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How can we prepare for large-scale inundation caused by torrential rain?
– The Latest Prediction Technique and It’s Use for Evacuation –

Masato Sekine
Professor, Faculty of Science and Engineering, Waseda University

Why does torrential rain cause disasters so frequently?

The 2016 typhoon season was almost over. Looking back, we were adversely affected again by heavy rain this year. Kumamoto Prefecture, which was struck by a major earthquake in April, was hit with a record-breaking rainfall in June when recovery had just gotten underway. In October, Mt. Aso-san erupted, meaning that the local residents had faced three natural disasters within just half a year. We Japanese have been placed in a very harsh natural environment. Going forward, we must gather the best of what scientific technology has to offer and use our knowledge in order to live in harmony with nature.

In recent years, torrential rain disasters have occurred almost every year. Have you noticed that this is the result of global climate change? According to the latest report by the Intergovernmental Panel on Climate Change (IPCC), the weather is becoming more and more extreme. Temperature rise as well as sea level rise resulting from climate change are well-recognized. In addition to these changes, there are concerns regarding the effects of temperature rise of sea water. You may have heard that when the temperature of sea waters near Japan increases, catch rates of fish, such as pacific saury, decrease. However, the effects of Climate Change do not stop there. It also generates typhoons on the Pacific Ocean much closer to Japan than before, and such huge typhoons could strike Japan without weakening their forces at all. This year, repeated typhoons hit Hokkaido, and unprecedented damage from torrential rain occurred. This also seems to be the result of the changes above, and it has been reported that areas not previously exposed to such disasters are now under the same level of risk. The rule of thumb that safety is guaranteed in places which have never faced natural disasters does not apply any longer. Furthermore, carelessly thinking that disasters only occur elsewhere and have nothing to do with you could put your life in danger.

Predicting inundation that could occur in urban areas

Inundation risk map:Results of numerical prediction in a situation where rain of the same intensity which occurred in Suginami ward in 2005 were to hit the entire area located the west of the Arakawa River. The Figure shows the inundation depths for each point.

Have you ever thought about what actions to take when you hear a flood or inundation warning announcement? Many people may be at a loss as to what they should do. This is because there are issues to be resolved by both the side of providing flood warnings and those receiving the information. In Sekine Laboratory at Waseda University, research involving detailed numerical predictions are being carried out to determine how inundation would occur and the inundation depth at each local point is estimated if Tokyo’s 23 wards were to be hit with torrential rain. The figure below is an example showing the outcome of such a prediction and could perhaps be the latest and most accurate inundation risk map. It is the computations of where torrential rain of the same intensity which occurred in Suginami ward in 2005 was to hit the entire west area of the Arakawa River (20 wards, specifically speaking). In these computations, all the detailed information about Tokyo are considered. The database have been already developed about road networks, city blocks, sewage systems and urban rivers. By using the database, each flow of water are solved faithfully based on the laws of dynamics. This prediction technique can be used to precisely evaluate inundation damage in urban areas that includes not only inundation during torrential rain but also large-scale inundation due to river embankment collapse. The results of such computation have been already notified to the persons concerned in Tokyo Metropolitan government, Subway Company and some underground spaces. They will be able to utilize this information for future inundation countermeasures effectively. Video image of inundation process obtained by the computation must be more useful than the still image for local residents to understand easily their inundation risk. Unfortunately, previous risk information, such as the hazard map provided by local government, did not offer this level of precision and was difficult to understand. We challenge to further enhance scientific evidence to allow people for gaining correct understanding of inundation risks and effectively utilize the information for resident evacuation.

How can inundation risk information be used to reduce the damage caused?

It is meaningless for accurate and comprehensible risk information to be simply presented in a one-way manner. It is essential that the residents who receive this information try to understand the situation that they are in and, if a disaster does occur, utilize their imagination to take actions that protect their life.

The existing systems would still be able to handle most instances of torrential rain in the future. However, this is not a universal solution— there will be situations where these systems alone would not be enough. In such cases, planned evacuation is necessary to ensure safety. We have created a convenient and comfortable society to this day. However, I believe that we have lost our instincts as animals to fear nature and be able to sense danger. What is required of us now is to retrieve these instincts.

For the Tokyo Olympic and Paralympic Games

The Tokyo Olympic and Paralympic Games will be held in 2020. The previous Tokyo Olympic Games in 1964 were held in October. But this time, they will be held between July and September during the rainy season. While it may be taken for granted that the events will run smoothly without a problem, is this truly the case? Of course, there are endless issues to consider once you start to worry, but we should be well prepared to respond to emergencies. I intend to further evolve the inundation prediction technique, which I have developed, and cooperate with researchers in the field of meteorology to create a "Real-time Flood and Inundation Forecast" that is coordinated with the torrential rain forecast. By 2020, I hope to provide and share information that can predict inundation. For instance, “In 30 minutes from now, Waseda-dori Street and Meiji-dori Street intersection will experience xx cm of inundation.” This information will be targeted towards not only Japanese people but also guests from overseas. I believe that the best act of "hospitality" that we can offer is by allowing our overseas guests to come and utilize risk information for a safer and more enjoyable stay in Tokyo.

Masato Sekine
Professor, Faculty of Science and Engineering, Waseda University

Professor Sekine graduated from Department of Civil Engineering of the School of Science and Engineering, Waseda University in March 1983, and immediately continued on to graduate school. He completed the doctoral course at the Graduate School of Science and Engineering and in March 1988 and obtained doctoral degree of Engineering. He worked as a Postdoctoral research fellow at the University of Minnesota, USA. Subsequently, he became Associate Professor in April 1992, and after that Professor at Department of Civil Engineering, Waseda University in April 2000. In accordance with the interschool reorganization, Professor Sekine became Professor of the Faculty of Science and Engineering (Department of Civil and Environmental Engineering, Waseda University) in April 2007.

[Affiliated academic societies]
Japan Society of Civil Engineers (JSCE), American Society of Civil Engineers (ASCE), International Association for Hydro-Environment Engineering and Research (IAHR), Japan Society for Natural Disaster Science

[Main Authored Works]
"Hydraulics of Movable Bed Flow" (Kyoritsu Shuppan, 2005), etc.

March 2004: Received the Best Paper Award of Hydraulic Engineering, JSCE, 2003.
March 2015: Received the Best Paper Award of Hydraulic Engineering, JSCE, 2014.
March 2016: Received the Best Paper Award of Hydraulic Engineering, JSCE, 2015.