Natural disasters in Japan

Mount Aso 4 pyroclastic flow and the spread of Aso 4 tephra. The pyroclastic flow reaches almost the whole area of Kyushu, and volcanic ash is deposited over 15 cm in a wide area from Kyushu to southern Hokkaido.

There are three VEI-7 volcanoes in Japan. These are the Aira Caldera, Kikai Caldera and Aso Caldera.

Mount Aso is the largest active volcano in Japan. Mount Aso had four eruptions 300,000 to 90,000 years ago. It emitted huge amounts of volcanic ash that covered all of Kyushu and up to Yamaguchi Prefecture.

Japan has experienced several major mudslide events, often due to other natural disasters.

On September 14, 1984, the Otaki earthquake (magnitude 6.8) in Nagano Prefecture, Japan, caused a major mudslide on the south face of Mount Ontake, which reached speeds of 80~100 km/h.[4] Heavy rain over several days prior to the quake contributed to causing the mudslide, known as “Ontake Kuzure,” which took the lives of 29 people. After the disaster, barriers against mudslide were erected in nine locations at the foot of Mt. Ontake.[5]

In September 2011, Tropical Storm Talas (2011), which formed and was named on August 25, made landfall over Japan, bringing heavy rain to the mountainous Kii Peninsula, and causing a mudslide, called “Shinsohokai,” in Wakayama and Nara. "Shinsohokai" means that not only soil collapses, but also bedrock. In this case, houses were covered with earth and sand from the mudslide and there were many injured and killed.[6]

In June 2010, Hiroshima experienced heavy rains which caused severe damage, including mudslides and river floods; Shōbara had 64 mm rain in an hour and Hiroshima had 30–50 mm on June 12, 20-40mm on the 13th and 20-50mm on the 14th. Five people were killed and six injured; 1,787 houses were destroyed.[7]

Flood management has been important in Japan for generations. The first form of flood management dates back to the 4th century when the Manda Levee (embankment) was built at the Yodo River. Here a narrow path runs along the old river near the inhabited areas. Today there is even a monument.[11] Japan has many large-scale projects to protect its inhabitants from the water. For example, there is a lot of emphasis on breaking high waves and a lot of research is being done into the behavior of tsunamis.[12] After the devastating tsunami of 2011, the Japanese government decided to protect all vulnerable villages and towns along the coast by building sea walls that range from 8.5 meters to 24 meters in height. As much as eleven billion is spent on building this 400-kilometer-long concrete wall.[13] Inland, flood management in Japan is also important against river flooding. Many of these projects are also recognizable to us. They are working on widening the river channels and building dikes. This is to limit the rise in a river's water level by increasing its cross-section and to prevent the land from flooding in times of flooding. They have drainage basins, which are used to temporarily store the overflow of the river during a flood and to drain it again after the risk of flooding has passed. This prevents the downstream water levels from rising. They also have pumping stations, here pumps are used to drain rainwater into the river in areas where the water level in the river is higher than the land. They also dredge in Japan. This is done to lower the water level in a river by deepening or dredging the river bed. Another important way of flood management is to create so-called shortcuts or shortcuts. The length of a river channel is shortened by straightening bends, so that flood water can flow through it quickly. And a flood management approach widely used by Japan is dams, such as the 186 meter high Kurobe Dam in Toyama. These dams are huge, so it often takes years to build them. The function of these dams is to regulate the water flow upstream at the time of a flood, so that the flow is reduced downstream.[14]