River flood risk “Earlier grasp” Research progresses December 24 at 1:35

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As the damage caused by the typhoon becomes more severe, research is being conducted to narrow down rivers with high risk of flooding more than half a day before the typhoon approaches. Evacuation immediately before the flooding of the river approaches is life-threatening, so it is expected that the dangers will be grasped at an early stage, leading to early evacuation.

Prediction of river flooding risk Current status is "3 hours ahead"

Regarding the risk of flooding of rivers, for large rivers, the Meteorological Agency collaborated with the Ministry of Land, Infrastructure, Transport and Tourism and the prefectures on `` flooding risk information '' and `` flood warning information '' based on observation data of water level and prediction of rainfall. Flood Forecast "has been announced.

On the other hand, for small and medium-sized rivers for which flood forecasts cannot be announced, the Japan Meteorological Agency has released a "risk distribution" that indicates the risk of flooding by coloring rivers on a map based on rainfall forecasts.

However, the prediction is up to "3 hours ahead", and when the danger is grasped, there are cases where evacuation is difficult because rain has already intensified or the water level has risen, and how to grasp the danger quickly is Is an issue.

The risk of flooding

Associate professor Takahiro Sayama of Kyoto University, who specializes in flood forecasting, is working on research that can predict flooding more than half a day ago to overcome these issues.

Associate Professor Sayama used the Meteorological Agency's rainfall forecast up to 39 hours ahead to calculate the amount of water flowing into each small and medium-sized river nationwide, the "runoff".

"Runoff" is calculated by dividing the amount of water flowing in the river by the area of ​​the upstream basin. According to Associate Professor Sayama, it is easy for floods to occur in small and medium-sized rivers if they exceed 30 mm per hour.

As the risk of flooding increased, Associate Professor Sayama showed the color of the river on a map that changed from yellow to blue to purple.

As a result, an analysis based on the forecast by the Japan Meteorological Agency at 9:00 am on October 12, about half a day before the typhoon landed, showed that the mountainous area in the western part of the Kanto region, the Ashio Mountains in Tochigi Prefecture, Miyagi Prefecture, In some parts of Fukushima Prefecture, the purple color has increased, indicating that the "spill volume" exceeded 30 mm.

Many of the small and medium-sized rivers, which had turned from blue to purple, were actually being damaged by flooding and collapse.

At present, there are issues such as long calculation times, but Associate Professor Sayama says that if this prediction can be made early, it can be used for early evacuation.

Associate Professor Sayama said, "Because we know the danger of floods at an earlier stage than before, if we can calculate in real time, it may lead to early evacuation of residents."