Topic 5: Probabilistic Prediction of Flood Hazards

山崎 大/Yamazaki Dai
Associate Professor, Institute of Industrial Science, University of Tokyo


Q1 What do you study?
I study global terrestrial water dynamics, which is about the flow of water over land across the entire globe.
Specifically, where, how much and what kind of water is there on land, including rivers, wetlands, groundwater and soil moisture, and how it moves. Our research aims to understand the dynamics of terrestrial water, mainly by developing models and performing computer simulations.
Will the amount of water in rivers increase or decrease in the future? How will groundwater change from weeks to centuries in the future? These are some of the topics of interest. Understanding, modelling, and predicting the movement of various terrestrial waters is a central theme of my research.
Q2 Can you describe the CaMa-Flood model you have developed?
Our research team is working on developing a model which can efficiently simulate water flow in all rivers in the world.
Essentially, river water flow is a very local phenomenon. To perform precise river flow simulation, it is necessary to represent local-scale topography of river basins in detail. Therefore, the simulation of this river flow has been often carried out at local scales. Our challenge is on how to model river flow simulation on the global scale, i.e., simultaneous simulation of all rivers on the planet.

Figure: Global river topography data: (left) channel width, (right) floodplain topography from Yamazaki et al 2019 Water Resources Research

If you solve the detailed flow in each river one by one, it requires tremendous amount of data and computations. This, simulation takes long time even when we use supercomputers, making some applications difficult such as real-time prediction. Our research team is building a global river model named CaMa-Flood, which is designed to reduce the computational cost by appropriately approximation complex water flow while minimizing degradation of the accuracy.
Figure: Mekong River flooding simulation using CaMa-Flood
The capacity of CaMa-Flood is not limited to river flood forecasting. This model can also be applied to many other problems. This broad applicability of CaMa-Flood makes my research fun. For example, global river model can be used both for flood forecast and drought monitoring purposes. The applications of CaMa-Flood model also cover multiple temporal scale. For example, we can use CaMa-Flood for short-term flooding forecast on hours or weekly scales, or for climate change risk assessment on decadal to even centennial scales.

Another strength of CaMa-Flood is its ability to calculate the river water flow across the entire planet, and it is now used as a component of Earth System Model which is used for future climate projection. The main difference between CaMa-Flood and the other river models is that CaMa-Flood can be used not only as a tool to assess how river water cycle and human society will be affected by climate change but also as a component of a climate projection model (i.e., applicable both for Climate Impact Assessment and Climate Projection studies).
Q3 What do you want to contribute to this Moonshot project?
In this project, it is necessary to accurately estimate the impact of controlling typhoon on economic and humanity damage in real-time.
One of the most important impacts of typhoons on our society is river flooding. Therefore, I think that we can contribute in terms of real-time probabilistic flood impact forecasting by using the high-speed and highly accurate river simulations by CaMa-Flood.

However, we do not think that the current CaMa-Flood has sufficient skill for this purpose. Human societies had been already adopted to flood hazards, for example by building levees and dams. In developed countries, it is possible that even once-in-100-year flood hazard has little impact on human society, because of protection by levees and dams. Nevertheless, the current CaMa-Flood does not represent these flood protection effects, which may lead to overestimation of economic damage and human losses. Therefore, we believe that flood defense facilities such as levees and dams need to be well represented in CaMa-Flood to accurately estimate the damage caused by floods, especially those are smaller than the current design flood levels.

As a typhoon caused hazards in many regions with different occurrence probability levels, and as the typhoon forecast itself cannot be deterministic, we have to estimate damage caused by typhoon hazards based on probabilistic approach. A challenge for this Moonshot project to estimate the wholistic damage by typhoon considering multiple regions and multiple occurrence probability in real time, to enable discussion on optimum control of typhoon for the society.
Q4 One of the major themes of the Moonshot project is weather control. How can CaMa-Flood contribute to realize this?
Even if weather control or typhoon control itself becomes technically feasible, I think there will be a problem of practical use of the technology unless there is a social consensus on whether typhoon control is acceptable.
If only the intensity of a typhoon becomes weaker by applying controlling measures, we can reactively be easy to accept the use of the technology. On the other hand, there may be cases where the track of typhoon changes and it causes damage in a different region other than originally expected. In such cases, we need to estimate the changes in expected damage before and after the typhoon control for multiple regions. After considering the all impacts induced by weather control including its benefits and side effects, we can eventually decide whether typhoon control is socially acceptable or not.
I think it is necessary to examine and discuss these issues at a stage before the technology is ready to be installed. When implementing weather control, it is necessary to choose what scenarios could be realized by controls, including impact on society in each scenario. CaMa-Flood allows high-speed simulations of flood hazards for various scenarios. I think CaMa-Flood can contribute to support decision making on Typhoon control, by quickly simulating how, who, and what kind of damage could be caused before and after the typhoon control,
Q5 How do you plan to develop your research in the far future? What kind of things do you hope to do?
In terms of the part related to weather control studies, weather forecasting involves uncertainties under all conditions, and I think it is very hard for people to make decisions under such uncertainties.
I believe society will be able to handle the uncertainties better, if we can realize a world in which people can objectively make difficult decisions with the support of science, i.e. providing reliable future data through model simulations. I think this would be good for humanity as it would make society more capable of dealing with uncertainty.
Q6 What kind of world do you think will exist in 2050, Moonshot's target year?
I hope that we will have a society where people are more honest about what they want to do.
In 2050, I wish the simulation technology needed to predict damage from floods and the other natural disasters will have been established. Prediction of the magnitude of damage by natural hazard is still difficult now, thus I would be happy if we can achieve the disaster impact prediction, and I hope we are loving in a future society where we know in advance where and how much damage is expected and how we should respond.

From the viewpoint of climate change research, I believe that we will achieve carbon neutrality under the Paris Agreement by 2050. I hope that both technical and social problems related to climate change will be solved. I would like to see a society where people can travel freely without worrying about greenhouse gas emissions, or where people can use energy without concerns on global warming.

It is very hard for human beings to live with patience, and eventually we reach our limits of patience. Even now, we have to cut back on many things due to the climate change, we have to stop doing these activities because they are not good for us, and we have to live our daily lives under such constraints. I hope that these restrictions will be solved technologically and we will have a society where we can do what we want to do more honestly.

Thank you very much.
    Moonshot Goal8 Realization of a society safe from the threat of extreme winds and rains by controlling and modifying the weather by 2050.
Coupling/Control Systems