a general heavy rain and flood warning the night before the mudslides. But they did not issue a special warning that could have led to evacuation, a classification created just last year for natural disasters, because the area of the predicted rainfall was small and it was not expected to last long.

Are mudslides regular occurrences?

Japan has, historically, been especially prone to mud and landslides due to its topography and heavy rainfall. About 200 people are killed annually due to rain-related disasters.

The Land, Infrastructure, Transport and Tourism Ministry estimates there are between 700 and 1,400 landslides every year.

As of early August, before the Hiroshima mudslides, the ministry said 497 sediment-related disasters (landslides, rockslides, and collapsed cliffs) occurred nationwide. Kagoshima and Kochi prefectures, with their mountains and narrow valleys and gorges, had over 50 incidents each, one-fifth of the total. Nine prefectures, mostly in the Kansai, Chubu, and Tohoku regions, had no incidents.

So just how vulnerable are other areas of the country to landslides?

Figures vary, depending on who is doing the calculations and what those doing the calculating define as “dangerous” or “potentially dangerous.” But in designated caution zones, there are certain restrictions on construction, and local governments are urged to shore up the areas by building retaining walls, dams or through other measures.

However, restrictions on building houses are weak, and even designating areas as having a risk of mudslides can often be difficult, as local residents and property owners in and around the area complain such an official designation will cause the value of their property to drop.

Facing criticism that the prefectural government had given the caution zone designation to only about 30 percent of the locations in Hiroshima that have been recognized as potentially dangerous, Hiroshima Gov. Hidehiko Yuzaki said the bureaucratic process is tough and brings few tangible benefits to area residents.

Who is responsible for designating and enforcing potentially dangerous zones?

The central government’s law directs local governments on what to do, but it’s the local governments that are actually responsible for surveying locations and classifying them into either caution zones or special caution zones, where the risk is higher. Again, though, political pressures from landowners, a lack of funds to carry out such surveys and other reasons mean that only 70 percent of the more than half a million places nationwide officially identified as being at risk have actually been surveyed and classified.

Given the unpredictability of rain and landslides, and the fact that in Hiroshima most of the landslides occurred in areas that had not been designated as hazardous, local and national officials are skeptical as to how much can be done beforehand.

To what extent is the increase in mudslides due to the changing climate?

Evidence from climate change experts suggests there is a strong connection between increased severe weather events, such as intense, localized downpours, and increased weather-related disasters.

In a July 2013 report on Japan’s measures to adapt to climate change, Takuya Nomoto of the Environment Ministry’s global environment bureau noted that disaster risks due to heavy rain were predicted to increase because over the past century there has been a gradual increase in the number of days with more than 200 mm of rain an hour.

A separate report by World Wide Fund for Nature (WWF) released in 2008 and based on Japanese government predictions, warned that climate change means not only hotter weather by century’s end, but also that precipitation between June and September is expected to increase 17 to 19 percent over current levels.

How can people take precautions against mudslides?

First, check with your prefecture to see if the area you’re living in or close to it is in a designated hazard zone. If so, find out what precautions local officials recommend — registration of emergency contact details with the fire department, for example — and where the designated evacuation shelters are.

After the disasters of March 11, 2011, it became more common for people to prepare emergency kits, but those living in potentially risky areas may also wish to have a backpack full of water, food and medical supplies ready in case a quick evacuation becomes necessary.

In a time of both misinformation and too much information, quality journalism is more crucial than ever.
By subscribing, you can help us get the story right.