岸由二 慶應義塾大学名誉教授 npoTRネット代表理事
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Watershed Flood Control: Flood Control Measures in an Era of Climate Change Crisis
YUJI KISHI, Professor Emeritus, Keio University, Representative Director, npoTR net
April 18, 2022


Every year, large-scale floods and landslides have occurred, including the torrential rains in northern Kyushu in July 2017, the torrential rains in western Japan in 2006, the typhoon in eastern Japan in 2007, and the torrential rains in July 2020. The author, who has been involved in citizen activities along the Tsurumi River for 41 years, examines the past, present, and future of basin flood control.

Conventional flood control measures reaching their limits


Japan's flood control measures have reached a historic turning point: in 2020, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) announced a policy called "watershed flood control. The term "watershed" here does not refer to "flood plains. A flood plain is a plain created by sediments carried by a river, and is a low-lying area that is flooded when a river overflows during heavy rains. A "watershed," on the other hand, refers to the landforms and ecosystems that convert rainwater into rivers. The policy of the plan was that Japan's future flood control should "take into account the structure and ecosystem of the watershed, and the human use of the watershed as a whole. However, floods and landslides occur when large amounts of rainwater collect in watersheds during heavy rains. Why is the policy that "flood control should be carried out within a watershed" now a historical turning point? An explanation is in order.

Conventional flood control measures reaching their limits


Conventional flood control measures were based on the development and management of rivers and water systems in accordance with the River Law and the development and management of water supply facilities in accordance with the Sewerage Law. Rivers, water systems, and sewerage facilities are only part of a watershed. Within a watershed, there are green areas, farmlands, and urban facilities, and during heavy rains, rainwater is collected from the entire watershed, including these areas.


Conventional methods have been devised to provide greater flexibility in land use in the watershed. By concentrating the control of flooding and flood damage on river and sewerage projects, this has eased flood control restrictions on land use, such as the management of greenery and agricultural land, as well as residential and industrial use, and has supported the rapid urbanization of the archipelago. However, this division of labor is now failing to function adequately. Inadequate budgets for river and sewerage improvement projects are not the only reasons for this. There is an increasing risk that river and sewerage improvement alone will not be sufficient to cope with the unexpectedly heavy rains expected to be brought about by climate change.

Utilize the effects of green space and rice paddies for recreational water use


In recent years, the Japanese archipelago has been hit by a series of torrential floods in major rivers and severe landslides in hilly and mountainous areas. Although it is not yet possible to determine that this is a real and valid sign of climate change (global warming), it is true that awareness is rapidly spreading that the time has come to build a new adaptation system to cope with the heavy rainfall crisis. The new Japanese version of manifestation of this is the proposal for watershed flood control to adapt to the era of global warming and heavy rainfall.


In addition to river and sewage improvements, the new method also includes the development of a "green infrastructure" that takes advantage of the water retention and water utilization effects provided by the green areas within the watershed. The "landscape-based flood retarding " concept, which is based on the expectation of the floodplain space, such as rice paddies, to provide a flood control effect, was also emphasized. The policy of global warming adaptation through the entire development and utilization of the topography and ecosystem of the basin, and more specifically, the living areas that develop there, was clarified. In this way, the policy of global warming adaptation through the development and utilization of the topography and ecosystem of the watershed, and more specifically, the living areas that develop there, was clarified.

The goals for adaptation measures can be summarized in the following three points.


Control the magnitude of floods and other disasters caused by torrential rains
Limit the extent of flooding and other disasters
Facilitate post-disaster reconstruction


Revisions to related laws and regulations are underway, and the first-ever basin flood control plans have already been published under the name "Basin Flood Control Measures Project" for the nation's 109 major Class A water systems. However, none of the plans have yet taken the form of a comprehensive plan that includes "river and sewerage improvement," "green infrastructure planning," "flood control plan for overflows," and "review of urban planning itself. After successive revisions, a comprehensive law such as the Law for the Promotion of River Basin Flood Control may eventually be implemented, and basin development plans for each major river basin may become the law.

Tsurumi River, which has already been working on watershed flood control



While this is the situation in the country as a whole, there is actually a first-class water system that has been ahead of the curve in "basin flood control" for 41 years under the name of "comprehensive flood control," and has achieved results. It is the Tsurumi River basin(=watershed), which runs along the border between Tokyo and Kanagawa Prefectures. The Tsurumi River system, whose watershed is within a 30-kilometer radius of the centers of Tokyo, Kawasaki, and Yokohama, was subjected to rapid development pressure after World War II, mainly due to residential use.

By the mid-1970s, the area was flooded by thousands of homes due to rainfall that could not be called "heavy rainfall," and it was faced with a situation in which flood control by ordinary river and sewer management was reaching its limits. Under these circumstances, the River Bureau of the then Ministry of Construction (now the Ministry of Land, Infrastructure, Transport and Tourism) established the "Tsurumi River The Comprehensive Flood Control Measures for the Basin and the measures was launched in 1980. Apart from the name, it is in principle the same measure as the 2020 watershed flood control.


In the 41 years since then, the Tsurumi River watershed has undergone large-scale river improvement, including the construction of embankments, dredging, and the establishment of five large and small recreational areas, as well as sewerage system improvement, including the installation of large-capacity rainwater harvesting pipes, and green conservation has progressed throughout the watershed.


In the 1950s and 1960s, heavy damage with the size of 20,000 houses inundated under and above floor level was caused by the average two-day rainfall of ca.300 mm, in the Tsurumi River basin. Since 1990, the basin has been free from river flooding even after several torrential rains of the same magnitude.


In my opinion, the current flood control situation in the Tsurumi River basin is such that there is no danger of large-scale external flooding even in a once-in-50-year rainstorm. Although those history may not be applicable to other river basins because of the fact that the Tsurumi River is a typical urban water system with no dams and no vast rice paddies, the 41 years of experience in comprehensive flood control in the Tsurumi River basin will surely serve as a great reference for future flood control in watersheds nationwide.

Even higher hurdles, watershed management in an era of climate change


Nevertheless, "comprehensive flood control" in the Tsurumi River basin and the future "watershed flood control" are not identical as measures. Comprehensive flood control" is an urban watershed measure to cope with rapid urbanization, while "watershed flood control" presented in 2020 has a specific mission of adaptation to the era of global warming and heavy rainfall, without distinguishing between urban rivers and natural rivers. If the flood control plan is designed to cope with urbanization, it should be able to cope with the heavy rains that are expected to occur once every 100 to 200 years. However, flood control in the era of warming rainfall will have to take into account even greater amounts of rainfall, localized extreme rainfall, sea level rise, and storm surges. Watershed management in the era of warming and torrential rainfall is a hurdle much higher than the past to overcome.


Currently, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) has published inundation hazard maps for some important rivers in the event of an expected maximum heavy rainfall (once in 1,000 years). Basin flood control should be a disaster prevention and mitigation measure with this in mind. The basin flood control project, which will be tested and revised, will be unprecedented in the Tsurumi River basin, and will require cooperation among the national government, local governments, and the private sector.


Incidentally, the basic policy for river improvement of the Tsurumi River system assumes a torrential downpour once in 150 years, not once in 1,000 years, but even with a torrential downpour once in 150 years, the Tsurumi River basin is expected to inevitably be submerged several meters downstream if the parallel Tama River overflows at the same time. The expected submergence depth in the downstream area at the time of the expected maximum heavy rainfall is 3 to 10 meters. It is still unimaginable what kind of technological innovations, watershed cooperation, and urban reforms will be required to realize watershed flood control to adapt to such a situation and to improve the watershed environment beyond that.


If lessons can be drawn from the practice of comprehensive flood control in the Tsurumi River basin, it is the importance and difficulty of coordination among the national government, local governments, businesses, and citizens involved in the basin. It is important to keep in mind that coordination among the various stakeholders involved in basin flood control is easier said than done.


In addition to the comprehensive flood control efforts that the Tsurumi River has been undertaking to date, there is still no comprehensive, legal framework that requires the cooperation of various stakeholders in both planning and implementation of river basin flood control projects that will be undertaken in rivers across the country. It will probably take years or more for a law to be enacted to promote basin flood control, and for an organization to be established in each major river basin to integrate the entities involved and promote planning. The sense of urgency to promote global warming mitigation measures is stressed all around the nation in Japan, and a comparable sense of mission are now strongly expected from the national government agencies to promote river basin flood control project.

岸 由二 KISHI Yūji

View full list of biographies and writings
Evolutionary ecologist. Professor Emeritus at Keio University. Formerly a professor at Keio University. Doctor of Science. He has been promoting citizen activities in the Tsurumi River Watershed and Tama-Miura Hills area with the mission of creating a disaster-resistant and biodiversity-friendry urban culture/structure based on watershed thinking. Formerly a member of the National River Committee of the Ministry of Land, Infrastructure, Transport and Tourism. He is a member of the Tsurumi River Basin Water Committee and the representative director of the NPO Tsurumi River Basin Networking. Author of "Watershed Thinking for Survival" (Chikuma Primer Shinsho, 2021), "A Small Revolution of Selfish Gene" (Yasaka Shobo, 2019), "A Gaze to Nature" (Kinokuniya Shoten, 1996), etc. Co-translations include Richard Dawkins, The Selfish Gene (Ibid., 1991).< /p>

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