Gigantic Rainwater Storage Pipes of Tokyo Metropolis to Prepare for Climate Change

Date of interview	October 16, 2020
Targets	Planning and Coordination Division, Bureau of Sewerage, Tokyo Metropolitan Government Chikao Okuda, Director in Charge of Emergency Rainwater Measures; Takashi Sakuma, Deputy Director; and Shusuke Wakasa, Senior Staff Member, Planning Section

Please give us a summary on the rainfall inundation measures that Tokyo Metropolitan Government has worked on so far.

Mr. Sakuma: If we talk about the recent heavy rains, there was an extremely heavy localized torrential rain that exceeded 50 mm per hour around the Jonan area, which includes Setagaya and Ota Wards, in July 2013. Such extreme torrential rainfalls are called guerrilla rainstorms in general, and have the characteristic of causing heavy rainfall in a localized area over a short period of time. The sewers have a role to quickly eliminate the rainwater into rivers and the sea, and measures are being required to handle such heavy rainfall that occurs in a short period of time in particular.
If we cannot eliminate the rainwater that precipitated into rivers or the sea, or if we cannot take it into the sewer pipes, the roads become inundated and cause inundation in houses below or above floor level. This is called “rainfall inundation.” Inundation damages occurred in over 700 houses in July 2013. 87% of the floods and inundations that occur in Tokyo in recent years are rainwater inundations. Since “river water inundation” occurs more frequently in rural areas, rainwater inundation can be considered a characteristic of urban areas.

Mr. Okuda: As inundation measures, storage pipes that are shaped as pipes and regulating reservoirs that are shaped as ponds are possible. While the function to store rainwater is the same for both, pipes can drain the water into rivers, etc. later. The pond type is selected more often if a large land can be secured and the areas where the risk of inundation is high can be identified.

Mr. Sakuma: In recent years, rainfalls exceeding 50 mm per hour have occurred quite frequently, and we are also taking measures to raise the capacity level to 75 mm per hour. Since there are limits to our budget, time, and the number of contractors, it is difficult to modify all the locations to the capacity for 75 mm per hour. We therefore place priority on areas with large-scale underground shopping areas such as Tokyo Station, Shibuya Station, and Shinjuku Station, which would suffer large inundation damages due to entry of rainwater, and establish storage facilities, etc. to handle the rainfall of 75 mm per hour.

In addition, a frequent case is that the capacity of the sewer pipes that were established in the past can no longer handle the rainfall of 50 mm per hour as rainwater flow into the sewer pipes increased when the lands that used to be dry fields were developed with urban structures. In some areas where sewer pipes were inserted into what used to be creeks or channels to establish the sewerage facilities, the rainwater flows backward, spills over the ground, and causes inundation when the water level inside the sewer pipes rises because the depth at which the sewer pipes are buried is too shallow. We are currently working on additional measures by specifying both of these types as priority regions.

Mr. Okuda: The flow-out coefficient, which is the rate of rainfall that goes into the sewer pipes instead of permeating into the ground, was set to 50% when sewerage facilities were first constructed. However, the current flow-out coefficient with advanced urban development is 80%, and we have completed only around 70% of sewers that have the capacity for it. Since it takes a long period to establish the facilities, we focus on establishing them with areas where rainwater tends to gather such as low ground and the bottom of a slope as priority regions to mitigate the inundation damages as early as possible.

Please tell us about the characteristics of the Wada-Yayoi Trunk Sewer.

Mr. Sakuma: The Wada-Yayoi Trunk Sewer is one of the largest sewer storage facilities in Japan, with storage capacity of 150,000 m2, diameter 8.5 m, and extension 2.2 km. Since we started storing here in FY 2007, it has contributed considerably to the mitigation of inundation damages. Its storage effect was used to the maximum in the East Japan Typhoon in 2019, and it reached the full capacity for the first time since its establishment.

What were the particular points that required creative efforts, and what were the difficulties?

Mr. Sakuma: I will first talk about our creative efforts. A large-scale facility will surely demonstrate its effectiveness once it is completed, but it takes a long period until it is completed. To deliver the inundation damage mitigation effect from an early stage, we utilize the partially completed facilities such as the trunk sewer as provisional storage pipes. We also install bypass pipes and additional street rainwater inlets in cooperation with the road administrator as small-scale measures.
Next are the difficulties we faced. Securing the land for vertical shafts, etc. for the tunnel construction to establish the sewer pipes is essential for large-scale facilities, and we do this upon gaining cooperation from the corresponding wards to form a consensus with the residents. The residents are happy as the effects are clearly demonstrated when we take measures in areas that are vulnerable to inundation damages, and many are cooperative when we start the construction. However, we borrow parks belonging to wards, etc. as the land to establish vertical shafts for deep underground constructions, and we carefully provide explanation and ensure their understanding, since the users of the parks will be inconvenienced if the construction requires a long period.

We hear that you won the Minister of Land, Infrastructure, Transport and Tourism Grand Prize. Can you tell us more about it?

Mr. Wakasa: It was in FY 2020, and we received the Minister of Land, Infrastructure, Transport and Tourism Award ‘Junkai no Michi Wastewater System Prize.’ An emergency heavy rainfall warning was issued for the first time in Tokyo due to the East Japan Typhoon in 2019, and we were awarded for making a great contribution to mitigating the inundation damages during the storm.
There are 56 storage facilities over the 23 wards, and we were able to store water to 60% of the total capacity for rainwater storage, which is 600,000 m³. We also have 70 rainwater pumps to help discharge water in low ground places where water cannot be discharged into rivers, etc. without help. These are capable of discharging rainwater by 110,000 m3 per minute. We were awarded for delivering the maximum stocking effect and dramatically reducing inundation. The fact that there has been no large inundation damage after we started storing indicates how much contribution it makes to the prevention of inundation damages.

It is expected that heavy rainfall over a short period of time will increase in frequency due to climate change in recent years. What measures are you considering in the field?

Mr. Sakuma: Hardware measures take time and cannot be implemented at once. However, rain will not wait. As a software measure, we therefore publish the rainfall information from the rainfall radar by the Bureau of Sewerage as ‘Tokyo Amesh.’ It became available to view on smartphones in 2017. It is quite high-performance and indicates even extremely small amounts of rain.

Mr. Okuda: We have about 60 million accesses per year. I hope that people use it for self-help, such as preparing sandbags, water stop boards, etc. or moving furniture while checking Amesh.

Mr. Sakuma: The metropolitan government also publishes the results of runoff analysis simulation on computers, etc. in case the maximum possible rainfall that can be expected with 153 mm per hour occurs as the inundation hazard area map. Each ward, city, town, or village prepares a flood hazard map based on this and distributes it to the residents.

While it is expected that climate change will progress further in the future, what measures is the Tokyo Metropolitan Government planning to take? And how do you cooperate with other concerned departments and bureaus?

Mr. Sakuma: For measures in 2020 and later based on the Basic Policy for Intense Rainfalls of the Tokyo Metropolitan Government, the government prepared the ‘Tokyo Metropolitan Government Action Plan for Heavy Rainfall’ in January 2020 as an action plan for the inundation measures for about 5 years into the future in cooperation with other concerned bureaus of the metropolitan government in light of the situation of torrential rains in recent years. We will continue the development of safe and secure urban areas while looking ahead to the 2030s and reinforcing cooperation within the metropolitan government such as the river department and the urban planning department in addition to the sewerage department.
We are constantly making adjustments and promoting cooperation with other concerned departments. Just because the sewers have been established, discharging from sewers into the rivers as much as possible will result in the rivers overflowing if the river improvements are not finished. We therefore discuss this with the river administrator and take measures such as adjusting the amount of water discharge from sewers into the rivers to suit the capacity of the river. We cooperate with the road administrator for roads. For example, rainwater that falls on ward roads is taken from the street rainwater inlets installed by the wards into the sewers. However, rainwater that flows down a slope often runs down the slope instead of going into the street rainwater inlet because they are accelerated, and causes inundation damages at the bottom of the slope. To facilitate the rainwater intake, we ask them to increase the number of street rainwater inlets or replace the inlet lid with grating, which is a mesh type, so that it is easier to take in the rainwater.
We are also asked for cooperation by the urban planning department in providing subsidies or instructions so that rainwater storage facilities or facilities to permeate rainwater are established at the same time when there is construction of a large building or redevelopment of an area.

Please tell us briefly about the sense of satisfaction in working in your current positions and your future prospects.

Mr. Sakuma: Many residents live in Tokyo, and the functions that support the national government and economy are also concentrated here. Sewerage operates 24 hours a day and 365 days a year without rest in order to support these activities. I feel the sense of satisfaction in operating the sewerage stably into the future and providing high-quality services as an essential worker supporting the society. I hope to continue working on the development of safe and secure urban areas by actively and effectively taking measures against inundation from both hardware and software aspects so that the lives and assets of Tokyo residents are protected from inundation damages and the urban activities of the capital city are supported.

Mr. Wakasa: Functions of sewerage cannot be stopped in order for people to continue their lives. That I can be involved in such an essential work gives me a great sense of satisfaction. In addition, the sewer facilities like the Wada-Yayoi Trunk Sewer that was visited in this interview are quite large when you see it with your own eyes, even though they stay invisible in your normal life. Being able to get involved in planning, etc. of such large-scale facilities also gives me satisfaction as an engineer.

Mr. Okuda: As much as 180 billion yen is spent by the 23 wards of Tokyo alone in sewerage projects to construct the sewer facilities like the Wada-Yayoi Trunk Sewer. Since we are essential workers, human resource development and succession of skills are also necessary so that young employees can work for the residents alongside the experienced employees who know much about sewers and the mid-level employees in the prime of life. I hope to continue my job to protect the safety and security of the residents of Tokyo, while also considering such things as a manager in the metropolitan government.