Clarify correlation between accumulated mound of soil and huge Atami landslide

An accumulated mound of soil, situated around the uppermost portion of a river, is believed to have increased the damage caused by the mudslide in Atami, Shizuoka Prefecture. The causal relationship between the mound of soil and the slope collapse must be clarified, and the information used for future countermeasures.

The mudslide stretched about two kilometers, overrode a soil erosion control dam upstream and swallowed up a residential area more than 100 meters wide downstream. More than 120 houses and other buildings were damaged and many people remain unaccounted for.

There were 50,000 cubic meters of accumulated soil in the mound at the starting point of the mudslide. A business in the past reportedly carried construction waste soil to that spot. The scale of the mudslide was estimated at 100,000 cubic meters of soil, with the Shizuoka prefectural government saying the mound of soil “likely led to extensive damage.”

When a layer of soil is laid on an original slope, rainwater soaks into the space between them and the mound of soil tends to collapse easily. This time, there is a possibility that the mound of soil collapsed first, triggering a large-scale mudslide. It is necessary to conduct a thorough investigation into whether measures, including the discharging of water, were taken adequately.

Safety measures for an accumulated mound of soil associated with residential land development are required under the Regulation of Residential Land Development Law. However, in the case of the Atami mudslide, the land at the site was not subject to the regulation because it was not residential land. It is vital to reconfirm whether the current system is still appropriate.

In the surrounding area, solar panels have been installed and residential land development is in progress. If trees are cut down, the water retention capacity of soil decreases. It is also important to examine the impact of development in forests.

Accidents involving collapsing waste soil have occurred in many parts of the country.

According to the Land, Infrastructure, Transport and Tourism Ministry, 14 such accidents occurred in 11 prefectures, including Osaka and Yamanashi, between 2001 and 2014, resulting in fatalities. There is no end to companies that illegally dump waste soil in mountainous forests and vacant land, and the reality is that a sufficient monitoring system is lacking.

Land minister Kazuyoshi Akaba expressed his view to inspect all accumulated mounds of soil nationwide. It is essential to evaluate not only the danger of the mounds of soil themselves but also the risk of causing mudslides.

In the mudslide this time, the prefectural and municipal governments announced the names of 64 people whose whereabouts could not be confirmed. This is because many houses in the disaster-hit area are used as vacation homes, making it difficult to confirm the safety of the missing. As a result of the announcement, the whereabouts of more than 40 people reportedly have been confirmed.

It can be said that the appropriate disclosure of information has led to prompt confirmation of the safety of the missing.

In recent years, in the event of a natural disaster, there has been a noticeable trend of local governments withholding the names of the dead and missing, and only announcing the number of people, on the grounds of the protection of personal information and the inability to obtain the consent of families.

The publication of names, however, is useful in narrowing down the scope of a search in time-sensitive situations. In the future, local governments should make it a principle to actively publicize names under such circumstances.

— The original Japanese article appeared in The Yomiuri Shimbun on July 7, 2021.