[Tateyama Caldera] History of Tateyama’s Erosion Control Works

The Tateyama Caldera is a huge depression stretching 6.5km east to west, 4.5km north to south, and with an elevation difference of 500 to 1,700m (1,640 to 5,577ft).

It has been filled with soil and sand from numerous collapses in the past, and every time there was heavy rain, the soil and sand would flow out onto the plains, causing great damage.

Erosion control works, which is called sabo in Japanese, began in the Meiji era to protect the Toyama Plain, and have been ongoing for over a century now.

Hietsu Earthquake

In April 1858, an activity on the Atotsugawa Fault caused a large earthquake measuring 7.3 to 7.6 on the Richter scale.

This earthquake caused Mt. Otonbi and Mt. Kotonbi, located on the south side of the Tateyama caldera, to collapse. Large amounts of rocks and earth and sand dammed the flow of the river inside the caldera, creating many dammed lakes. This phenomenon is locally called “Tonbi Kuzure” or “Tonbi Collapse”.

About two months after the Tonbi-kuzure, the dammed lake collapsed twice. Large-scale debris flows caused great damage to the Joganji River and surrounding areas.

Toyama Prefecture is now known as a prefecture with few natural disasters, but in the past it was a disaster-prone land that frequently suffered major disasters such as the Tonbi-kuzure.

Toyama Prefecture has terrain with an elevation difference of 3,000m in a short distance, and with many days of snow and rain, there are many rapid rivers.

Erosion control work started in 1906

In the early Meiji period, Toyama Prefecture was still part of Ishikawa Prefecture.

However, the threat to residents from vast mountainous areas such as Mt. Tateyama was great, and flood control and erosion control (sabo) works were given priority as serious political issues, leading to the separation into present-day Toyama Prefecture.

At the time, Toyama Prefecture began sabo works due to the frequent occurrence of landslides, but the work did not progress smoothly on its own. As a result, the Tateyama Erosion Control Works Office took over the work as a direct national project.

People involved in the erosion control works

Masao Akagi

In 1926, Masao Akagi became the first director of the Tateyama Erosion Control Works Office. He was a leading figure in sabo construction, and worked tirelessly on the construction of the Shiraiwa Sabo Dam and other erosion control works on the Jōganji River.

He came up with bold ideas for sabo-related policies, and completed the Tateyama Sabo System, which was said to be a difficult project.

Johannes de Rijke

Born in the Netherlands, he worked as a civil engineer in the civil engineering department of the Ministry of the Interior in 1873. He was involved in flood control projects for the Kiso Three Rivers, Yodo River, and Joganji River. In 1891, De Rijke saw the collapsed caldera for the first time and was overwhelmed by the sight. De Rijke succeeded in stopping the flow of sediment from the mountain using a construction method called a megalithic dam.

When he saw the caldera collapse site, the sight was beyond his imagination and he muttered, “We’ll have to wrap the entire mountain in steel plates…”