At the UN conference "COP26" that closed the other day, it was pointed out that disasters would intensify due to global warming, but it became clear that the tsunami could have a major impact.


Experts estimated the impact of sea level rise in areas where damage is expected due to the Nankai Trough giant earthquake, and found that the rise in "run-up height" when a tsunami runs up land is the same as the rise in sea level in some places. It turned out to be more than doubled.

Experts point out that "for the future, we need to consider measures according to the terrain and region."

Regarding the rise in sea level, the Japan Meteorological Agency and others have not taken any additional measures, and if the temperature rises by about 4 degrees at the end of this century, it may rise by about 71 cm on average near Japan.



A research group at the International Research Institute of Disaster Science, Tohoku University has estimated how the effects of the tsunami will change in Wakayama, Tokushima, and Kochi, which are expected to be severely damaged by the Nankai Trough giant earthquake.



In the event of a magnitude 9 class earthquake, the total flooded area increased by 17% in Tokushima, 8% in Wakayama, and 4% in Kochi.



Furthermore, the run-up height at which the tsunami runs up the land may be higher than the rise of the sea level, and in the urban area of ​​Wakayama City where the altitude is gradually rising, about 6 meters to 7.6 meters, more than 1.5 meters. It became expensive.



This difference is more than double the 71 cm rise in sea level.

Associate Professor Sappashi Anawat of the research group analyzed that the water depth was maintained even when the tsunami reached the land, so it ran up without slowing down too much, and said that the destructive power of the tsunami would also increase.



Associate Professor Anawat also said that urban areas with gentle slopes and places surrounded by steep mountains behind them are susceptible to rising sea levels. I would like you to consider not only heavy rains and typhoons, but also the effects of tsunamis. Depending on the terrain, the damage may be greater, so it is necessary to investigate in detail. "

Sea level rise, why does it have a big impact on run-up height?

Why does rising sea level have a major impact on run-up height?



Professor Fumihiko Imamura of the International Research Institute of Disaster Science, Tohoku University, who is familiar with the mechanism of the tsunami, points out that the characteristics of the tsunami itself have an effect.



The tsunami is extremely fast when traveling in deep water, reaching 800 km / h offshore, but slows down due to resistance from land when the water depth becomes shallow.



However, at a depth of about 10 meters and a speed of about 40 km / h, even if you run at full speed, you will be caught up.



When the sea level rises, even if the tsunami could not run up at present, the speed will not slow down due to the sufficient water depth, and it is expected that the slope will run up at once.

Expert “May increase the destructive power of the tsunami”

In addition, Professor Imamura points out that the destructive power of the tsunami will increase.



Regarding the simulation results of Wakayama City, he said, "In addition to the water depth being more than doubled at the tip, the flow velocity is likely to be more than doubled, and the degree of destruction in the vicinity may be greater."



Since the destructive force of a tsunami is greatly affected by the flow velocity, the force applied to the seawall on the coast may change, and it is necessary to study the impact in more detail.

Maintenance of seawalls, etc. Specific height for each coast

In November last year, the national government changed the "Coastal Conservation Basic Policy" that provides guidelines for the development of seawalls, etc., saying that the level of protection against tsunamis, storm surges, and storm surges should be set based on the effects of climate change.



We are requesting prefectures to revise the "Coastal Conservation Basic Plan," which stipulates specific heights of seawalls for each coast, by the end of July Reiwa.